fast-export: fix surprising behavior with --first-parent

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

This commit makes fast-export output the same commits as rev-list --first-parent, and makes --reverse not have an effect on which commits are output.

The fix involves removing logic within fast-export which was responsible for ensuring that parents are processed before their children, which was what was exiting early due to missing second parents. This is replaced by setting 'reverse = 1' before revision walking, which, in conjuction with topo_order, allows for delegating the ordering of commits to revision.c. The reverse flag is set after parsing rev-list arguments to avoid having it disabled.

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

This is my first time patching git, so I probably need some guidance on my approach. :)

builtin/fast-export.c +0 -25

-static int has_unshown_parent(struct commit *commit)
-{
-	struct commit_list *parent;
-
-	for (parent = commit->parents; parent; parent = parent->next)
-		if (!(parent->item->object.flags & SHOWN) &&
-		    !(parent->item->object.flags & UNINTERESTING))
-			return 1;
-	return 0;
-}
-
 struct anonymized_entry {
 	struct hashmap_entry hash;
 	const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
 	return strbuf_detach(&out, NULL);
 }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-			struct string_list *paths_of_changed_objects)
-{
-	struct commit *commit;
-	while (commits->nr) {
-		commit = (struct commit *)object_array_pop(commits);
-		if (has_unshown_parent(commit)) {
-			/* Queue again, to be handled later */
-			add_object_array(&commit->object, NULL, commits);
-			return;
-		}
-		handle_commit(commit, revs, paths_of_changed_objects);
-	}
-}

static void handle_tag(const char *name, struct tag *tag) {

builtin/fast-export.c +0 -1

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
 int cmd_fast_export(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info revs;
-	struct object_array commits = OBJECT_ARRAY_INIT;
 	struct commit *commit;
 	char *export_filename = NULL,
 	     *import_filename = NULL,

builtin/fast-export.c +0 -0

 	revs.diffopt.format_callback = show_filemodify;
 	revs.diffopt.format_callback_data = &paths_of_changed_objects;
 	revs.diffopt.flags.recursive = 1;
 	while ((commit = get_revision(&revs))) {

Quoted 19 lines · William Sprent via GitGitGadget

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

This commit makes fast-export output the same commits as rev-list --first-parent, and makes --reverse not have an effect on which commits are output.

The fix involves removing logic within fast-export which was responsible for ensuring that parents are processed before their children, which was what was exiting early due to missing second parents. This is replaced by setting 'reverse = 1' before revision walking, which, in conjuction with topo_order, allows for delegating the ordering of commits to revision.c. The reverse flag is set after parsing rev-list arguments to avoid having it disabled.

Quoted 8 lines

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

This is my first time patching git, so I probably need some guidance on my approach. :)

Hi, thanks for your first contribution to git. This is a rather shallow review, a deeper one is much deserved.

Quoted 1 line

I notice that you're removing code in builtin/fast-export.c, presumably

we have code in revision.c that does the same thing. It would really help a reviewer for you to dig a bit into the relevant commit history and note it in the commit message.

I.e. could revision.c always do this, and this was always needless duplication, or at time X it was needed, but as of Y revision.c learned to do this, and callers A, B and C were adjusted, but just not this missed call D? etc.

Quoted 32 lines

builtin/fast-export.c +0 -25

-static int has_unshown_parent(struct commit *commit)
-{
-	struct commit_list *parent;
-
-	for (parent = commit->parents; parent; parent = parent->next)
-		if (!(parent->item->object.flags & SHOWN) &&
-		    !(parent->item->object.flags & UNINTERESTING))
-			return 1;
-	return 0;
-}
-
  struct anonymized_entry {
  	struct hashmap_entry hash;
  	const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
  	return strbuf_detach(&out, NULL);
  }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-			struct string_list *paths_of_changed_objects)
-{
-	struct commit *commit;
-	while (commits->nr) {
-		commit = (struct commit *)object_array_pop(commits);
-		if (has_unshown_parent(commit)) {
-			/* Queue again, to be handled later */
-			add_object_array(&commit->object, NULL, commits);
-			return;
-		}
-		handle_commit(commit, revs, paths_of_changed_objects);
-	}
-}

...

