Sparse Index: Design, Format, Tests

From: Derrick Stolee <[email protected]>

This begins a long effort to update the index format to allow sparse directory entries. This should result in a significant improvement to Git commands when HEAD contains millions of files, but the user has selected many fewer files to keep in their sparse-checkout definition.

Currently, the index format is only updated in the presence of extensions.sparseIndex instead of increasing a file format version number. This is temporary, and index v5 is part of the plan for future work in this area.

The design document details many of the reasons for embarking on this work, and also the plan for completing it safely.

Signed-off-by Derrick Stolee <[email protected]>

---
 Documentation/technical/index-format.txt |   7 +
 Documentation/technical/sparse-index.txt | 167 +++++++++++++++++++++++
 2 files changed, 174 insertions(+)
 create mode 100644 Documentation/technical/sparse-index.txt

Documentation/technical/index-format.txt +7 -0

index b633482b1bdf..387126582556 100644
@@ -44,6 +44,13 @@ Git index format
   localization, no special casing of directory separator '/'). Entries
   with the same name are sorted by their stage field.
 
+  An index entry typically represents a file. However, if sparse-checkout
+  is enabled in cone mode (`core.sparseCheckoutCone` is enabled) and the
+  `extensions.sparseIndex` extension is enabled, then the index may
+  contain entries for directories outside of the sparse-checkout definition.
+  These entries have mode `0040000`, include the `SKIP_WORKTREE` bit, and
+  the path ends in a directory separator.
+
   32-bit ctime seconds, the last time a file's metadata changed
     this is stat(2) data
 

Documentation/technical/sparse-index.txt new file +37 -0

new file mode 100644
index 000000000000..9070836f0655
@@ -0,0 +1,167 @@
+Git Sparse-Index Design Document
+================================
+
+The sparse-checkout feature allows users to focus a working directory on
+a subset of the files at HEAD. The cone mode patterns, enabled by
+`core.sparseCheckoutCone`, allow for very fast pattern matching to
+discover which files at HEAD belong in the sparse-checkout cone.
+
+Three important scale dimensions for a Git worktree are:
+
+* `HEAD`: How many files are present at `HEAD`?
+
+* Populated: How many files are within the sparse-checkout cone.
+
+* Modified: How many files has the user modified in the working directory?
+
+We will use big-O notation -- O(X) -- to denote how expensive certain
+operations are in terms of these dimensions.
+
+These dimensions are ordered by their magnitude: users (typically) modify
+fewer files than are populated, and we can only populate files at `HEAD`.
+These dimensions are also ordered by how expensive they are per item: it
+is expensive to detect a modified file than it is to write one that we
+know must be populated; changing `HEAD` only really requires updating the
+index.
+
+Problems occur if there is an extreme imbalance in these dimensions. For
+example, if `HEAD` contains millions of paths but the populated set has
+only tens of thousands, then commands like `git status` and `git add` can
+be dominated by operations that require O(`HEAD`) operations instead of
+O(Populated). Primarily, the cost is in parsing and rewriting the index,
+which is filled primarily with files at `HEAD` that are marked with the
+`SKIP_WORKTREE` bit.
+
+The sparse-index intends to take these commands that read and modify the
+index from O(`HEAD`) to O(Populated). To do this, we need to modify the
+index format in a significant way: add "sparse directory" entries.

Documentation/technical/sparse-index.txt new file +15 -0

+index format and its in-memory data structure. There are many consumers in
+the codebase that expect to iterate through all of the index entries and
+see only files. In addition, they expect to see all files at `HEAD`. One
+way to handle this is to parse trees to replace a sparse-directory entry
+with all of the files within that tree as the index is loaded. However,
+parsing trees is slower than parsing the index format, so that is a slower
+operation than if we left the index alone.
+
+The implementation plan below follows four phases to slowly integrate with
+the sparse-index. The intention is to incrementally update Git commands to
+interact safely with the sparse-index without significant slowdowns. This
+may not always be possible, but the hope is that the primary commands that
+users need in their daily work are dramatically improved.
+
+Phase I: Format and initial speedups

Documentation/technical/sparse-index.txt new file +16 -0

+contains sparse-directory entries.
+
+To start, we use a new repository extension, `extensions.sparseIndex`, to
+allow inserting sparse-directory entries into indexes with file format
+versions 2, 3, and 4. This prevents Git versions that do not understand
+the sparse-index from operating on one, but it also prevents other
+operations that do not use the index at all. A new format, index v5, will
+be introduced that includes sparse-directory entries by default. It might
+also introduce other features that have been considered for improving the
+index, as well.
+
+Next, consumers of the index will be guarded against operating on a
+sparse-index by inserting calls to `ensure_full_index()` or
+`expand_index_to_path()`. After these guards are in place, we can begin
+leaving sparse-directory entries in the in-memory index structure.
+

Documentation/technical/sparse-index.txt new file +10 -0

+
+Phase II: Careful integrations
+------------------------------
+
+This phase focuses on ensuring that all index extensions and APIs work
+well with a sparse-index. This requires significant increases to our test
+coverage, especially for operations that interact with the working
+directory outside of the sparse-checkout definition. Some of these
+behaviors may not be the desirable ones, such as some tests already
+marked for failure in `t1092-sparse-checkout-compatibility.sh`.

Documentation/technical/sparse-index.txt new file +6 -0

+Phase III: Important command speedups
+-------------------------------------
+
+At this point, the patterns for testing and implementing sparse-directory
+logic should be relatively stable. This phase focuses on updating some of
+the most common builtins that use the index to operate as O(Populated).

Documentation/technical/sparse-index.txt new file +12 -0

+This phase is the first where parallel work might be possible without too
+much conflicts between topics.
+
+Phase IV: The long tail
+-----------------------
+
+This last phase is less a "phase" and more "the new normal" after all of
+the previous work.
+
+To start, the `command_requires_full_index` option could be removed in
+favor of expanding only when hitting an API guard.
+

On Tue, Feb 23, 2021 at 12:14 PM Derrick Stolee via GitGitGadget
<[email protected]> wrote:>> +This addition of sparse-directory entries violates expectations about the

Violates current expectations, yes. Documentation tends to live a long time, and I suspect that 2-3 years from now reading this sentence might be jarring since we'll have modified the code to have an updated set of expectations. Maybe a simple "As of time of writing, ..." at the beginning of the sentence here? Or maybe I'm just being overly worried...

Documentation/technical/sparse-index.txt line 98

Quoted 4 lines

Documentation/technical/sparse-index.txt new file +4 -0

+To complete this phase, the commands `git status` and `git add` will be
+integrated with the sparse-index so that they operate with O(Populated)
+performance. They will be carefully tested for operations within and
+outside the sparse-checkout definition.

Good plan so far, but there's something else bugging me a little here. One thing we noticed with our usage of sparse-checkout is that although unimportant _tracked_ files go away, leftover build files and other untracked files stick around. So, although 'git status' shouldn't have to check the tracked files anymore, it is still going to have to look at each of the *untracked* files and compare to .gitignore files in order to correctly classify each file as ignored or just plain untracked. Our sparsify tool has for a long time tried to warn about such files when changing the sparsity patterns/modules and had an --remove-old-ignores option for clearing out ignored files that are found within directories that are sparse (Meaning the directories where all files under them are marked SKIP_WORKTREE.). I was never sure whether a warning was enough, or if pushing that option more made sense, but about a month ago my colleagues made the tool just auto-invoke that option from other sparsify invocations. To my knowledge, there have been no complaints about that being automatically turned on; but there were complaints/confusion before about the directories being left around. (Of course, non-ignored files are still left around by that option.)

I'm worried that since sparse-checkout doesn't do anything to help with all these untracked/ignored files, we might not get all the performance improvements we want from a git status with sparse directories. We'll be dropping from walking O(2*HEAD) files (1 source
+ 1 object file) down to O(HEAD) files (just the object files) rather than actually getting down to O(Populated).

Quoted 6 lines

Documentation/technical/sparse-index.txt new file +6 -0

+Phase III: Important command speedups
+-------------------------------------
+
+At this point, the patterns for testing and implementing sparse-directory
+logic should be relatively stable. This phase focuses on updating some of
+the most common builtins that use the index to operate as O(Populated).

Documentation/technical/sparse-index.txt

Quoted 10 lines

Documentation/technical/sparse-index.txt new file +10 -0

+Here is a potential list of commands that could be valuable to integrate
+at this point:
+
+* `git commit`
+* `git checkout`
+* `git merge`
+* `git rebase`
+
+Along with `git status` and `git add`, these commands cover the majority
+of users' interactions with the working directory.

Sounds like a good plan as well.

I hope we get to make this specific to the merge-ort backend. It localizes the index-related code to (a) a call to unpack_trees() called from checkout-like code (which would probably automatically be handled by your updates to git checkout), and (b) a single function named record_conflicted_index_entries(). I feel it should be pretty easy to update.

In contrast, the idea of attempting to update merge-recursive with this kind of change sounds overwhelming.

Documentation/technical/sparse-index.txt line 144

Quoted 10 lines

In addition, we can

Documentation/technical/sparse-index.txt new file +9 -0

+integrate with these commands:
+
+* `git grep`
+* `git rm`
+
+These have been proposed as some whose behavior could change when in a
+repo with a sparse-checkout definition. It would be good to include this
+behavior automatically when using a sparse-index. Some clarity is needed
+to make the behavior switch clear to the user.

Is this leftover from before recent events? I think this portion of the document should just be stricken.

I argued about how these were buggy as-is due SKIP_WORKTREE always having been an incomplete implementation of an idea at [1], but didn't hear a further response from you. I'm curious if you disagreed with my reasoning, or it just slipped through the cracks in a busy schedule and this portion of the document was leftover from before. In my opinion, both commands are just buggy and should be fixed for general sparse-checkout usage cases, not just for sparse-index.

Anyway, that's a long way of saying I think this section of your document is already obsolete. (Which is a good thing -- less work to do to get sparse-index working. Thanks, Matheus!).

