Fix commit-graph/graft/replace/shallow combo

The commit-graph file stores a condensed version of the commit history. This helps speed up several operations involving commit walks. This feature does not work well if those commits "change" using features like commit grafts, replace objects, or shallow clones.

Since the commit-graph feature is optional, hidden behind the 'core.commitGraph' config option, and requires manual maintenance (until ds/commit-graph-fsck' is merged), these issues only arise for expert users who decided to opt-in.

This RFC is a first attempt at rectify the issues that come up when these features interact. I have not succeeded entirely, as I will discuss below.

The first two "DO NOT MERGE" commits are not intended to be part of the main product, but are ways to help the full test suite run while computing and consuming commit-graph files. This is acheived by calling write_commit_graph_reachable() from git fetch and git commit. I believe this is the only dependence on ds/commit-graph-fsck. The other commits should only depend on ds/commit-graph-lockfile-fix.

Running the full test suite after these DO NO MERGE commits results in the following test failures, which I categorize by type of failure.

The following tests are red herrings. Most work by deleting a commit from the object database and trying to demonstrate a failure. Others rely on how certain objects are loaded. These are not bugs, but will add noise if you run the tests suite with this patch.

t0410-partial-clone.sh Failed tests: 5, 8
t5307-pack-missing-commit.sh Failed tests: 3-4
t6011-rev-list-with-bad-commit.sh Failed test: 4
t6024-recursive-merge.sh Failed test: 4

The following tests are fixed in "commit-graph: enable replace-object and grafts".

t6001-rev-list-graft.sh Failed tests: 3, 5, 7, 9, 11, 13
t6050-replace.sh Failed tests: 11-15, 23-24, 29

The following tests involve shallow clones.

t5500-fetch-pack.sh Failed tests: 30-31, 34, 348-349
t5537-fetch-shallow.sh Failed tests: 4-7, 9
t5802-connect-helper.sh Failed test: 3

These tests are mostly fixed by three commits:

* commit-graph: avoid writing when repo is shallow
* fetch: destroy commit graph on shallow parameters
* commit-graph: revert to odb on missing parents

Each of these cases cover a different interaction that occurs with shallow clones. Some are due to a commit becoming shallow, while others are due to a commit becoming unshallow (and hence invalidating its generation number).

After these changes, there is one test case that still fails, and I cannot understand why:

t5500-fetch-pack.sh Failed test: 348

This test fails when performing a git fetch --shallow-exclude. When I halt the test using t5500-fetch-pack.sh -d -i and navigate to the directory to replay the fetch it performs as expected. After struggling with it for a while, I figured I should just put this series up for discussion. Maybe someone with more experience in shallow clones can point out the obvious issues I'm having.

Thanks,
-Stolee

Derrick Stolee (6):
DO NOT MERGE: compute commit-graph on every commit DO NOT MERGE: write commit-graph on every fetch
commit-graph: enable replace-object and grafts
commit-graph: avoid writing when repo is shallow
fetch: destroy commit graph on shallow parameters
commit-graph: revert to odb on missing parents

 builtin/commit.c  |  5 +++++
 builtin/fetch.c   | 10 +++++++++
 builtin/gc.c      |  2 +-
 builtin/replace.c |  3 +++
 commit-graph.c    | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++----
 commit-graph.h    |  9 ++++++++
 commit.c          |  5 +++++
 environment.c     |  2 +-
 8 files changed, 95 insertions(+), 6 deletions(-)

2.16.2.338.gcfe06ae955

Also enable core.commitGraph and gc.commitGraph. Helps to test the commit-graph feature with the rest of the test infrastructure.

---
 builtin/commit.c | 5 +++++
 builtin/gc.c     | 2 +-
 environment.c    | 2 +-
 3 files changed, 7 insertions(+), 2 deletions(-)

index e8e8d13be4..8751b816c1 100644
@@ -32,6 +32,7 @@
 #include "column.h"
 #include "sequencer.h"
 #include "mailmap.h"
+#include "commit-graph.h"
 
 static const char * const builtin_commit_usage[] = {
 	N_("git commit [<options>] [--] <pathspec>..."),