Quoted 11 lines

static void handle_tag(const char *name, struct tag *tag) {

builtin/fast-export.c +0 -1

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
  int cmd_fast_export(int argc, const char **argv, const char *prefix)
  {
  	struct rev_info revs;
-	struct object_array commits = OBJECT_ARRAY_INIT;
  	struct commit *commit;
  	char *export_filename = NULL,
  	     *import_filename = NULL,

builtin/fast-export.c int cmd_fast_export(int argc, const char **argv, const char *prefix) · line 1257

Quoted 4 lines

builtin/fast-export.c +1 -0

@@ -1281,19 +1254,14 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
  	get_tags_and_duplicates(&revs.cmdline);
 
+	revs.reverse = 1;

Is the placement of revs.reverse = 1 here important, or could it go earlier after init_revision_sources() when we assign some other values ir revs?

builtin/fast-export.c int cmd_fast_export(int argc, const char **argv, const char *prefix) · line 1259

Quoted 2 lines

builtin/fast-export.c +0 -0

  	if (prepare_revision_walk(&revs))
  		die("revision walk setup failed");

A light reading of prepare_revision_walk() suggests it could come after, but maybe I'm entirely wrong.

Quoted 4 lines

builtin/fast-export.c +0 -0

  	revs.diffopt.format_callback = show_filemodify;
  	revs.diffopt.format_callback_data = &paths_of_changed_objects;
  	revs.diffopt.flags.recursive = 1;
  	while ((commit = get_revision(&revs))) {

builtin/fast-export.c int cmd_fast_export(int argc, const char **argv, const char *prefix) · line 1266

Quoted 10 lines

builtin/fast-export.c +1 -7

-		if (has_unshown_parent(commit)) {
-			add_object_array(&commit->object, NULL, &commits);
-		}
-		else {
-			handle_commit(commit, &revs, &paths_of_changed_objects);
-			handle_tail(&commits, &revs, &paths_of_changed_objects);
-		}
+		handle_commit(commit, &revs, &paths_of_changed_objects);
  	}
 

Yay code deletion, good if it works (I didn't check).

Since this is just a one-statement while-loop we can also remove its braces now.

Quoted 1 line

Quoted 1 line

+test_expect_success 'fast-export --first-parent outputs all

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 756

Quoted 2 lines

t/t9350-fast-export.sh +2 -0

+	git init first-parent &&
+	cd first-parent &&

Do any such "cd" in a sub-shell:

	git init x &&
	(
     		cd x &&
                 ...
	)

Otherwise the next test after you is going to run in anotherdirectory.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 766

Quoted 10 lines

t/t9350-fast-export.sh +10 -0

+	test_commit init &&
+	git checkout -b topic1 &&
+	test_commit file2 file2.txt &&
+	git checkout main &&
+	git merge topic1 --no-ff &&
+
+	git checkout -b topic2 &&
+	test_commit file3 file3.txt &&
+	git checkout main &&
+	git merge topic2 --no-ff &&

Just a nit. I'd use "test_commit A", "test_commit B" etc. when the filenames etc. aren't important. There's no subsequent reference here, so I assume they're not.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 770

Quoted 3 lines

t/t9350-fast-export.sh +3 -0

+	test_commit branch-head &&
+
+	git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main > expected &&

nit; >expected, not > expected is the usual style.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 773

Quoted 3 lines

t/t9350-fast-export.sh +3 -0

+
+	git fast-export main -- --first-parent > first-parent-export &&
+	git fast-export main -- --first-parent --reverse > first-parent-reverse-export &&

ditto:

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 774

Quoted 1 line

t/t9350-fast-export.sh +1 -0

+	git init import && cd import &&

ditto earlier "cd" comment.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 775

Quoted 1 line

t/t9350-fast-export.sh +1 -0

+	cat ../first-parent-export | git fast-import &&

Instead of "cat x | prog" do "prog <x".