Quoted 12 lines

Documentation/technical/sparse-index.txt new file +12 -0

+This phase is the first where parallel work might be possible without too
+much conflicts between topics.
+
+Phase IV: The long tail
+-----------------------
+
+This last phase is less a "phase" and more "the new normal" after all of
+the previous work.
+
+To start, the `command_requires_full_index` option could be removed in
+favor of expanding only when hitting an API guard.
+

Documentation/technical/sparse-index.txt line 167

Quoted 10 lines

Documentation/technical/sparse-index.txt new file +10 -0

+There are many Git commands that could use special attention to operate as
+O(Populated), while some might be so rare that it is acceptable to leave
+them with additional overhead when a sparse-index is present.
+
+Here are some commands that might be useful to update:
+
+* `git sparse-checkout set`
+* `git am`
+* `git clean`
+* `git stash`

Oh, man, git stash is definitely in need of work. It's still a

minimalistic transliteration of shell to C, complete with lots of process forking and piping output between various low-level commands. It might be interesting to rewrite this in terms of the merge machinery, though its separate stashing of staged stuff, unstaged stuff, and possibly untracked stuff means that there is a sequence of two or three merges needed and interesting failure handling to do if those merges fail, especially if the user uses --index. But I digress...

Anyway, overall, very nicely written and planned out. Thanks for taking the time to write this all up.

Quoted 9 lines · Elijah Newren

On Tue, Feb 23, 2021 at 12:14 PM Derrick Stolee via GitGitGadget
<[email protected]> wrote:>> +This addition of sparse-directory entries violates expectations about the

Violates current expectations, yes. Documentation tends to live a long time, and I suspect that 2-3 years from now reading this sentence might be jarring since we'll have modified the code to have an updated set of expectations. Maybe a simple "As of time of writing, ..." at the beginning of the sentence here? Or maybe I'm just being overly worried...

I was hoping that the phrase "this addition of" places this statement in a moment of time where sparse-directory entries didn't exist, but now they will. I'm open to alternatives and will give this some thought.

Quoted 26 lines

Documentation/technical/sparse-index.txt line 98

Quoted 5 lines

Documentation/technical/sparse-index.txt new file +4 -0

+To complete this phase, the commands `git status` and `git add` will be
+integrated with the sparse-index so that they operate with O(Populated)
+performance. They will be carefully tested for operations within and
+outside the sparse-checkout definition.

Good plan so far, but there's something else bugging me a little here. One thing we noticed with our usage of sparse-checkout is that although unimportant _tracked_ files go away, leftover build files and other untracked files stick around. So, although 'git status' shouldn't have to check the tracked files anymore, it is still going to have to look at each of the *untracked* files and compare to .gitignore files in order to correctly classify each file as ignored or just plain untracked. Our sparsify tool has for a long time tried to warn about such files when changing the sparsity patterns/modules and had an --remove-old-ignores option for clearing out ignored files that are found within directories that are sparse (Meaning the directories where all files under them are marked SKIP_WORKTREE.). I was never sure whether a warning was enough, or if pushing that option more made sense, but about a month ago my colleagues made the tool just auto-invoke that option from other sparsify invocations. To my knowledge, there have been no complaints about that being automatically turned on; but there were complaints/confusion before about the directories being left around. (Of course, non-ignored files are still left around by that option.)

Quoted 6 lines

I'm worried that since sparse-checkout doesn't do anything to help with all these untracked/ignored files, we might not get all the performance improvements we want from a git status with sparse directories. We'll be dropping from walking O(2*HEAD) files (1 source
+ 1 object file) down to O(HEAD) files (just the object files) rather than actually getting down to O(Populated).

This definitely seems like a valuable _enhancement_ to sparse-checkout that could occur in parallel.

For a workaround in the moment: is "git clean -xdf" sufficient to help these users?

Quoted 26 lines

Quoted 6 lines

Documentation/technical/sparse-index.txt new file +6 -0

+Phase III: Important command speedups
+-------------------------------------
+
+At this point, the patterns for testing and implementing sparse-directory
+logic should be relatively stable. This phase focuses on updating some of
+the most common builtins that use the index to operate as O(Populated).

Documentation/technical/sparse-index.txt

Quoted 10 lines

Documentation/technical/sparse-index.txt new file +10 -0

+Here is a potential list of commands that could be valuable to integrate
+at this point:
+
+* `git commit`
+* `git checkout`
+* `git merge`
+* `git rebase`
+
+Along with `git status` and `git add`, these commands cover the majority
+of users' interactions with the working directory.

Sounds like a good plan as well.

I hope we get to make this specific to the merge-ort backend. It localizes the index-related code to (a) a call to unpack_trees() called from checkout-like code (which would probably automatically be handled by your updates to git checkout), and (b) a single function named record_conflicted_index_entries(). I feel it should be pretty easy to update.

Quoted 2 lines

In contrast, the idea of attempting to update merge-recursive with this kind of change sounds overwhelming.

Yes, I'm hoping to eventually say "if you are in a sparse-checkout, then you should use ORT by default." Then, if someone opts to do merge-recursive instead, then they pay the index expansion cost.

While this is very clear in my head, it might be worth mentioning that explicitly here.

Quoted 14 lines

Documentation/technical/sparse-index.txt line 144

Quoted 10 lines

In addition, we can

Documentation/technical/sparse-index.txt new file +9 -0

+integrate with these commands:
+
+* `git grep`
+* `git rm`
+
+These have been proposed as some whose behavior could change when in a
+repo with a sparse-checkout definition. It would be good to include this
+behavior automatically when using a sparse-index. Some clarity is needed
+to make the behavior switch clear to the user.

Is this leftover from before recent events? I think this portion of the document should just be stricken.

Quoted 7 lines

I argued about how these were buggy as-is due SKIP_WORKTREE always having been an incomplete implementation of an idea at [1], but didn't hear a further response from you. I'm curious if you disagreed with my reasoning, or it just slipped through the cracks in a busy schedule and this portion of the document was leftover from before. In my opinion, both commands are just buggy and should be fixed for general sparse-checkout usage cases, not just for sparse-index.

This is definitely a case of "I've been too busy to read those topics in detail." I figured that there was something going on that was relevant to the sparse-checkout and would affect the sparse-index, but I shelved it in my mind until I had space to think about it directly.

Quoted 3 lines

Anyway, that's a long way of saying I think this section of your document is already obsolete. (Which is a good thing -- less work to do to get sparse-index working. Thanks, Matheus!).

Thank you for your summary, which helps a lot. Thanks, Matheus, too! If those fixes already include coverage for the behavior, then I'll see if they also translate to sparse-index tests easily.

I feel like a lot of these later contributions will be more about adding tests than actually writing a lot of code.

Quoted 24 lines

Quoted 12 lines

Documentation/technical/sparse-index.txt new file +12 -0

+This phase is the first where parallel work might be possible without too
+much conflicts between topics.
+
+Phase IV: The long tail
+-----------------------
+
+This last phase is less a "phase" and more "the new normal" after all of
+the previous work.
+
+To start, the `command_requires_full_index` option could be removed in
+favor of expanding only when hitting an API guard.
+

Documentation/technical/sparse-index.txt line 167

Quoted 10 lines

Documentation/technical/sparse-index.txt new file +10 -0

+There are many Git commands that could use special attention to operate as
+O(Populated), while some might be so rare that it is acceptable to leave
+them with additional overhead when a sparse-index is present.
+
+Here are some commands that might be useful to update:
+
+* `git sparse-checkout set`
+* `git am`
+* `git clean`
+* `git stash`

Oh, man, git stash is definitely in need of work. It's still a

Quoted 8 lines

minimalistic transliteration of shell to C, complete with lots of process forking and piping output between various low-level commands. It might be interesting to rewrite this in terms of the merge machinery, though its separate stashing of staged stuff, unstaged stuff, and possibly untracked stuff means that there is a sequence of two or three merges needed and interesting failure handling to do if those merges fail, especially if the user uses --index. But I digress...

I would prefer to leave 'git stash' alone, but it's used by enough people that I need to care about it eventually.

Quoted 2 lines

Anyway, overall, very nicely written and planned out. Thanks for taking the time to write this all up.

Thanks for your detailed comments!
-Stolee

From: Derrick Stolee <[email protected]>

Create a test script that takes the default performance test (the Git codebase) and multiplies it by 256 using four layers of duplicated trees of width four. This results in nearly one million blob entries in the index. Then, we can clone this repository with sparse-checkout patterns that demonstrate four copies of the initial repository. Each clone will use a different index format or mode so peformance can be tested across the different options.

Note that the initial repo is stripped of submodules before doing the copies. This preserves the expected data shape of the sparse index, because directories containing submodules are not collapsed to a sparse directory entry.

Run a few Git commands on these clones, especially those that use the index (status, add, commit).

Here are the results on my Linux machine:

Test
--------------------------------------------------------------

2000.2: git status (full-index-v3)             0.37(0.30+0.09)
2000.3: git status (full-index-v4)             0.39(0.32+0.10)
2000.4: git add -A (full-index-v3)             1.42(1.06+0.20)
2000.5: git add -A (full-index-v4)             1.26(0.98+0.16)
2000.6: git add . (full-index-v3)              1.40(1.04+0.18)
2000.7: git add . (full-index-v4)              1.26(0.98+0.17)
2000.8: git commit -a -m A (full-index-v3)     1.42(1.11+0.16)
2000.9: git commit -a -m A (full-index-v4)     1.33(1.08+0.16)

It is perhaps noteworthy that there is an improvement when using index version 4. This is because the v3 index uses 108 MiB while the v4 index uses 80 MiB. Since the repeated portions of the directories are very short (f3/f1/f2, for example) this ratio is less pronounced than in similarly-sized real repositories.