@@ -1623,5 +1624,9 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	UNLEAK(err);
 	UNLEAK(sb);
+
+	if (core_commit_graph)
+		write_commit_graph_reachable(get_object_directory(), 1);
+
 	return 0;
 }

index efd214a59f..999c09d8e2 100644
@@ -35,7 +35,7 @@ static int aggressive_depth = 50;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
-static int gc_commit_graph = 0;
+static int gc_commit_graph = 1;
 static int detach_auto = 1;
 static timestamp_t gc_log_expire_time;
 static const char *gc_log_expire = "1.day.ago";

index 8853e2f0dd..fdb2d56d90 100644
@@ -62,7 +62,7 @@ enum push_default_type push_default = PUSH_DEFAULT_UNSPECIFIED;
 enum object_creation_mode object_creation_mode = OBJECT_CREATION_MODE;
 char *notes_ref_name;
 int grafts_replace_parents = 1;
-int core_commit_graph;
+int core_commit_graph = 1;
 int core_apply_sparse_checkout;
 int merge_log_config = -1;
 int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */

2.16.2.338.gcfe06ae955

THIS IS ONLY FOR TESTING TO INCREASE TEST COVERAGE

---
 builtin/fetch.c | 4 ++++
 1 file changed, 4 insertions(+)

index 8ee998ea2e..af896e4b74 100644
@@ -20,6 +20,7 @@
 #include "utf8.h"
 #include "packfile.h"
 #include "list-objects-filter-options.h"
+#include "commit-graph.h"
 
 static const char * const builtin_fetch_usage[] = {
 	N_("git fetch [<options>] [<repository> [<refspec>...]]"),

@@ -1480,6 +1481,9 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 
 	close_all_packs();
 
+	if (core_commit_graph)
+		write_commit_graph_reachable(get_object_directory(), 1);
+
 	argv_array_pushl(&argv_gc_auto, "gc", "--auto", NULL);
 	if (verbosity < 0)
 		argv_array_push(&argv_gc_auto, "--quiet");

2.16.2.338.gcfe06ae955

Create destroy_commit_graph() method to delete the commit-graph file when history is altered by a replace-object call. If the commit-graph is rebuilt after that, we will load the correct object while reading the commit-graph.

When parsing a commit, first check if the commit was grafted. If so, then ignore the commit-graph for that commit and insted use the parents loaded by parsing the commit buffer and comparing against the graft file.

---
 builtin/replace.c |  3 +++
 commit-graph.c    | 20 +++++++++++++++++++-
 commit-graph.h    |  9 +++++++++
 commit.c          |  5 +++++
 4 files changed, 36 insertions(+), 1 deletion(-)

index 9f01f3fc7f..d553aadcdc 100644
@@ -15,6 +15,7 @@
 #include "parse-options.h"
 #include "run-command.h"
 #include "tag.h"
+#include "commit-graph.h"
 
 static const char * const git_replace_usage[] = {
 	N_("git replace [-f] <object> <replacement>"),

@@ -468,6 +469,8 @@ int cmd_replace(int argc, const char **argv, const char *prefix)
 		usage_msg_opt("--raw only makes sense with --edit",
 			      git_replace_usage, options);
 
+	destroy_commit_graph(get_object_directory());
+
 	switch (cmdmode) {
 	case MODE_DELETE:
 		if (argc < 1)

index e9195dfb17..95af4ed519 100644
@@ -6,6 +6,7 @@
 #include "pack.h"
 #include "packfile.h"
 #include "commit.h"
+#include "dir.h"
 #include "object.h"
 #include "refs.h"
 #include "revision.h"

@@ -240,15 +241,22 @@ static struct commit_list **insert_parent_or_die(struct commit_graph *g,
 {
 	struct commit *c;
 	struct object_id oid;
+	const unsigned char *real;
 
 	if (pos >= g->num_commits)
 		die("invalid parent position %"PRIu64, pos);
 
 	hashcpy(oid.hash, g->chunk_oid_lookup + g->hash_len * pos);
+
+	real = lookup_replace_object(oid.hash);
+
 	c = lookup_commit(&oid);
 	if (!c)
 		die("could not find commit %s", oid_to_hex(&oid));
-	c->graph_pos = pos;
+
+	if (!hashcmp(real, oid.hash))
+		c->graph_pos = pos;
+
 	return &commit_list_insert(c, pptr)->next;
 }
 