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 778

Quoted 3 lines

t/t9350-fast-export.sh +2 -0

+	git rev-list --format="%ad%B" --topo-order --all --no-commit-header > actual &&
 
+	test $(git rev-list --all | wc -l) -eq 4 &&

Instead:

git rev-list --all >tmp &&
test_line_count = 4 tmp

(for some value of tmp)

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 781

Quoted 3 lines

t/t9350-fast-export.sh +3 -0

+	test_cmp ../expected actual &&
+	test_cmp ../first-parent-export ../first-parent-reverse-export
+'

Maybe some of the CD-ing around here wouldu be easier by not doing that and instead running e.g.:

     git -C subdir fast-export >file-not-in-subdir &&
     ...

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

This commit makes fast-export output the same commits as rev-list --first-parent, and makes --reverse not have an effect on which commits are output.

The fix involves removing logic within fast-export which was responsible for ensuring that parents are processed before their children, which was what was exiting early due to missing second parents. This is replaced by setting 'reverse = 1' before revision walking, which, in conjuction with topo_order, allows for delegating the ordering of commits to revision.c. The reverse flag is set after parsing rev-list arguments to avoid having it disabled.

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

This is my first time patching git, so I probably need some guidance on my approach. :)

I've noticed that git fast-export exhibits some odd behavior when passed the --first-parent flag. In the repository I was working on, it would only output the initial commit before exiting. Moreover, adding the --reverse flag causes it to behave differently and instead output all commits in the first parent line that only have one parent. My expectation is more or less that git fast-export should output the same set of commits as git rev-list would output given the same arguments, which matches how it acts when given revision ranges.

It seems like this behavior comes from the fact that has_unshown_parents will never be false for any merge commits encountered when fast-export is called with --first-parent. This causes the while loop to follow the pattern of pushing all commits into the "commits" queue until the initial commit is encountered, at which point it calls handle_tail which falls through on the first merge commit, causing most of the commits to be unhandled.

My impression is that this logic only serves to ensure that parents are processed before their children, so in my patch I've opted to fix the issue by delegating that responsibility to revision.c by adding the reverse flag before performing the revision walk. From what I can see, this should be equivalent to what the previous logic is trying to achieve, but I can also see that there could be risk in these changes.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1084%2Fwilliams-unity%2Ffast-export%2Ffirst-parent-issues-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1084/williams-unity/fast-export/first-parent-issues-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1084

 builtin/fast-export.c  | 36 ++----------------------------------
 t/t9350-fast-export.sh | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 34 deletions(-)

builtin/fast-export.c +1 -27

index 8e2caf72819..50f8c224b6e 100644
@@ -107,18 +107,6 @@ static int parse_opt_reencode_mode(const struct option *opt,
 
 static struct decoration idnums;
 static uint32_t last_idnum;
-
-static int has_unshown_parent(struct commit *commit)
-{
-       struct commit_list *parent;
-
-       for (parent = commit->parents; parent; parent = parent->next)
-               if (!(parent->item->object.flags & SHOWN) &&
-                   !(parent->item->object.flags & UNINTERESTING))
-                       return 1;
-       return 0;
-}
-
 struct anonymized_entry {
        struct hashmap_entry hash;
        const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
        return strbuf_detach(&out, NULL);
 }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-                       struct string_list *paths_of_changed_objects)
-{
-       struct commit *commit;
-       while (commits->nr) {
-               commit = (struct commit *)object_array_pop(commits);
-               if (has_unshown_parent(commit)) {
-                       /* Queue again, to be handled later */
-                       add_object_array(&commit->object, NULL, commits);
-                       return;
-               }
-               handle_commit(commit, revs, paths_of_changed_objects);
-       }
-}
 
 static void handle_tag(const char *name, struct tag *tag)
 {

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
 int cmd_fast_export(int argc, const char **argv, const char *prefix)
 {
        struct rev_info revs;
-       struct object_array commits = OBJECT_ARRAY_INIT;
        struct commit *commit;
        char *export_filename = NULL,
             *import_filename = NULL,

@@ -1281,19 +1254,14 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
        get_tags_and_duplicates(&revs.cmdline);
 