Signed-off-by Derrick Stolee <[email protected]>

---
 t/perf/p2000-sparse-operations.sh | 87 +++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100755 t/perf/p2000-sparse-operations.sh

t/perf/p2000-sparse-operations.sh new file +7 -0

new file mode 100755
index 000000000000..52597683376e
@@ -0,0 +1,87 @@
+#!/bin/sh
+
+test_description="test performance of Git operations using the index"
+
+. ./perf-lib.sh
+
+test_perf_default_repo

t/perf/p2000-sparse-operations.sh new file +65 -1

+       git commit -m "remove submodules" &&
+
+       echo bogus >a &&
+       cp a b &&
+       git add a b &&
+       git commit -m "level 0" &&
+       BLOB=$(git rev-parse HEAD:a) &&
+       OLD_COMMIT=$(git rev-parse HEAD) &&
+       OLD_TREE=$(git rev-parse HEAD^{tree}) &&
+
+       for i in $(test_seq 1 4)
+       do
+               cat >in <<-EOF &&
+                       100755 blob $BLOB       a
+                       040000 tree $OLD_TREE   f1
+                       040000 tree $OLD_TREE   f2
+                       040000 tree $OLD_TREE   f3
+                       040000 tree $OLD_TREE   f4
+               EOF
+               NEW_TREE=$(git mktree <in) &&
+               NEW_COMMIT=$(git commit-tree $NEW_TREE -p $OLD_COMMIT -m "level $i") &&
+               OLD_TREE=$NEW_TREE &&
+               OLD_COMMIT=$NEW_COMMIT || return 1
+       done &&
+
+       git sparse-checkout init --cone &&
+       git branch -f wide $OLD_COMMIT &&
+       git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-index-v3 &&
+       (
+               cd full-index-v3 &&
+               git sparse-checkout init --cone &&
+               git sparse-checkout set $SPARSE_CONE &&
+               git config index.version 3 &&
+               git update-index --index-version=3
+       ) &&
+       git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-index-v4 &&
+       (
+               cd full-index-v4 &&
+               git sparse-checkout init --cone &&
+               git sparse-checkout set $SPARSE_CONE &&
+               git config index.version 4 &&
+               git update-index --index-version=4
+       )
+'
+
+test_perf_on_all () {
+       command="$@"
+       for repo in full-index-v3 full-index-v4
+       do
+               test_perf "$command ($repo)" "
+                       (
+                               cd $repo &&
+                               echo >>$SPARSE_CONE/a &&
+                               $command
+                       )
+               "
+       done
+}
+
+test_perf_on_all git status
+test_perf_on_all git add -A
+test_perf_on_all git add .
+test_perf_on_all git commit -a -m A
+
+test_done
--

gitgitgadget

On Tue, Feb 23, 2021 at 12:14 PM Derrick Stolee via GitGitGadget <[email protected]> wrote:

t/perf/p2000-sparse-operations.sh new file +7 -0

+test_expect_success 'setup repo and indexes' '
+       git reset --hard HEAD &&
+       # Remove submodules from the example repo, because our
+       # duplication of the entire repo creates an unlikly data shape.
+       git config --file .gitmodules --get-regexp "submodule.*.path" >modules &&
+       rm -f .gitmodules &&
+       git add .gitmodules &&

Why not git rm [-f] .gitmodules instead of these two commands? Is there something special about .gitmodules that requires this special handling?

t/perf/p2000-sparse-operations.sh line 22

Quoted 5 lines

t/perf/p2000-sparse-operations.sh new file +5 -0

+       for module in $(awk "{print \$2}" modules)
+       do
+               git rm $module || return 1
+       done &&
+       git add . &&

What does the git add . do? I don't see any changes there weren't already git-add'ed or git-rm'ed.

From: Derrick Stolee <[email protected]>

Upcoming changes will introduce modifications to the index format that allow sparse directories. It will be useful to have a mechanism for converting those sparse index files into full indexes by walking the tree at those sparse directories. Name this method ensure_full_index() as it will guarantee that the index is fully expanded.

This method is not implemented yet, and instead we focus on the scaffolding to declare it and call it at the appropriate time.

Add a 'command_requires_full_index' member to struct repo_settings. This will be an indicator that we need the index in full mode to do certain index operations. This starts as being true for every command, then we will set it to false as some commands integrate with sparse indexes.

If 'command_requires_full_index' is true, then we will immediately expand a sparse index to a full one upon reading from disk. This suffices for now, but we will want to add more callers to ensure_full_index() later.

Signed-off-by Derrick Stolee <[email protected]>

---
 Makefile        |  1 +
 repo-settings.c |  8 ++++++++
 repository.c    | 11 ++++++++++-
 repository.h    |  2 ++
 sparse-index.c  |  8 ++++++++
 sparse-index.h  |  7 +++++++
 6 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 sparse-index.c
 create mode 100644 sparse-index.h

Makefile +1 -0

index 5a239cac20e3..3bf61699238d 100644
@@ -980,6 +980,7 @@ LIB_OBJS += setup.o
 LIB_OBJS += shallow.o
 LIB_OBJS += sideband.o
 LIB_OBJS += sigchain.o
+LIB_OBJS += sparse-index.o
 LIB_OBJS += split-index.o
 LIB_OBJS += stable-qsort.o
 LIB_OBJS += strbuf.o

repo-settings.c +8 -0

index f7fff0f5ab83..d63569e4041e 100644
@@ -77,4 +77,12 @@ void prepare_repo_settings(struct repository *r)
                UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_KEEP);
 
        UPDATE_DEFAULT_BOOL(r->settings.fetch_negotiation_algorithm, FETCH_NEGOTIATION_DEFAULT);
+
+       /*
+        * This setting guards all index reads to require a full index
+        * over a sparse index. After suitable guards are placed in the
+        * codebase around uses of the index, this setting will be
+        * removed.
+        */
+       r->settings.command_requires_full_index = 1;
 }

repository.c +10 -1

index c98298acd017..a8acae002f71 100644
@@ -10,6 +10,7 @@
 #include "object.h"
 #include "lockfile.h"
 #include "submodule-config.h"
+#include "sparse-index.h"
 
 /* The main repository */
 static struct repository the_repo;

@@ -261,6 +262,8 @@ void repo_clear(struct repository *repo)
 
 int repo_read_index(struct repository *repo)
 {
+       int res;
+
        if (!repo->index)
                repo->index = xcalloc(1, sizeof(*repo->index));
 

@@ -270,7 +273,13 @@ int repo_read_index(struct repository *repo)
        else if (repo->index->repo != repo)
                BUG("repo's index should point back at itself");
 
-       return read_index_from(repo->index, repo->index_file, repo->gitdir);
+       res = read_index_from(repo->index, repo->index_file, repo->gitdir);
+
+       prepare_repo_settings(repo);
+       if (repo->settings.command_requires_full_index)
+               ensure_full_index(repo->index);
+
+       return res;
 }
 
 int repo_hold_locked_index(struct repository *repo,

repository.h +2 -0

index b385ca3c94b6..e06a23015697 100644
@@ -41,6 +41,8 @@ struct repo_settings {
        enum fetch_negotiation_setting fetch_negotiation_algorithm;
 
        int core_multi_pack_index;
+
+       unsigned command_requires_full_index:1;
 };
 
 struct repository {

sparse-index.c new file +8 -0

new file mode 100644
index 000000000000..82183ead563b
@@ -0,0 +1,8 @@
+#include "cache.h"
+#include "repository.h"
+#include "sparse-index.h"
+
+void ensure_full_index(struct index_state *istate)
+{
+       /* intentionally left blank */
+}

sparse-index.h new file +7 -1

new file mode 100644
index 000000000000..09a20d036c46
@@ -0,0 +1,7 @@
+#ifndef SPARSE_INDEX_H__
+#define SPARSE_INDEX_H__
+
+struct index_state;
+void ensure_full_index(struct index_state *istate);
+
+#endif
--

gitgitgadget

From: Derrick Stolee <[email protected]>

We will mark an in-memory index_state as having sparse directory entries with the sparse_index bit. These currently cannot exist, but we will add a mechanism for collapsing a full index to a sparse one in a later change. That will happen at write time, so we must first allow parsing the format before writing it.

Commands or methods that require a full index in order to operate can call ensure_full_index() to expand that index in-memory. This requires parsing trees using that index's repository.

Sparse directory entries have a specific 'ce_mode' value. The macro S_ISSPARSEDIR(ce->ce_mode) can check if a cache_entry 'ce' has this type. This ce_mode is not possible with the existing index formats, so we don't also verify all properties of a sparse-directory entry, which are:

1. ce->ce_mode == 0040000
2. ce->flags & CE_SKIP_WORKTREE is true
3. ce->name[ce->namelen - 1] == '/' (ends in dir separator)
4. ce->oid references a tree object.

These are all semi-enforced in ensure_full_index() to some extent. Any deviation will cause a warning at minimum or a failure in the worst case.