@@ -1019,3 +1027,13 @@ int verify_commit_graph(struct commit_graph *g)
 
 	return verify_commit_graph_error;
 }
+
+void destroy_commit_graph(const char *obj_dir)
+{
+	char *graph_name;
+	close_commit_graph();
+
+	graph_name = get_commit_graph_filename(obj_dir);
+	remove_path(graph_name);
+	FREE_AND_NULL(graph_name);
+}

index 9a06a5f188..1d17da1582 100644
@@ -56,4 +56,13 @@ void write_commit_graph(const char *obj_dir,
 
 int verify_commit_graph(struct commit_graph *g);
 
+/*
+ * Delete the commit-graph file in the given object directory.
+ *
+ * WARNING: this deletes data, so should only be used when
+ * performing history-altering actions like replace-object
+ * or grafts.
+ */
+void destroy_commit_graph(const char *obj_dir);
+
 #endif

index 6eaed0174c..2fe31cde77 100644
@@ -403,6 +403,11 @@ int parse_commit_internal(struct commit *item, int quiet_on_missing, int use_com
 		return -1;
 	if (item->object.parsed)
 		return 0;
+
+	prepare_commit_graft();
+	if (commit_graft_pos(item->object.oid.hash) >= 0)
+		use_commit_graph = 0;
+
 	if (use_commit_graph && parse_commit_in_graph(item))
 		return 0;
 	buffer = read_sha1_file(item->object.oid.hash, &type, &size);

2.16.2.338.gcfe06ae955

Derrick Stolee <[email protected]> writes:

First, this commit seems to try to do two things at once, and in its current form it should be split into adding destroy_commit_graph() and into grafts / replacements check. Those are separate and unconnected features, and should be in separate patches, in my opinion.

I'm not sure if this is the best solution. It would work both for invalidation, and for turning off the feature, but in my opinion there are better solutions for both:
- in case of invalidation, wouldn't it be better to re-create the file?
- in case of turning off, wouldn't it be better to simply set core_commit_graph to false?

Nevertheless I think destroy_commit_graph(), however it would be named at the end, would be a good to have.

Unless uyou turn off using generation numbers and other such indices, this is not enough to prevent from generating incorrect results if current commit-graph file doesn't agree with current altered view of the project history.

Take a simple example of joining two histories using replacements (git-replace) or grafts. Numbers denote generation numbers:

Original history:

    A<---B<---C             <===  master
    1    2    3


    a<---b<---c<---d<---e   <===  historical
    1    2    3    4    5

Altered view of history:

    a<---b<---c<---d<---e<---[A]<---B<---C   <===  master

In the new view of history, 'e' is reachable from 'C'. But going by pre-altering generation numbers, gen_{pre}(e) = 5 > gen_{pre}(C) = 3. This means that we don't even arrive at '[A]', where replacement object or graft changed parents compared to what's loaded from commit-graph file, isn't it?

Shallow clones do not interact well with the commit-graph feature for several reasons. Instead of doing the hard thing to fix those interactions, instead prevent reading or writing a commit-graph file for shallow repositories.

---
 commit-graph.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

index 95af4ed519..80e377b90f 100644
@@ -208,6 +208,9 @@ static void prepare_commit_graph(void)
 		return;
 	prepare_commit_graph_run_once = 1;
 
+	if (is_repository_shallow())
+		return;
+
 	obj_dir = get_object_directory();
 	prepare_commit_graph_one(obj_dir);
 	prepare_alt_odb();

@@ -711,6 +714,15 @@ void write_commit_graph(const char *obj_dir,
 	int num_extra_edges;
 	struct commit_list *parent;
 
+	/*
+	 * Shallow clones are not supproted, as they create bad
+	 * generation skips as they are un-shallowed.
+	 */
+	if (is_repository_shallow()) {
+		warning("writing a commit-graph in a shallow repository is not supported");
+		return;
+	}
+
 	oids.nr = 0;
 	oids.alloc = approximate_object_count() / 4;
 

2.16.2.338.gcfe06ae955

Makes sense.

The latter instead would want to vanish, I would guess.

Do you mean that we should call destroy_commit_graph() if we detect a shallow repository during write_commit_graph(), then I can make that change.

I think Junio meant here the "instead" word, because you have it twice in the second sentence of quoted paragraph.

Jakub Narębski

No, I was just having trouble with reading "Instead of doing X, instead do Y".

The commit-graph feature is not compatible with history-rewriting features such as shallow clones. When running a 'git fetch' with any of the shallow/unshallow options, destroy the commit-graph file and override core.commitGraph to be false.