+       revs.reverse = 1;
        if (prepare_revision_walk(&revs))
                die("revision walk setup failed");
        revs.diffopt.format_callback = show_filemodify;
        revs.diffopt.format_callback_data = &paths_of_changed_objects;

builtin/fast-export.c +0 -0

        }
 
        handle_tags_and_duplicates(&extra_refs);

t/t9350-fast-export.sh +30 -0

index 409b48e2442..bd08101b81d 100755
@@ -750,4 +750,34 @@ test_expect_success 'merge commit gets exported with --import-marks' '
        )
 '
 
+
+test_expect_success 'fast-export --first-parent outputs all revisions output by revision walk' '
+       git init first-parent &&
+       cd first-parent &&
+       test_commit init &&
+       git checkout -b topic1 &&
+       test_commit file2 file2.txt &&
+       git checkout main &&
+       git merge topic1 --no-ff &&
+
+       git checkout -b topic2 &&
+       test_commit file3 file3.txt &&
+       git checkout main &&
+       git merge topic2 --no-ff &&
+
+       test_commit branch-head &&
+
+       git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main > expected &&
+
+       git fast-export main -- --first-parent > first-parent-export &&
+       git fast-export main -- --first-parent --reverse > first-parent-reverse-export &&
+       git init import && cd import &&
+       cat ../first-parent-export | git fast-import &&
+
+       git rev-list --format="%ad%B" --topo-order --all --no-commit-header > actual &&
+       test $(git rev-list --all | wc -l) -eq 4 &&
+       test_cmp ../expected actual &&
+       test_cmp ../first-parent-export ../first-parent-reverse-export
+'
+
 test_done

base-commit: cd3e606211bb1cf8bc57f7d76bab98cc17a150bc --
gitgitgadget

Quoted 3 lines · William Sprent via GitGitGadget

From: William Sprent <[email protected]>
...

Wow, interesting. I did some related work that I never submitted at
https://github.com/newren/git/commit/08f799b4667de1c755c78e1ea1657f946b588556
-- in that commit, I also noticed that fast-export did not seem careful enough about making sure to process all commits, and came up with a much different fix. However, in my case, I didn't know how to trigger the problems in fast-export without some additional changes I was trying to make, and since I never got those other changes working, I didn't think it was worth submitting my fix. My solution is more complex, in part because it was designed to handle ordering requirements & recommendations other than just ancestry. However, we don't currently have any additional ordering requirements (the current code only considers ancestry), so your solution is much simpler. If I do at some point get my other changes working, I'd need to re-introduce the queue and handle_tail() and my changes to these, but that can always be done later if and when I get those other changes working.

Interestingly, Dscho added both the --reverse ability to revision.c and the commits object_array and the handle_tail() stuff in fast-export.c. Both were done in the same year, and --reverse came first. See 9c5e66e97da8 (Teach revision machinery about --reverse,

2007-01-20) and f2dc849e9c5f (Add 'git fast-export', the sister of 'git fast-import', 2007-12-02). It's not clear why revs.reverse = 1 wasn't used previously, although it certainly didn't occur to me either and I think it would have been an alternative solution to my first ever git.git contribution -- 784f8affe4df (fast-export: ensure we traverse commits in topological order, 2009-02-10). (Though rev.reverse = 1 wouldn't have provided the same speedups that rev.topo_order=1 provided.)

I can't think of any cases where this would cause any problems, and it seems like using rev.reverse is a pretty clean solution. Just in case I'm missing something, though, it would be nice to get Dscho to also comment on this. I'm cc'ing him.

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

Quoted 6 lines · William Sprent via GitGitGadget

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

I do not doubt we would want to make the command behave sensibly with all options it accepts, but let me first ask a more basic and possibly stupid question.

What is "git fast-export --first-parent <options>" supposed to do differently from "git fast-export <options>" (with the same set of options other than "--first-parent")? Should it omit merge commits altogether, pretending that the first single-parent ancestor it finds on the first parent chain is a direct parent of a single-parent descendant, e.g. if the real history were with two single-parente commits A and B, with two merges M and N, on the mainline, making the resulting commits into a single strand of two pearls, with A and B before and after the rewrite to have the same tree objects?

     ---A---M---N---B             ---A---B
           /   /           ==>
          X   Y

Or should it pretend merge commits have only their first parent as their parents, i.e.