Signed-off-by Derrick Stolee <[email protected]>

---
 cache.h        |  7 +++-
 read-cache.c   |  9 +++++
 sparse-index.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 109 insertions(+), 2 deletions(-)

cache.h +0 -0

index d92814961405..1336c8d7435e 100644

cache.h +5 -1

+
 /* Forward structure decls */
 struct pathspec;
 struct child_process;

@@ -319,7 +321,8 @@ struct index_state {
                 drop_cache_tree : 1,
                 updated_workdir : 1,
                 updated_skipworktree : 1,
-                fsmonitor_has_run_once : 1;
+                fsmonitor_has_run_once : 1,
+                sparse_index : 1;
        struct hashmap name_hash;
        struct hashmap dir_hash;
        struct object_id oid;

@@ -722,6 +725,8 @@ int read_index_from(struct index_state *, const char *path,
                    const char *gitdir);
 int is_index_unborn(struct index_state *);
 
+void ensure_full_index(struct index_state *istate);
+
 /* For use with `write_locked_index()`. */
 #define COMMIT_LOCK            (1 << 0)
 #define SKIP_IF_UNCHANGED      (1 << 1)

read-cache.c +0 -0

index 29144cf879e7..97dbf2434f30 100644

read-cache.c +7 -0

+
        istate->cache[nr] = ce;
        add_name_hash(istate, ce);
 }

@@ -2255,6 +2258,12 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
        trace2_data_intmax("index", the_repository, "read/cache_nr",
                           istate->cache_nr);
 
+       if (!istate->repo)
+               istate->repo = the_repository;
+       prepare_repo_settings(istate->repo);
+       if (istate->repo->settings.command_requires_full_index)
+               ensure_full_index(istate);
+
        return istate->cache_nr;
 
 unmap:

sparse-index.c +77 -1

index 82183ead563b..316cb949b74b 100644
@@ -1,8 +1,101 @@
 #include "cache.h"
 #include "repository.h"
 #include "sparse-index.h"
+#include "tree.h"
+#include "pathspec.h"
+#include "trace2.h"
+
+static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
+{
+       ALLOC_GROW(istate->cache, nr + 1, istate->cache_alloc);
+
+       istate->cache[nr] = ce;
+       add_name_hash(istate, ce);
+}
+
+static int add_path_to_index(const struct object_id *oid,
+                               struct strbuf *base, const char *path,
+                               unsigned int mode, int stage, void *context)
+{
+       struct index_state *istate = (struct index_state *)context;
+       struct cache_entry *ce;
+       size_t len = base->len;
+
+       if (S_ISDIR(mode))
+               return READ_TREE_RECURSIVE;
+
+       strbuf_addstr(base, path);
+
+       ce = make_cache_entry(istate, mode, oid, base->buf, 0, 0);
+       ce->ce_flags |= CE_SKIP_WORKTREE;
+       set_index_entry(istate, istate->cache_nr++, ce);
+
+       strbuf_setlen(base, len);
+       return 0;
+}
 
 void ensure_full_index(struct index_state *istate)
 {
-       /* intentionally left blank */
+       int i;
+       struct index_state *full;
+
+       if (!istate || !istate->sparse_index)
+               return;
+
+       if (!istate->repo)
+               istate->repo = the_repository;
+
+       trace2_region_enter("index", "ensure_full_index", istate->repo);
+
+       /* initialize basics of new index */
+       full = xcalloc(1, sizeof(struct index_state));
+       memcpy(full, istate, sizeof(struct index_state));
+
+       /* then change the necessary things */
+       full->sparse_index = 0;
+       full->cache_alloc = (3 * istate->cache_alloc) / 2;
+       full->cache_nr = 0;
+       ALLOC_ARRAY(full->cache, full->cache_alloc);
+
+       for (i = 0; i < istate->cache_nr; i++) {
+               struct cache_entry *ce = istate->cache[i];
+               struct tree *tree;
+               struct pathspec ps;
+
+               if (!S_ISSPARSEDIR(ce->ce_mode)) {
+                       set_index_entry(full, full->cache_nr++, ce);
+                       continue;
+               }
+               if (!(ce->ce_flags & CE_SKIP_WORKTREE))
+                       warning(_("index entry is a directory, but not sparse (%08x)"),
+                               ce->ce_flags);
+
+               /* recursively walk into cd->name */
+               tree = lookup_tree(istate->repo, &ce->oid);
+
+               memset(&ps, 0, sizeof(ps));
+               ps.recursive = 1;
+               ps.has_wildcard = 1;
+               ps.max_depth = -1;
+
+               read_tree_recursive(istate->repo, tree,
+                                   ce->name, strlen(ce->name),
+                                   0, &ps,

sparse-index.c +7 -1

+       istate->cache = full->cache;
+       istate->cache_nr = full->cache_nr;
+       istate->cache_alloc = full->cache_alloc;
+
+       free(full);
+
+       trace2_region_leave("index", "ensure_full_index", istate->repo);
 }
--

gitgitgadget

From: Derrick Stolee <[email protected]>

Add a new 'sparse-index' repo alongside the 'full-checkout' and 'sparse-checkout' repos in t1092-sparse-checkout-compatibility.sh. Also add run_on_sparse and test_sparse_match helpers. These helpers will be used when the sparse index is implemented.

Add GIT_TEST_SPARSE_INDEX environment variable to enable the sparse-index by default. This will be intended to use across the entire test suite, except that it will only affect cases where the sparse-checkout feature is enabled.

Signed-off-by Derrick Stolee <[email protected]>

---
 t/README                                 |  3 +++
 t/t1092-sparse-checkout-compatibility.sh | 24 ++++++++++++++++++++----
 2 files changed, 23 insertions(+), 4 deletions(-)

t/README +3 -0

index 593d4a4e270c..b98bc563aab5 100644
@@ -439,6 +439,9 @@ and "sha256".
 GIT_TEST_WRITE_REV_INDEX=<boolean>, when true enables the
 'pack.writeReverseIndex' setting.
 
+GIT_TEST_SPARSE_INDEX=<boolean>, when true enables index writes to use the
+sparse-index format by default.
+
 Naming Tests
 ------------
 

t/t1092-sparse-checkout-compatibility.sh +14 -4

index 3725d3997e70..71d6f9e4c014 100755
@@ -7,6 +7,7 @@ test_description='compare full workdir to sparse workdir'
 test_expect_success 'setup' '
        git init initial-repo &&
        (
+               GIT_TEST_SPARSE_INDEX=0 &&
                cd initial-repo &&
                echo a >a &&
                echo "after deep" >e &&

@@ -87,23 +88,32 @@ init_repos () {
 
        cp -r initial-repo sparse-checkout &&
        git -C sparse-checkout reset --hard &&
-       git -C sparse-checkout sparse-checkout init --cone &&
+
+       cp -r initial-repo sparse-index &&
+       git -C sparse-index reset --hard &&
 
        # initialize sparse-checkout definitions
-       git -C sparse-checkout sparse-checkout set deep
+       git -C sparse-checkout sparse-checkout init --cone &&
+       git -C sparse-checkout sparse-checkout set deep &&
+       GIT_TEST_SPARSE_INDEX=1 git -C sparse-index sparse-checkout init --cone &&
+       GIT_TEST_SPARSE_INDEX=1 git -C sparse-index sparse-checkout set deep
 }
 
 run_on_sparse () {
        (
                cd sparse-checkout &&
-               "$@" >../sparse-checkout-out 2>../sparse-checkout-err
+               GIT_TEST_SPARSE_INDEX=0 "$@" >../sparse-checkout-out 2>../sparse-checkout-err
+       ) &&
+       (
+               cd sparse-index &&
+               GIT_TEST_SPARSE_INDEX=1 "$@" >../sparse-index-out 2>../sparse-index-err
        )
 }
 
 run_on_all () {
        (
                cd full-checkout &&
-               "$@" >../full-checkout-out 2>../full-checkout-err
+               GIT_TEST_SPARSE_INDEX=0 "$@" >../full-checkout-out 2>../full-checkout-err
        ) &&
        run_on_sparse "$@"
 }

t/t1092-sparse-checkout-compatibility.sh +4 -1

+       test_cmp sparse-checkout-out sparse-index-out &&
+       test_cmp sparse-checkout-err sparse-index-err
+}
+
 test_expect_success 'status with options' '
        init_repos &&
        test_all_match git status --porcelain=v2 &&
--

gitgitgadget

From: Derrick Stolee <[email protected]>

This table is helpful for discovering data in the index to ensure it is being written correctly, especially as we build and test the sparse-index. This table includes an output format similar to 'git ls-tree', but should not be compared to that directly. The biggest reasons are that 'git ls-tree' includes a tree entry for every subdirectory, even those that would not appear as a sparse directory in a sparse-index. Further, 'git ls-tree' does not use a trailing directory separator for its tree rows.

This does not print the stat() information for the blobs. That could be added in a future change with another option. The tests that are added in the next few changes care only about the object types and IDs.

To make the option parsing slightly more robust, wrap the string comparisons in a loop adapted from test-dir-iterator.c.

Care must be taken with the final check for the 'cnt' variable. We continue the expectation that the numerical value is the final argument.

Signed-off-by Derrick Stolee <[email protected]>

---
 t/helper/test-read-cache.c | 50 ++++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 10 deletions(-)

t/helper/test-read-cache.c +0 -0

index 244977a29bdf..e4c3492f7d3e 100644

t/helper/test-read-cache.c +19 -11

+}
+
 int cmd__read_cache(int argc, const char **argv)
 {
+       struct repository *r = the_repository;
        int i, cnt = 1;
        const char *name = NULL;
+       int table = 0;
 
-       if (argc > 1 && skip_prefix(argv[1], "--print-and-refresh=", &name)) {
-               argc--;
-               argv++;
+       for (++argv, --argc; *argv && starts_with(*argv, "--"); ++argv, --argc) {
+               if (skip_prefix(*argv, "--print-and-refresh=", &name))
+                       continue;
+               if (!strcmp(*argv, "--table"))
+                       table = 1;
        }
 
-       if (argc == 2)
-               cnt = strtol(argv[1], NULL, 0);
+       if (argc == 1)
+               cnt = strtol(argv[0], NULL, 0);
        setup_git_directory();
        git_config(git_default_config, NULL);
+
        for (i = 0; i < cnt; i++) {
-               read_cache();
+               repo_read_index(r);
                if (name) {
                        int pos;
 
-                       refresh_index(&the_index, REFRESH_QUIET,
+                       refresh_index(r->index, REFRESH_QUIET,
                                      NULL, NULL, NULL);
-                       pos = index_name_pos(&the_index, name, strlen(name));
+                       pos = index_name_pos(r->index, name, strlen(name));
                        if (pos < 0)
                                die("%s not in index", name);
                        printf("%s is%s up to date\n", name,
-                              ce_uptodate(the_index.cache[pos]) ? "" : " not");
+                              ce_uptodate(r->index->cache[pos]) ? "" : " not");
                        write_file(name, "%d\n", i);
                }
-               discard_cache();
+               if (table)
+                       print_cache(r->index);
+               discard_index(r->index);
        }
        return 0;
 }
--

gitgitgadget

Quoted 35 lines · Derrick Stolee via GitGitGadget

From: Derrick Stolee <[email protected]>

This table is helpful for discovering data in the index to ensure it is being written correctly, especially as we build and test the sparse-index. This table includes an output format similar to 'git ls-tree', but should not be compared to that directly. The biggest reasons are that 'git ls-tree' includes a tree entry for every subdirectory, even those that would not appear as a sparse directory in a sparse-index. Further, 'git ls-tree' does not use a trailing directory separator for its tree rows.