---
 builtin/fetch.c | 6 ++++++
 1 file changed, 6 insertions(+)

index af896e4b74..2001dca881 100644
@@ -1452,6 +1452,12 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (update_shallow || depth || deepen_since || deepen_not.nr ||
+	    deepen_relative || unshallow || update_shallow || is_repository_shallow()) {
+		destroy_commit_graph(get_object_directory());
+		core_commit_graph = 0;
+	}
+
 	if (remote) {
 		if (filter_options.choice || repository_format_partial_clone)
 			fetch_one_setup_partial(remote);

2.16.2.338.gcfe06ae955

I associate "history rewriting" with changing objects in the history. For example interactive rebase or the BFG cleaner[1] / filter-branch to remove certain commits from other commits as parents. This history rewriting leads to different sha1s, and the commit graph feature is compatible with that in the sense that you can add all the new sha1s /commits to the graph and prune out the old unreferenced commits.

Shallow clones are not rewriting history IMHO, as the sha1s do not change. What changes is the assumption of presence of the parent commits (which makes it hard to compute the generation number), by the grafting trick, that "overlays" (?) history.

This is more of a nit, though.

[1] https://rtyley.github.io/bfg-repo-cleaner/

We do that *before* the actual fetch happens such that the improved negotiation of the future cannot even try to benefit from generation numbers?

We do it at fetch time instead of other local operations as that is an entry point to commit-graph incompatible features. Would this also be needed in clone?

I was about to ask if a more fine grained inclusion to lookup_commit would make sense, but that is explicitely called out in the cover letter as 'too hard for now'.

The commit-graph format includes a way to specify a parent is "missing" from the commit-graph (i.e. we do not have a record of that parent in our list of object IDs, and hence cannot provide a graph position). For mose cases, this does not occur due to the close_reachable() method adding all reachable commits. However, in a shallow clone, we will try to record the parents of a commit on the shallow boundary, but the parents are not in the repository.

The GRAPH_PARENT_MISSING value that is stored in the format is purposeful, especially for future plans to make the commit-graph file incremental or transporting sections of a commit-graph file across the network.

In the meantime, check if a commit has a missing parent while filling its details from the commit-graph. If a parent is missing, still assign the generation number and graph position for that item, but report that the commit-graph failed to fill the contents. Then the caller is responsible for filling the rest of the data from a commit buffer.

---
 commit-graph.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

index 80e377b90f..3e33d061fe 100644
@@ -278,17 +278,44 @@ static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 	struct commit_list **pptr;
 	const unsigned char *commit_data = g->chunk_commit_data + (g->hash_len + 16) * pos;
 
-	item->object.parsed = 1;
+	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
 	item->graph_pos = pos;
 
+	/*
+	 * If we have any edges marked as GRAPH_PARENT_MISSING, we must not parse any
+	 * more of this object and leave it to the commit buffer to parse.
+	 */
+	edge_value = get_be32(commit_data + g->hash_len);
+	if (edge_value == GRAPH_PARENT_MISSING)
+		return 0;
+	if (edge_value == GRAPH_PARENT_NONE)
+		goto continue_parsing;
+
+	edge_value = get_be32(commit_data + g->hash_len + 4);
+	if (edge_value == GRAPH_PARENT_MISSING)
+		return 0;
+	if (edge_value == GRAPH_PARENT_NONE)
+		goto continue_parsing;
+	if (!(edge_value & GRAPH_OCTOPUS_EDGES_NEEDED))
+		goto continue_parsing;
+
+	parent_data_ptr = (uint32_t*)(g->chunk_large_edges +
+			  4 * (uint64_t)(edge_value & GRAPH_EDGE_LAST_MASK));
+	do {
+		edge_value = get_be32(parent_data_ptr);
+		if (edge_value == GRAPH_PARENT_MISSING)
+			return 0;
+		parent_data_ptr++;
+	} while (!(edge_value & GRAPH_LAST_EDGE));
+	
+continue_parsing:
+	item->object.parsed = 1;
 	item->maybe_tree = NULL;
 
 	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
 	date_low = get_be32(commit_data + g->hash_len + 12);
 	item->date = (timestamp_t)((date_high << 32) | date_low);
 
-	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
-
 	pptr = &item->parents;
 
 	edge_value = get_be32(commit_data + g->hash_len);

2.16.2.338.gcfe06ae955

I applied these patches on top of ds/commit-graph-fsck (it would have been nice if you mentioned that it applies there) Running the test suite with all patches applied results in:

./t0410-partial-clone.sh (Wstat: 256 Tests: 15 Failed: 2)
Failed tests: 5, 8
./t5307-pack-missing-commit.sh (Wstat: 256 Tests: 5 Failed: 2)
Failed tests: 3-4
./t5500-fetch-pack.sh (Wstat: 256 Tests: 353 Failed: 1)
Failed test: 348
./t6011-rev-list-with-bad-commit.sh (Wstat: 256 Tests: 6 Failed: 1)
Failed test: 4
./t6024-recursive-merge.sh (Wstat: 256 Tests: 6 Failed: 1)
Failed test: 4

which you identified as 4x noise and t5500 as not understood.