     ---A---M---N---B             ---A---M---N---B
           /   /           ==>
          X   Y

"git fast-export --help" does not even mention "--first-parent" and pretend that any and all [<git-rev-list-args>...] are valid requests to make to the command, but I am wondering if that is what we intend to support in the first place. In builtin/fast-export.c, I do not see any attempt to do anything differently when "--first-parent" is requested. Perhaps we shouldn't be even taking "--first-parent" as an option to begin with.
The "--reverse" feels even more iffy. Are we reversing the history with such an export, i.e. pretending that parents are children and merges are forks?

     ---A---M---N---B             B---N---M---A---
           /   /           ==>         \   \
          X   Y                         X   Y

Or are we supposed to produce the same history in the end, just spewing older commits first in the output stream? I am not sure what purpose such a request serves---the "fast-import" downstream would need the same set of objects before it can create each commit anyway, so I am not sure what the point of giving "--reverse" is.

If there is no sensible interpretation for some of the options that are valid in rev-list in the context of "fast-export" command, should we just error out when we parse the command line, instead of producing nonsense output stream, I wonder.

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

This commit makes fast-export output the same commits as rev-list --first-parent, and makes --reverse not have an effect on which commits are output.

The fix involves removing logic within fast-export which was responsible for ensuring that parents are processed before their children, which was what was exiting early due to missing second parents. This is replaced by setting 'reverse = 1' before revision walking, which, in conjuction with topo_order, allows for delegating the ordering of commits to revision.c. The reverse flag is set after parsing rev-list arguments to avoid having it disabled.

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

I've noticed that git fast-export exhibits some odd behavior when passed the --first-parent flag. In the repository I was working on, it would only output the initial commit before exiting. Moreover, adding the --reverse flag causes it to behave differently and instead output all commits in the first parent line that only have one parent. My expectation is more or less that git fast-export should output the same set of commits as git rev-list would output given the same arguments, which matches how it acts when given revision ranges.

It seems like this behavior comes from the fact that has_unshown_parents will never be false for any merge commits encountered when fast-export is called with --first-parent. This causes the while loop to follow the pattern of pushing all commits into the "commits" queue until the initial commit is encountered, at which point it calls handle_tail which falls through on the first merge commit, causing most of the commits to be unhandled.

My impression is that this logic only serves to ensure that parents are processed before their children, so in my patch I've opted to fix the issue by delegating that responsibility to revision.c by adding the reverse flag before performing the revision walk. From what I can see, this should be equivalent to what the previous logic is trying to achieve, but I can also see that there could be risk in these changes.

Changes since v1:

* Moved revs.reverse assignment down to similar assignments below.
* Removed braces around single statement while loop.
* The test now only changes directory inside a sub-shell.
* Applied stylistic feedback on test such as: making redirections be on the form >FILE etc.

There were questions raised about whether it makes sense at all for fast-export to simply accept all git rev-list arguments whether they have an effect or not - in particular for a flag like --reverse. I think I agree that it is questionable behavior, or at least questionably documented, but I also think it is out of scope for this change.

I did consider teaching fast-export to complain if given --reverse, but it seemed inconsistent to me as it will gladly accept every other rev-list argument (for example, "git fast-export HEAD --show-signature --graph" works just fine).

cc Ævar Arnfjörð Bjarmason [email protected]

builtin/fast-export.c +0 -16

index 8e2caf72819..cb1d6473f12 100644
@@ -107,18 +107,6 @@ static int parse_opt_reencode_mode(const struct option *opt,
 
 static struct decoration idnums;
 static uint32_t last_idnum;
-
-static int has_unshown_parent(struct commit *commit)
-{
-       struct commit_list *parent;
-
-       for (parent = commit->parents; parent; parent = parent->next)
-               if (!(parent->item->object.flags & SHOWN) &&
-                   !(parent->item->object.flags & UNINTERESTING))
-                       return 1;
-       return 0;
-}
-
 struct anonymized_entry {
        struct hashmap_entry hash;
        const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
        return strbuf_detach(&out, NULL);
 }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-                       struct string_list *paths_of_changed_objects)
-{
-       struct commit *commit;