This does not print the stat() information for the blobs. That could be added in a future change with another option. The tests that are added in the next few changes care only about the object types and IDs.

To make the option parsing slightly more robust, wrap the string comparisons in a loop adapted from test-dir-iterator.c.

Care must be taken with the final check for the 'cnt' variable. We continue the expectation that the numerical value is the final argument.

Signed-off-by Derrick Stolee <[email protected]>

---
 t/helper/test-read-cache.c | 50 ++++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 10 deletions(-)

t/helper/test-read-cache.c +0 -0

index 244977a29bdf..e4c3492f7d3e 100644
@@ -2,35 +2,65 @@
 #include "cache.h"
 #include "config.h"
 

t/helper/test-read-cache.c line 7

Quoted 3 lines

t/helper/test-read-cache.c +3 -0

+static void print_cache_entry(struct cache_entry *ce)
+{
+       printf("%06o ", ce->ce_mode & 0777777);

This constant is curious. I think it's because you want to strip off the special in-memory bits of the ce_mode where git stores extra data, which would be everything beyond the first 16 bits (as noted in a comment near the beginning of cache.h). But here you keep the first 18 bits. Is CE_UPDATE and CE_REMOVE just 0 in the cases you've viewed so this works (but you really should use 0177777 or 0xFFFF), or am I off in my guess of what you're trying to do and you do want to see those two flags?

It also seems surprising to me that this constant isn't already defined somewhere in cache.h or as some variant of S_IFMT, though I'm not finding it.

t/helper/test-read-cache.c line 14

Quoted 7 lines

t/helper/test-read-cache.c +7 -0

+
+       if (S_ISSPARSEDIR(ce->ce_mode))
+               printf("tree ");
+       else if (S_ISGITLINK(ce->ce_mode))
+               printf("commit ");
+       else
+               printf("blob ");

Perhaps make use of the tree_type, commit_type, and blob_type global constants?

Quoted 5 lines

t/helper/test-read-cache.c +5 -0

+
+       printf("%s\t%s\n",
+              oid_to_hex(&ce->oid),
+              ce->name);
+}

t/helper/test-read-cache.c line 25

Quoted 6 lines

t/helper/test-read-cache.c +6 -0

+
+static void print_cache(struct index_state *cache)
+{
+       int i;
+       for (i = 0; i < the_index.cache_nr; i++)
+               print_cache_entry(the_index.cache[i]);

Why are you passing cache as a parameter, then ignoring it and using the_index?

From: Derrick Stolee <[email protected]>

As we modify the sparse-checkout definition, we perform index operations on a pattern_list that only exists in-memory. This allows easy backing out in case the index update fails.

However, if the index write itself cares about the sparse-checkout pattern set, we need access to that in-memory copy. Place a pointer to a 'struct pattern_list' in the index so we can access this on-demand. This will be used in the next change which uses the sparse-checkout definition to filter out directories that are outsie the sparse cone.

Signed-off-by Derrick Stolee <[email protected]>

---
 builtin/sparse-checkout.c | 17 ++++++++++-------
 cache.h                   |  2 ++
 2 files changed, 12 insertions(+), 7 deletions(-)

builtin/sparse-checkout.c +10 -7

index 2306a9ad98e0..e00b82af727b 100644
@@ -110,6 +110,8 @@ static int update_working_directory(struct pattern_list *pl)
        if (is_index_unborn(r->index))
                return UPDATE_SPARSITY_SUCCESS;
 
+       r->index->sparse_checkout_patterns = pl;
+
        memset(&o, 0, sizeof(o));
        o.verbose_update = isatty(2);
        o.update = 1;

@@ -138,6 +140,7 @@ static int update_working_directory(struct pattern_list *pl)
        else
                rollback_lock_file(&lock_file);
 
+       r->index->sparse_checkout_patterns = NULL;
        return result;
 }
 

@@ -517,19 +520,18 @@ static int modify_pattern_list(int argc, const char **argv, enum modify_type m)
 {
        int result;
        int changed_config = 0;
-       struct pattern_list pl;
-       memset(&pl, 0, sizeof(pl));
+       struct pattern_list *pl = xcalloc(1, sizeof(*pl));
 
        switch (m) {
        case ADD:
                if (core_sparse_checkout_cone)
-                       add_patterns_cone_mode(argc, argv, &pl);
+                       add_patterns_cone_mode(argc, argv, pl);
                else
-                       add_patterns_literal(argc, argv, &pl);
+                       add_patterns_literal(argc, argv, pl);
                break;
 
        case REPLACE:
-               add_patterns_from_input(&pl, argc, argv);
+               add_patterns_from_input(pl, argc, argv);
                break;
        }
 

@@ -539,12 +541,13 @@ static int modify_pattern_list(int argc, const char **argv, enum modify_type m)
                changed_config = 1;
        }
 
-       result = write_patterns_and_update(&pl);
+       result = write_patterns_and_update(pl);
 
        if (result && changed_config)
                set_config(MODE_NO_PATTERNS);
 
-       clear_pattern_list(&pl);
+       clear_pattern_list(pl);
+       free(pl);
        return result;
 }
 

cache.h +2 -1

index 1336c8d7435e..d75b352f38d3 100644
@@ -307,6 +307,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 struct split_index;
 struct untracked_cache;
 struct progress;
+struct pattern_list;
 
 struct index_state {
        struct cache_entry **cache;

@@ -332,6 +333,7 @@ struct index_state {
        struct mem_pool *ce_mem_pool;
        struct progress *progress;
        struct repository *repo;
+       struct pattern_list *sparse_checkout_patterns;
 };
 
 /* Name hashing */
--

gitgitgadget

From: Derrick Stolee <[email protected]>

If we have a full index, then we can convert it to a sparse index by replacing directories outside of the sparse cone with sparse directory entries. The convert_to_sparse() method does this, when the situation is appropriate.

For now, we avoid converting the index to a sparse index if:

1. the index is split.
2. the index is already sparse.
3. sparse-checkout is disabled.
4. sparse-checkout does not use cone mode.

Finally, we currently limit the conversion to when the GIT_TEST_SPARSE_INDEX environment variable is enabled. A mode using Git config will be added in a later change.

The trickiest thing about this conversion is that we might not be able to mark a directory as a sparse directory just because it is outside the sparse cone. There might be unmerged files within that directory, so we need to look for those. Also, if there is some strange reason why a file is not marked with CE_SKIP_WORKTREE, then we should give up on converting that directory. There is still hope that some of its subdirectories might be able to convert to sparse, so we keep looking deeper.

The conversion process is assisted by the cache-tree extension. This is calculated from the full index if it does not already exist. We then abandon the cache-tree as it no longer applies to the newly-sparse index. Thus, this cache-tree will be recalculated in every sparse-full-sparse round-trip until we integrate the cache-tree extension with the sparse index.

Some Git commands use the index after writing it. For example, 'git add' will update the index, then write it to disk, then read its entries to report information. To keep the in-memory index in a full state after writing, we re-expand it to a full one after the write. This is wasteful for commands that only write the index and do not read from it again, but that is only the case until we make those commands "sparse aware."

We can compare the behavior of the sparse-index in t1092-sparse-checkout-compability.sh by using GIT_TEST_SPARSE_INDEX=1 when operating on the 'sparse-index' repo. We can also compare the two sparse repos directly, such as comparing their indexes (when expanded to full in the case of the 'sparse-index' repo). We also verify that the index is actually populated with sparse directory entries.

The 'checkout and reset (mixed)' test is marked for failure when comparing a sparse repo to a full repo, but we can compare the two sparse-checkout cases directly to ensure that we are not changing the behavior when using a sparse index.

Signed-off-by Derrick Stolee <[email protected]>

---
 cache-tree.c                             |   3 +
 cache.h                                  |   2 +
 read-cache.c                             |  26 ++++-
 sparse-index.c                           | 139 +++++++++++++++++++++++
 sparse-index.h                           |   1 +
 t/t1092-sparse-checkout-compatibility.sh |  61 +++++++++-
 6 files changed, 227 insertions(+), 5 deletions(-)

cache-tree.c +3 -0

index 2fb483d3c083..5f07a39e501e 100644
@@ -6,6 +6,7 @@
 #include "object-store.h"
 #include "replace-object.h"
 #include "promisor-remote.h"
+#include "sparse-index.h"
 
 #ifndef DEBUG_CACHE_TREE
 #define DEBUG_CACHE_TREE 0

@@ -442,6 +443,8 @@ int cache_tree_update(struct index_state *istate, int flags)
        if (i)
                return i;
 
+       ensure_full_index(istate);
+
        if (!istate->cache_tree)
                istate->cache_tree = cache_tree();
 

cache.h +2 -0

index d75b352f38d3..e8b7d3b4fb33 100644
@@ -251,6 +251,8 @@ static inline unsigned int create_ce_mode(unsigned int mode)
 {
        if (S_ISLNK(mode))
                return S_IFLNK;
+       if (mode == S_IFDIR)
+               return S_IFDIR;
        if (S_ISDIR(mode) || S_ISGITLINK(mode))
                return S_IFGITLINK;
        return S_IFREG | ce_permissions(mode);

read-cache.c +1 -0

index 97dbf2434f30..67acbf202f4e 100644
@@ -25,6 +25,7 @@
 #include "fsmonitor.h"
 #include "thread-utils.h"
 #include "progress.h"
+#include "sparse-index.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 

read-cache.c +16 -1

                } else if (c == '\\' && protect_ntfs) {
                        if (is_ntfs_dotgit(path))
                                return 0;

@@ -3061,6 +3068,14 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l
                                 unsigned flags)
 {
        int ret;
+       int was_full = !istate->sparse_index;
+
+       ret = convert_to_sparse(istate);
+
+       if (ret) {
+               warning(_("failed to convert to a sparse-index"));
+               return ret;
+       }
 