$ GIT_TRACE=1 ./t5500-fetch-pack.sh -d -i -v -x
[...]
expecting success:
git -C shallow12 fetch --shallow-exclude one origin &&
git -C shallow12 log --pretty=tformat:%s origin/master >actual &&
test_write_lines three two >expected &&
test_cmp expected actual

++ git -C shallow12 fetch --shallow-exclude one origin
trace: built-in: git fetch --shallow-exclude one origin
trace: run_command: unset GIT_PREFIX; 'git-upload-pack '\''/u/git/t/trash directory.t5500-fetch-pack/shallow-exclude/.'\'''
trace: run_command: git --shallow-file pack-objects --revs --thin --stdout --shallow --progress --delta-base-offset --include-tag
trace: built-in: git pack-objects --revs --thin --stdout --shallow --progress --delta-base-offset --include-tag
remote: Counting objects: 4, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 4 (delta 0), reused 0 (delta 0)
trace: run_command: git --shallow-file unpack-objects --pack_header=2,4
trace: built-in: git unpack-objects --pack_header=2,4 Unpacking objects: 100% (4/4), done.
trace: run_command: git rev-list --objects --stdin --not --all --quiet
trace: built-in: git rev-list --objects --stdin --not --all --quiet
trace: run_command: unset GIT_PREFIX; 'git-upload-pack '\''/u/git/t/trash directory.t5500-fetch-pack/shallow-exclude/.'\'''
trace: run_command: git pack-objects --revs --thin --stdout --progress --delta-base-offset
trace: built-in: git pack-objects --revs --thin --stdout --progress --delta-base-offset
remote: Total 0 (delta 0), reused 0 (delta 0)
trace: run_command: git unpack-objects --pack_header=2,0
trace: built-in: git unpack-objects --pack_header=2,0
trace: run_command: git rev-list --objects --stdin --not --all --quiet
trace: built-in: git rev-list --objects --stdin --not --all --quiet From file:///u/git/t/trash directory.t5500-fetch-pack/shallow-exclude/.

 * [new tag]         one        -> one
 * [new tag]         two        -> two

run_processes_parallel: preparing to run up to 1 tasks
run_processes_parallel: done
trace: run_command: git gc --auto
trace: built-in: git gc --auto
++ git -C shallow12 log --pretty=tformat:%s origin/master
trace: built-in: git log '--pretty=tformat:%s' origin/master ++ test_write_lines three two
++ printf '%s\n' three two
++ test_cmp expected actual
++ diff -u expected actual

@@ -1,2 +1,3 @@
 three
 two
+one

error: last command exited with $?=1
not ok 348 - fetch exclude tag one
#

# git -C shallow12 fetch --shallow-exclude one origin &&
# git -C shallow12 log --pretty=tformat:%s origin/master >actual &&
# test_write_lines three two >expected &&
# test_cmp expected actual
#

What is "as expected" ?

When I insert a test_pause before that first line, such that:

test_expect_success 'fetch exclude tag one' '

    test_pause &&
    git -C shallow12 fetch --shallow-exclude one origin &&
    git -C shallow12 log --pretty=tformat:%s origin/master >actual &&
    test_write_lines three two >expected &&
    test_cmp expected actual

'

and then run

rm "shallow-exclude/.git/objects/info/commit-graph

the test works after exiting the test_pause.

Note how the shallow-exclude is the *remote* of the fetch.
So I think you also want to introduce the destruction
of commit graphs in upload-pack.c which is the remote counter part to fetch.

Why do you think these other tests are noice?

Thanks,
Stefan

Thanks for the details above. I agree that the solution is probably to disable the commit-graph during certain upload-pack parameters. Something is interfering there. I don't think the "destroy" pattern is right here.