builtin/fast-export.c +0 -1

 static void handle_tag(const char *name, struct tag *tag)
 {

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
 int cmd_fast_export(int argc, const char **argv, const char *prefix)
 {
        struct rev_info revs;
-       struct object_array commits = OBJECT_ARRAY_INIT;
        struct commit *commit;
        char *export_filename = NULL,
             *import_filename = NULL,

builtin/fast-export.c +0 -1

        revs.diffopt.format_callback = show_filemodify;
        revs.diffopt.format_callback_data = &paths_of_changed_objects;
        revs.diffopt.flags.recursive = 1;
-       while ((commit = get_revision(&revs))) {

builtin/fast-export.c +0 -0

        handle_tags_and_duplicates(&extra_refs);
        handle_tags_and_duplicates(&tag_refs);

t/t9350-fast-export.sh +5 -0

index 409b48e2442..df1a5b1013a 100755
@@ -750,4 +750,39 @@ test_expect_success 'merge commit gets exported with --import-marks' '
        )
 '
 
+
+test_expect_success 'fast-export --first-parent outputs all revisions output by revision walk' '
+       git init first-parent &&
+       (
+               cd first-parent &&

t/t9350-fast-export.sh +3 -0

+       )
+'
+
 test_done

... Here's what I think are the relevant points
(and yeah, it's lengthy):

The revision traversal machinery typically processes and returns all children before any parent. fast-export needs to operate in the reverse fashion, handling parents before any of their children in order to build up the history starting from the root commit(s). This would be a clear case where we could just use the revision traversal machinery's "reverse" option to achieve this desired affect.

However, this wasn't what the code did. It added its own array for queuing. The obvious hand-rolled solution would be to just push all the commits into the array and then traverse afterwards, but it didn't quite do that either. It instead attempted to process anything it could as soon as it could, and once it could, check whether it could process anything that had been queued. As far as I can tell, this was an effort to save a little memory in the case of multiple root commits since it could process some commits before queueing all of them. This involved some helper functions named has_unshown_parent() and handle_tail(). For typical invocations of fast-export, this alternative essentially amounted to a hand-rolled method of reversing the commits -- it was a bunch of work to duplicate the revision traversal machinery's "reverse" option.

This hand-rolled reversing mechanism is actually somewhat difficult to reason about. It takes some time to figure out how it ensures in normal cases that it will actually process all traversed commits (rather than just dropping some and not printing anything for them).

And it turns out there are some cases where the code does drop commits without handling them, and not even printing an error or warning for the user. Due to the has_unshown_parent() checks, some commits could be left in the array at the end of the "while...get_revision()" loop which would be unprocessed. This could be triggered for example with

    git fast-export main -- --first-parent
or non-sensical traversal rules such as
    git fast-export main -- --grep=Merge --invert-grep

While most traversals that don't include all parents should likely trigger errors in fast-export (or at least require being used in combination with --reference-excluded-parents), the --first-parent traversal is at least reasonable and it'd be nice if it didn't just drop commits. It'd also be nice to have a simpler "reverse traversal" mechanism. Use the "reverse" option of the revision traversal machinery to achieve both.

Even for the non-sensical traversal flags like the --grep one above, this would be an improvement. For example, in that case, the code previously would have silently truncated history to only those commits that do not have an ancestor containing "Merge" in their commit message. After this code change, that case would would include all

commits without "Merge" in their commit message -- but any commit that previously had a "Merge"-mentioning parent would lose that parent (likely resulting in many new root commits). While the new behavior is still odd, it is at least understandable given that --reference-excluded-parents is not the default.

Quoted 39 lines · Elijah Newren

... Here's what I think are the relevant points
(and yeah, it's lengthy):

The revision traversal machinery typically processes and returns all children before any parent. fast-export needs to operate in the reverse fashion, handling parents before any of their children in order to build up the history starting from the root commit(s). This would be a clear case where we could just use the revision traversal machinery's "reverse" option to achieve this desired affect.