        /*
         * TODO trace2: replace "the_repository" with the actual repo instance

@@ -3072,6 +3087,9 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l
        trace2_region_leave_printf("index", "do_write_index", the_repository,
                                   "%s", get_lock_file_path(lock));
 
+       if (was_full)
+               ensure_full_index(istate);
+
        if (ret)
                return ret;
        if (flags & COMMIT_LOCK)

@@ -3162,9 +3180,10 @@ static int write_shared_index(struct index_state *istate,
                              struct tempfile **temp)
 {
        struct split_index *si = istate->split_index;
-       int ret;
+       int ret, was_full = !istate->sparse_index;
 
        move_cache_to_base_index(istate);
+       convert_to_sparse(istate);
 
        trace2_region_enter_printf("index", "shared/do_write_index",
                                   the_repository, "%s", get_tempfile_path(*temp));

@@ -3172,6 +3191,9 @@ static int write_shared_index(struct index_state *istate,
        trace2_region_leave_printf("index", "shared/do_write_index",
                                   the_repository, "%s", get_tempfile_path(*temp));
 
+       if (was_full)
+               ensure_full_index(istate);
+
        if (ret)
                return ret;
        ret = adjust_shared_perm(get_tempfile_path(*temp));

sparse-index.c +33 -0

index 316cb949b74b..cb1f85635fbc 100644
@@ -4,6 +4,145 @@
 #include "tree.h"
 #include "pathspec.h"
 #include "trace2.h"
+#include "cache-tree.h"
+#include "config.h"
+#include "dir.h"
+#include "fsmonitor.h"
+
+static struct cache_entry *construct_sparse_dir_entry(
+                               struct index_state *istate,
+                               const char *sparse_dir,
+                               struct cache_tree *tree)
+{
+       struct cache_entry *de;
+
+       de = make_cache_entry(istate, S_IFDIR, &tree->oid, sparse_dir, 0, 0);
+
+       de->ce_flags |= CE_SKIP_WORKTREE;
+       return de;
+}
+
+/*
+ * Returns the number of entries "inserted" into the index.
+ */
+static int convert_to_sparse_rec(struct index_state *istate,
+                                int num_converted,
+                                int start, int end,
+                                const char *ct_path, size_t ct_pathlen,
+                                struct cache_tree *ct)
+{
+       int i, can_convert = 1;
+       int start_converted = num_converted;
+       enum pattern_match_result match;
+       int dtype;
+       struct strbuf child_path = STRBUF_INIT;
+       struct pattern_list *pl = istate->sparse_checkout_patterns;

sparse-index.c +14 -0

+
+       for (i = start; can_convert && i < end; i++) {
+               struct cache_entry *ce = istate->cache[i];
+
+               if (ce_stage(ce) ||
+                   !(ce->ce_flags & CE_SKIP_WORKTREE))
+                       can_convert = 0;
+       }
+
+       if (can_convert) {
+               struct cache_entry *se;
+               se = construct_sparse_dir_entry(istate, ct_path, ct);
+
+               istate->cache[num_converted++] = se;

sparse-index.c +72 -0

+                * entry.
+                */
+               base = ce->name + ct_pathlen;
+               slash = strchr(base, '/');
+
+               if (slash)
+                       pos = cache_tree_subtree_pos(ct, base, slash - base);
+
+               if (pos < 0) {
+                       istate->cache[num_converted++] = ce;
+                       i++;
+                       continue;
+               }
+
+               strbuf_setlen(&child_path, 0);
+               strbuf_add(&child_path, ce->name, slash - ce->name + 1);
+
+               span = ct->down[pos]->cache_tree->entry_count;
+               count = convert_to_sparse_rec(istate,
+                                             num_converted, i, i + span,
+                                             child_path.buf, child_path.len,
+                                             ct->down[pos]->cache_tree);
+               num_converted += count;
+               i += span;
+       }
+
+       strbuf_release(&child_path);
+       return num_converted - start_converted;
+}
+
+int convert_to_sparse(struct index_state *istate)
+{
+       if (istate->split_index || istate->sparse_index ||
+           !core_apply_sparse_checkout || !core_sparse_checkout_cone)
+               return 0;
+
+       /*
+        * For now, only create a sparse index with the
+        * GIT_TEST_SPARSE_INDEX environment variable. We will relax
+        * this once we have a proper way to opt-in (and later still,
+        * opt-out).
+        */
+       if (!git_env_bool("GIT_TEST_SPARSE_INDEX", 0))
+               return 0;
+
+       if (!istate->sparse_checkout_patterns) {
+               istate->sparse_checkout_patterns = xcalloc(1, sizeof(struct pattern_list));
+               if (get_sparse_checkout_patterns(istate->sparse_checkout_patterns) < 0)
+                       return 0;
+       }
+
+       if (!istate->sparse_checkout_patterns->use_cone_patterns) {
+               warning(_("attempting to use sparse-index without cone mode"));
+               return -1;
+       }
+
+       if (cache_tree_update(istate, 0)) {
+               warning(_("unable to update cache-tree, staying full"));
+               return -1;
+       }
+
+       remove_fsmonitor(istate);
+
+       trace2_region_enter("index", "convert_to_sparse", istate->repo);
+       istate->cache_nr = convert_to_sparse_rec(istate,
+                                                0, 0, istate->cache_nr,
+                                                "", 0, istate->cache_tree);
+       istate->drop_cache_tree = 1;
+       istate->sparse_index = 1;
+       trace2_region_leave("index", "convert_to_sparse", istate->repo);
+       return 0;
+}
 
 static void set_index_entry(struct index_state *istate, int nr, struct cache_entry *ce)
 {

sparse-index.h +1 -0

index 09a20d036c46..64380e121d80 100644
@@ -3,5 +3,6 @@
 
 struct index_state;
 void ensure_full_index(struct index_state *istate);
+int convert_to_sparse(struct index_state *istate);
 
 #endif

t/t1092-sparse-checkout-compatibility.sh +0 -0

index 4d789fe86b9d..ca87033d30b0 100755

t/t1092-sparse-checkout-compatibility.sh +6 -2

+GIT_TEST_SPLIT_INDEX=0
+
 . ./test-lib.sh
 
 test_expect_success 'setup' '

@@ -121,15 +124,49 @@ run_on_all () {
 test_all_match () {
        run_on_all "$@" &&
        test_cmp full-checkout-out sparse-checkout-out &&
-       test_cmp full-checkout-err sparse-checkout-err
+       test_cmp full-checkout-out sparse-index-out &&
+       test_cmp full-checkout-err sparse-checkout-err &&
+       test_cmp full-checkout-err sparse-index-err
 }
 
 test_sparse_match () {
-       run_on_sparse $* &&
+       run_on_sparse "$@" &&
        test_cmp sparse-checkout-out sparse-index-out &&
        test_cmp sparse-checkout-err sparse-index-err
 }
 

t/t1092-sparse-checkout-compatibility.sh +41 -2

+
+       GIT_TEST_SPARSE_INDEX=1 git -C sparse-index sparse-checkout set folder1 &&
+
+       test-tool -C sparse-index read-cache --table >cache &&
+       for dir in deep folder2 x
+       do
+               TREE=$(git -C sparse-index rev-parse HEAD:$dir) &&
+               grep "040000 tree $TREE $dir/" cache \
+                       || return 1
+       done &&
+
+       GIT_TEST_SPARSE_INDEX=1 git -C sparse-index sparse-checkout set deep/deeper1 &&
+
+       test-tool -C sparse-index read-cache --table >cache &&
+       for dir in deep/deeper2 folder1 folder2 x
+       do
+               TREE=$(git -C sparse-index rev-parse HEAD:$dir) &&
+               grep "040000 tree $TREE $dir/" cache \
+                       || return 1
+       done
+'
+
 test_expect_success 'expanded in-memory index matches full index' '
        init_repos &&
        test_sparse_match test-tool read-cache --expand --table

@@ -137,6 +174,7 @@ test_expect_success 'expanded in-memory index matches full index' '
 
 test_expect_success 'status with options' '
        init_repos &&
+       test_sparse_match ls &&
        test_all_match git status --porcelain=v2 &&
        test_all_match git status --porcelain=v2 -z -u &&
        test_all_match git status --porcelain=v2 -uno &&

@@ -273,6 +311,17 @@ test_expect_failure 'checkout and reset (mixed)' '
        test_all_match git reset update-folder2
 '
 
+# Ensure that sparse-index behaves identically to
+# sparse-checkout with a full index.
+test_expect_success 'checkout and reset (mixed) [sparse]' '
+       init_repos &&
+
+       test_sparse_match git checkout -b reset-test update-deep &&
+       test_sparse_match git reset deepest &&
+       test_sparse_match git reset update-folder1 &&
+       test_sparse_match git reset update-folder2
+'
+
 test_expect_success 'merge' '
        init_repos &&
 

@@ -309,14 +358,20 @@ test_expect_success 'clean' '
        test_all_match git status --porcelain=v2 &&
        test_all_match git clean -f &&
        test_all_match git status --porcelain=v2 &&
+       test_sparse_match ls &&
+       test_sparse_match ls folder1 &&
 
        test_all_match git clean -xf &&
        test_all_match git status --porcelain=v2 &&
+       test_sparse_match ls &&
+       test_sparse_match ls folder1 &&
 
        test_all_match git clean -xdf &&
        test_all_match git status --porcelain=v2 &&
+       test_sparse_match ls &&
+       test_sparse_match ls folder1 &&
 
-       test_path_is_dir sparse-checkout/folder1
+       test_sparse_match test_path_is_dir folder1
 '
 
 test_done
--

gitgitgadget

read-cache.c int verify_path(const char *path, unsigned mode) · line 1010

Quoted 3 lines · Derrick Stolee via GitGitGadget

read-cache.c +3 -0

+                       /*
+                        * allow terminating directory separators for
+                        * sparse directory enries.

enries -> entries

read-cache.c int verify_path(const char *path, unsigned mode) · line 1013

Quoted 3 lines

read-cache.c +3 -0

+                        */
+                       if (c == '\0')
+                               return S_ISDIR(mode);

Yaay, much simpler (than the RFC version).

sparse-index.c line 49

Quoted 9 lines

sparse-index.c +9 -0

+       /*
+        * Is the current path outside of the sparse cone?
+        * Then check if the region can be replaced by a sparse
+        * directory entry (everything is sparse and merged).
+        */
+       match = path_matches_pattern_list(ct_path, ct_pathlen,
+                                         NULL, &dtype, pl, istate);
+       if (match != NOT_MATCHED)
+               can_convert = 0;

Not sure if you saw my comments on the flow control at
https://lore.kernel.org/git/CABPp-BE9wPwmC0=pA4p1_QSRDHrO8RzqfJQdE2NxYZsYL_Rcig@mail.gmail.com/
(the typos elsewhere seem to still be present). If you saw it and decided against it, that's fine, just wanted the idea to at least be floated.