Take t5307-pack-missing-commit.sh for a typical example: a repo is constructed, a pack is made, and then that pack is manipulated to "remove" a commit. That missing commit is "detected" by running rev-list. Except if we are running the commit-graph on every 'git commit' call, the rev-list now succeeds since we are not looking at the packfile for the commit details any more!

This is what I mean by the tests being "noise". Adding the "DO NOT MERGE" commits only interrupted the test mechanism for unrelated behavior.

Thanks,

-Stolee

I like to think about those features as providing an overlay for the commit graph (similar to overlay filesystems, like overlayfs); in the case of git-replace quite literally.

I will be calling all those features "history-altering features", for short.

What I would like to see here in cover letter is a generic description of _strategy_ to deal with those history-altering features.

From what I understand you have the following options for each of replacements, shallow clones and grafts:
- turn off serialized commit-graph if given history-altering feature is present, as if core.commitGraph was set to false
- invalidate and optionally refresh commit-graph file if given history-altering feature is present (maybe only if it changed the view of the history, is such check is possible)

For automatic invalidation you would need to have either:
- cover all possible ways by which given history-altering feature can change the view of history, or
- keep state of history-altering change for which commit-graph was created (e.g. in proposed VAL4 chunk) and compare with current view of history if it changed

For automatic turning off you would need only to check if history-altering feature is present.

Let us examine each of those history-altering features that Git supports:

* shallow clone

- shallow clone usually means having shorter history, so serialized commit-graph is not needed as much; also changing the depth screws-up assumptions about generation numbers

- there are only a few entry points changing the view of history,

    namely fetch and push with shallow options (--depth=<depth>,
    --deepen=<depth>, --shallow-since=<date>, --update-shallow,
    --shallow-exclude=<revision>, --unshallow)

- it is easy to check for presence of shallow clone feature by chacking of $GIT_DIR/shallow exists and is not empty

- different contents of $GIT_DIR/shallow means different view of history

- NOTE: internally uses grafts mechanism (emulated grafts)

* replacements (replace objects, git-replace)

- git-replace can be used to join current development repository with historical repository, in which case we would certainly want to make use of serialied commit graph; on the other hand the replacements do not necessary alter the view of the history

- theoretically you could create replacement refs by hand, but in practice there are TWO ways of getting them:

- using git-replace command to create, edit/change and delete replacement objects'
'
- fetching or having pushed-to refs in refs/replace/* namespace

- you need to check if there are any refs in refs/replace/* namespace to check if the feature is used (but it doesn't necessarily mean that it altered project history)

- changed list of refs in refs/replace/* namespace (which you can get with for-each-ref command/API) does not necessarily mean that the view of the history changed: you can replace non-commit object, you can replace commits and not change their parents; it is not as easy as checking if file exists

- NOTE: the feature can be turned off manually with GIT_NO_REPLACE_OBJECTS environment variable and with --no-replace-objects option to git wrapper.

Also when pushing, fetching and fsck-ing this feature is turned off and refs in refs/replace/* namespace are treated as ordinary refs.

This may mean that we would want to create commit-graph with replacements for ordinary non-bare repository, and without replacements for server-side bare repository.

* grafts

- subset of uses of git-replace, older and *obsolete* feature (because it is unsafe to use; that is you need to be careful with it).

- edited by hand, no automatic entry points

- if $GIT_DIR/info/grafts file is present, then feature is enabled (barring some corner cases, like empty file or file consisting only of comments)

- changed contents of this file means changed view of history (well, except for reordering lines, or removing/adding empty lines and comments)

That's clever way of increasing coverage.

Does this means that those tests should be in the end (i.e. when core.commitGraph is turned on by defult) be simply run with core.commitGraph explicitly disabled for the test?

Would it make sense to split this commit into two dealing separately with replace objects and with grafts? Or do they use the same underlying API?

O.K. It might be good idea to add separate test that does the same, but with git-replace instead of grafts, though.

The t6050-replace.sh does not test changing the DAG of revision (excluding changing the SHA-1 of commit), if I read it correctly. It would be good to test using git-replace to change committer date, and to change parents: shortening or lengthening history (e.g. emulating joining two independent lines of development in the latter case).

See also comment about t6001 above.

Seems O.K. from the subject, but I'd have to check the details.

I wonder if reverting to using object database is a good solution, and if it wouldn't be better to invalidate / delete commit-graph file instead...

Why do not simply check if repository is shallow?