Review

Quoted 2 lines

(quoted diff) +2 -0

+               /*
+                * Detect if this is a normal entry oustide of any subtree

s/oustide/outside/

Quoted 5 lines

t/t1092-sparse-checkout-compatibility.sh +5 -0

+test_expect_success 'sparse-index contents' '
+       init_repos &&
+
+       test-tool -C sparse-index read-cache --table >cache &&
+       for dir in folder1 folder2 x

t/t1092-sparse-checkout-compatibility.sh run_on_all () { · line 147

Quoted 5 lines

t/t1092-sparse-checkout-compatibility.sh +5 -0

+       do
+               TREE=$(git -C sparse-index rev-parse HEAD:$dir) &&
+               grep "040000 tree $TREE $dir/" cache \
+                       || return 1
+       done &&

Thanks for making the output look more like ls-tree output; it's easier to parse that way, at least for me.

I mostly read over the range-diff since it was much shorter. You've addressed a number of questions/comments I had on the RFC version, but there's still some I didn't see a response to so I reposted them here.

From: Derrick Stolee <[email protected]>

The index_pos_by_traverse_info() currently throws a BUG() when a directory entry exists exactly in the index. We need to consider that it is possible to have a directory in a sparse index as long as that entry is itself marked with the skip-worktree bit.

The negation of the 'pos' variable must be conditioned to only when it starts as negative. This is identical behavior as before when the index is full.

Signed-off-by Derrick Stolee <[email protected]>

---
 unpack-trees.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

unpack-trees.c +6 -4

index 4dd99219073a..b324eec2a5d1 100644
@@ -746,9 +746,12 @@ static int index_pos_by_traverse_info(struct name_entry *names,
        strbuf_make_traverse_path(&name, info, names->path, names->pathlen);
        strbuf_addch(&name, '/');
        pos = index_name_pos(o->src_index, name.buf, name.len);
-       if (pos >= 0)
-               BUG("This is a directory and should not exist in index");
-       pos = -pos - 1;
+       if (pos >= 0) {
+               if (!o->src_index->sparse_index ||
+                   !(o->src_index->cache[pos]->ce_flags & CE_SKIP_WORKTREE))
+                       BUG("This is a directory and should not exist in index");
+       } else
+               pos = -pos - 1;
        if (pos >= o->src_index->cache_nr ||
            !starts_with(o->src_index->cache[pos]->name, name.buf) ||
            (pos > 0 && starts_with(o->src_index->cache[pos-1]->name, name.buf)))
--

gitgitgadget

Quoted 11 lines · Derrick Stolee via GitGitGadget

From: Derrick Stolee <[email protected]>

The index_pos_by_traverse_info() currently throws a BUG() when a directory entry exists exactly in the index. We need to consider that it is possible to have a directory in a sparse index as long as that entry is itself marked with the skip-worktree bit.

The negation of the 'pos' variable must be conditioned to only when it starts as negative. This is identical behavior as before when the index is full.

Same comment on the second paragraph as I made in the RFC series --
https://lore.kernel.org/git/CABPp-BGPJgA4guWHVm3AVS=hM0fTixUpRvJe5i9NnHT-3QJMfw@mail.gmail.com/.
I apologize if I'm repeating stuff you chose to not change, but I didn't see a response and given the three typos left in previous patches, I'm unsure whether it was unaddressed on purpose or on accident.

Quoted 14 lines · Elijah Newren

Quoted 11 lines · Derrick Stolee via GitGitGadget

From: Derrick Stolee <[email protected]>

The index_pos_by_traverse_info() currently throws a BUG() when a directory entry exists exactly in the index. We need to consider that it is possible to have a directory in a sparse index as long as that entry is itself marked with the skip-worktree bit.

The negation of the 'pos' variable must be conditioned to only when it starts as negative. This is identical behavior as before when the index is full.

Quoted 6 lines

Same comment on the second paragraph as I made in the RFC series --
https://lore.kernel.org/git/CABPp-BGPJgA4guWHVm3AVS=hM0fTixUpRvJe5i9NnHT-3QJMfw@mail.gmail.com/.
I apologize if I'm repeating stuff you chose to not change, but I didn't see a response and given the three typos left in previous patches, I'm unsure whether it was unaddressed on purpose or on accident.

Yes, I dropped this one. How about this?

The 'pos' variable is assigned a negative value if an exact match is not found. Since a directory name can be an exact match, it is no longer an error to have a nonnegative 'pos' value.

From: Derrick Stolee <[email protected]>

Previously, we enabled the sparse index format only using GIT_TEST_SPARSE_INDEX=1. This is not a feasible direction for users to actually select this mode. Further, sparse directory entries are not understood by the index formats as advertised.

We _could_ add a new index version that explicitly adds these capabilities, but there are nuances to index formats 2, 3, and 4 that are still valuable to select as options. For now, create a repo extension, "extensions.sparseIndex", that specifies that the tool reading this repository must understand sparse directory entries.

This change only encodes the extension and enables it when GIT_TEST_SPARSE_INDEX=1. Later, we will add a more user-friendly CLI mechanism.

Signed-off-by Derrick Stolee <[email protected]>

---
 Documentation/config/extensions.txt |  7 ++++++
 cache.h                             |  1 +
 repo-settings.c                     |  7 ++++++
 repository.h                        |  3 ++-
 setup.c                             |  3 +++
 sparse-index.c                      | 38 +++++++++++++++++++++++++----
 6 files changed, 53 insertions(+), 6 deletions(-)

Documentation/config/extensions.txt +0 -0

index 4e23d73cdcad..5c86b3648732 100644

cache.h +1 -0

index e8b7d3b4fb33..eea61fba7568 100644
@@ -1053,6 +1053,7 @@ struct repository_format {
        int worktree_config;
        int is_bare;
        int hash_algo;
+       int sparse_index;
        char *work_tree;
        struct string_list unknown_extensions;
        struct string_list v1_only_extensions;

repo-settings.c +7 -0

index d63569e4041e..9677d50f9238 100644
@@ -85,4 +85,11 @@ void prepare_repo_settings(struct repository *r)
         * removed.
         */
        r->settings.command_requires_full_index = 1;
+
+       /*
+        * Initialize this as off.
+        */
+       r->settings.sparse_index = 0;
+       if (!repo_config_get_bool(r, "extensions.sparseindex", &value) && value)
+               r->settings.sparse_index = 1;
 }

repository.h +2 -1

index e06a23015697..a45f7520fd9e 100644
@@ -42,7 +42,8 @@ struct repo_settings {
 
        int core_multi_pack_index;
 
-       unsigned command_requires_full_index:1;
+       unsigned command_requires_full_index:1,
+                sparse_index:1;
 };
 
 struct repository {

setup.c +3 -0

index c04cd25a30df..cd8394564613 100644
@@ -500,6 +500,9 @@ static enum extension_result handle_extension(const char *var,
                        return error("invalid value for 'extensions.objectformat'");
                data->hash_algo = format;
                return EXTENSION_OK;
+       } else if (!strcmp(ext, "sparseindex")) {
+               data->sparse_index = 1;
+               return EXTENSION_OK;
        }
        return EXTENSION_UNKNOWN;
 }

sparse-index.c +33 -6

index 14029fafc750..97b0d0c57857 100644
@@ -102,19 +102,47 @@ static int convert_to_sparse_rec(struct index_state *istate,
        return num_converted - start_converted;
 }
 
+static int enable_sparse_index(struct repository *repo)
+{
+       const char *config_path = repo_git_path(repo, "config.worktree");
+
+       if (upgrade_repository_format(1) < 0) {
+               warning(_("unable to upgrade repository format to enable sparse-index"));
+               return -1;
+       }
+       git_config_set_in_file_gently(config_path,
+                                     "extensions.sparseIndex",
+                                     "true");
+
+       prepare_repo_settings(repo);
+       repo->settings.sparse_index = 1;
+       return 0;
+}
+
 int convert_to_sparse(struct index_state *istate)
 {
        if (istate->split_index || istate->sparse_index ||
            !core_apply_sparse_checkout || !core_sparse_checkout_cone)
                return 0;
 
+       if (!istate->repo)
+               istate->repo = the_repository;
+
+       /*
+        * The GIT_TEST_SPARSE_INDEX environment variable triggers the
+        * extensions.sparseIndex config variable to be on.
+        */
+       if (git_env_bool("GIT_TEST_SPARSE_INDEX", 0)) {
+               int err = enable_sparse_index(istate->repo);
+               if (err < 0)
+                       return err;
+       }
+
        /*
-        * For now, only create a sparse index with the
-        * GIT_TEST_SPARSE_INDEX environment variable. We will relax
-        * this once we have a proper way to opt-in (and later still,
-        * opt-out).
+        * Only convert to sparse if extensions.sparseIndex is set.
         */
-       if (!git_env_bool("GIT_TEST_SPARSE_INDEX", 0))
+       prepare_repo_settings(istate->repo);
+       if (!istate->repo->settings.sparse_index)
                return 0;
 
        if (!istate->sparse_checkout_patterns) {
--

gitgitgadget

Quoted 8 lines · Derrick Stolee via GitGitGadget

From: Derrick Stolee <[email protected]>

Previously, we enabled the sparse index format only using GIT_TEST_SPARSE_INDEX=1. This is not a feasible direction for users to actually select this mode. Further, sparse directory entries are not understood by the index formats as advertised.

Quoted 5 lines

We _could_ add a new index version that explicitly adds these capabilities, but there are nuances to index formats 2, 3, and 4 that are still valuable to select as options. For now, create a repo extension, "extensions.sparseIndex", that specifies that the tool reading this repository must understand sparse directory entries.

This commit is unchanged from the RFC series, but given your comments in the design document about how you do intend to create an index format v5 now, do you want to reference that here?

Documentation/config/extensions.txt extensions.objectFormat:: · line 15

Quoted 6 lines

Documentation/config/extensions.txt +6 -0

+extensions.sparseIndex::
+       When combined with `core.sparseCheckout=true` and
+       `core.sparseCheckoutCone=true`, the index may contain entries
+       corresponding to directories outside of the sparse-checkout
+       definition. Versions of Git that do not understand this extension
+       do not expect directory entries in the index.

I had a wording suggestion for this paragraph in the RFC series --
https://lore.kernel.org/git/CABPp-BFEJE82k4VgkR=Jf7V7sZxZzo2pHMfAGshhi9_vV6iK0w@mail.gmail.com/.
Let me know if you just decided to leave it out so I don't bug you about stuff you already considered.

Documentation/git-sparse-checkout.txt To avoid interfering with other worktrees, it first enables the · line 58

Quoted 2 lines · Derrick Stolee via GitGitGadget

Documentation/git-sparse-checkout.txt +2 -0

+that is not completely understood by other tools. Enabling sparse index
+enables the `extensions.spareseIndex` config value, which might cause

s/sparese/sparse

Documentation/git-sparse-checkout.txt To avoid interfering with other worktrees, it first enables the · line 61

Quoted 3 lines

Documentation/git-sparse-checkout.txt +3 -0

+other tools to stop working with your repository. If you have trouble with
+this compatibility, then run `git sparse-checkout sparse-index disable` to
+remove this config and rewrite your index to not be sparse.

While I'm commenting on this..:

There are several "layers" here, for lack of a better term. "Enabling foo enables bar which may cause baz. If you fail due to baz, try dropping bar by dropping foo." If I remove any mention of the config variable from your text, I get the following.

Enabling sparse index might cause other tools to stop working with your repository. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse.

I'm tempted to suggest such a rewrite to relieve readers of knowing of the middle step, which you could say is more of an implementation detail. But if we think that the symptoms / error messages might involve "extensions.sparseIndex" or, as would be the case with an older Git installation,

fatal: unknown repository extensions found:
sparseindex

maybe there is some value in mentioning the config item by name. Just thinking out loud, really, and I don't have any strong opinion. I only came here to point out the typo in the docs.

Quoted 6 lines · Martin Ågren

Documentation/git-sparse-checkout.txt To avoid interfering with other worktrees, it first enables the · line 58

Quoted 2 lines · Derrick Stolee via GitGitGadget

Documentation/git-sparse-checkout.txt +2 -0

+that is not completely understood by other tools. Enabling sparse index
+enables the `extensions.spareseIndex` config value, which might cause

s/sparese/sparse

Thanks!

Quoted 22 lines

Documentation/git-sparse-checkout.txt To avoid interfering with other worktrees, it first enables the · line 61

Quoted 3 lines

Documentation/git-sparse-checkout.txt +3 -0

+other tools to stop working with your repository. If you have trouble with
+this compatibility, then run `git sparse-checkout sparse-index disable` to
+remove this config and rewrite your index to not be sparse.

While I'm commenting on this..:

There are several "layers" here, for lack of a better term. "Enabling foo enables bar which may cause baz. If you fail due to baz, try dropping bar by dropping foo." If I remove any mention of the config variable from your text, I get the following.

Enabling sparse index might cause other tools to stop working with your repository. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse.

I'm tempted to suggest such a rewrite to relieve readers of knowing of the middle step, which you could say is more of an implementation detail. But if we think that the symptoms / error messages might involve "extensions.sparseIndex" or, as would be the case with an older Git installation,

Quoted 6 lines

fatal: unknown repository extensions found:
sparseindex

maybe there is some value in mentioning the config item by name. Just thinking out loud, really, and I don't have any strong opinion. I only came here to point out the typo in the docs.

I agree that the layers are confusing. We could rearrange and have a similar flow to what you recommend by mentioning the extension at the end:

**WARNING:** Using a sparse index requires modifying the index in a way that is not completely understood by other tools. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse. Older versions of Git will not understand the sparseIndex repository extension and may fail to interact with your repository until it is disabled.

Thanks,
-Stolee

Quoted 36 lines · Derrick Stolee

Quoted 16 lines · Martin Ågren

There are several "layers" here, for lack of a better term. "Enabling foo enables bar which may cause baz. If you fail due to baz, try dropping bar by dropping foo." If I remove any mention of the config variable from your text, I get the following.

Enabling sparse index might cause other tools to stop working with your repository. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse.

I'm tempted to suggest such a rewrite to relieve readers of knowing of the middle step, which you could say is more of an implementation detail. But if we think that the symptoms / error messages might involve "extensions.sparseIndex" or, as would be the case with an older Git installation,

Quoted 6 lines

fatal: unknown repository extensions found:
sparseindex

maybe there is some value in mentioning the config item by name. Just thinking out loud, really, and I don't have any strong opinion. I only came here to point out the typo in the docs.

I agree that the layers are confusing. We could rearrange and have a similar flow to what you recommend by mentioning the extension at the end:

**WARNING:** Using a sparse index requires modifying the index in a way that is not completely understood by other tools. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse. Older versions of Git will not understand the sparseIndex repository extension and may fail to interact with your repository until it is disabled.

Quoted 2 lines

Thanks,
-Stolee

This looks pretty good to me, but could we change the first sentence to read "...modifying the index in a way that may not yet be understood by external tools." ? I'm worried "other tools" might make people worry about different builtin commands (e.g. fast-export, log). I also prefer "may" and "yet" because I suspect most external tools (e.g. git filter-repo just to name a personal example) won't need to read an index format and will thus be unaffected, and any tools that do read the index format will probably eventually learn how to work with the new format.

Quoted 40 lines · Elijah Newren

Quoted 36 lines · Derrick Stolee

Quoted 16 lines · Martin Ågren

There are several "layers" here, for lack of a better term. "Enabling foo enables bar which may cause baz. If you fail due to baz, try dropping bar by dropping foo." If I remove any mention of the config variable from your text, I get the following.

Enabling sparse index might cause other tools to stop working with your repository. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse.

I'm tempted to suggest such a rewrite to relieve readers of knowing of the middle step, which you could say is more of an implementation detail. But if we think that the symptoms / error messages might involve "extensions.sparseIndex" or, as would be the case with an older Git installation,

Quoted 6 lines

fatal: unknown repository extensions found:
sparseindex

maybe there is some value in mentioning the config item by name. Just thinking out loud, really, and I don't have any strong opinion. I only came here to point out the typo in the docs.

I agree that the layers are confusing. We could rearrange and have a similar flow to what you recommend by mentioning the extension at the end:

**WARNING:** Using a sparse index requires modifying the index in a way that is not completely understood by other tools. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse. Older versions of Git will not understand the sparseIndex repository extension and may fail to interact with your repository until it is disabled.

Quoted 2 lines

Thanks,
-Stolee

Quoted 9 lines

This looks pretty good to me, but could we change the first sentence to read "...modifying the index in a way that may not yet be understood by external tools." ? I'm worried "other tools" might make people worry about different builtin commands (e.g. fast-export, log). I also prefer "may" and "yet" because I suspect most external tools (e.g. git filter-repo just to name a personal example) won't need to read an index format and will thus be unaffected, and any tools that do read the index format will probably eventually learn how to work with the new format.

I can make the change, but I do want to point out that the current use of a repository extension _does_ mean that tools that (correctly) interact with a Git repository should fail even if they don't try to access the index file. This is only something to make this work until we introduce a new index file format version and then can drop the extension.

"git filter-repo" _should_ be safe because it's really just shelling to Git, right? I'm more concerned about tools like libgit2.

I agree that the layers are confusing. We could rearrange and have a similar flow to what you recommend by mentioning the extension at the end:

**WARNING:** Using a sparse index requires modifying the index in a way that is not completely understood by other tools. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse. Older versions of Git will not understand the sparseIndex repository extension and may fail to interact with your repository until it is disabled.

Quoted 11 lines · Derrick Stolee

I agree that the layers are confusing. We could rearrange and have a similar flow to what you recommend by mentioning the extension at the end:

**WARNING:** Using a sparse index requires modifying the index in a way that is not completely understood by other tools. If you have trouble with this compatibility, then run git sparse-checkout sparse-index disable to rewrite your index to not be sparse. Older versions of Git will not understand the sparseIndex repository extension and may fail to interact with your repository until it is disabled.

I like it. I find this easier to read than the previous version. That said, is git sparse-index sparse-checkout disable really the way to do this? I don't see a "sparse-index" subcommand of git-sparse-checkout. ... Hmm, no, after building and installing your patches, I get

  $ git sparse-checkout sparse-index disable
  usage: git sparse-checkout (init|list|set|add|reapply|disable) <options>

Should that be git sparse-checkout init --no-sparse-index? I just tried that on a fresh, empty repo. It seems to work in the sense that it drops the config item. I'm guessing re-initing a sparse checkout is a safe and sane thing to do?

I don't find any tests for this. If re-initing should be ok and in particular if it should allow toggling the use of sparse index, it might be good having a test. At a minimum to see that the command passes and that the config item goes away? And check that the actual index is rewritten back to the "old" format? (Sorry if you have that already and I'm just bad at finding it.)