However, this wasn't what the code did. It added its own array for queuing. The obvious hand-rolled solution would be to just push all the commits into the array and then traverse afterwards, but it didn't quite do that either. It instead attempted to process anything it could as soon as it could, and once it could, check whether it could process anything that had been queued. As far as I can tell, this was an effort to save a little memory in the case of multiple root commits since it could process some commits before queueing all of them. This involved some helper functions named has_unshown_parent() and handle_tail(). For typical invocations of fast-export, this alternative essentially amounted to a hand-rolled method of reversing the commits -- it was a bunch of work to duplicate the revision traversal machinery's "reverse" option.

This hand-rolled reversing mechanism is actually somewhat difficult to reason about. It takes some time to figure out how it ensures in normal cases that it will actually process all traversed commits (rather than just dropping some and not printing anything for them).

And it turns out there are some cases where the code does drop commits without handling them, and not even printing an error or warning for the user. Due to the has_unshown_parent() checks, some commits could be left in the array at the end of the "while...get_revision()" loop which would be unprocessed. This could be triggered for example with

    git fast-export main -- --first-parent
or non-sensical traversal rules such as
    git fast-export main -- --grep=Merge --invert-grep

Quoted 7 lines

While most traversals that don't include all parents should likely trigger errors in fast-export (or at least require being used in combination with --reference-excluded-parents), the --first-parent traversal is at least reasonable and it'd be nice if it didn't just drop commits. It'd also be nice to have a simpler "reverse traversal" mechanism. Use the "reverse" option of the revision traversal machinery to achieve both.

The above is a very helpful and understandable explanation of what is going on. I am a bit puzzled by the very last part, though. By "It'd also be nice to have a simpler 'reverse traversal' mechanism", do you mean that the end users have need to control the direction the traversal goes (in other words, they use "git fast-export" for some thing, and "git fast-export --reverse" to achieve some other things)? Or do you just mean that we need to do a reverse traversal but that is already available in the revision traversal machinery, and not using it and rolling our own does not make sense?

Quoted 5 lines

Even for the non-sensical traversal flags like the --grep one above, this would be an improvement. For example, in that case, the code previously would have silently truncated history to only those commits that do not have an ancestor containing "Merge" in their commit message. After this code change, that case would would include all

"would would" -> "would"

Quoted 5 lines

commits without "Merge" in their commit message -- but any commit that previously had a "Merge"-mentioning parent would lose that parent (likely resulting in many new root commits). While the new behavior is still odd, it is at least understandable given that --reference-excluded-parents is not the default.

Nicely written.

Quoted 12 lines · William Sprent via GitGitGadget

From: William Sprent <[email protected]>

When invoking git-fast-export with the --first-parent flag on a branch with merges, fast-export would early-out on processing the first merge on the branch. If combined with --reverse, fast-export would instead output all single parent commits on the branch.

This commit makes fast-export output the same commits as rev-list --first-parent, and makes --reverse not have an effect on which commits are output.

Quoted 7 lines

The fix involves removing logic within fast-export which was responsible for ensuring that parents are processed before their children, which was what was exiting early due to missing second parents. This is replaced by setting 'reverse = 1' before revision walking, which, in conjuction with topo_order, allows for delegating the ordering of commits to revision.c. The reverse flag is set after parsing rev-list arguments to avoid having it disabled.

This explains how you discovered the issue, but seems to focus on these particular flags, leaving the reader wondering whether there are other flags that are also mishandled that need additional care, and whether this fix might break things for other sets of flags. It took me quite a bit of reasoning to satisfy myself on those points; it's actually somewhat complex. May I suggest an alternative commit message based on that? Here's what I think are the relevant points (and yeah, it's lengthy):

The revision traversal machinery typically processes and returns all children before any parent. fast-export needs to operate in the reverse fashion, handling parents before any of their children in order to build up the history starting from the root commit(s). This would be a clear case where we could just use the revision traversal machinery's "reverse" option to achieve this desired affect.

However, this wasn't what the code did. It added its own array for queuing. The obvious hand-rolled solution would be to just push all the commits into the array and then traverse afterwards, but it didn't quite do that either. It instead attempted to process anything it could as soon as it could, and once it could, check whether it could process anything that had been queued. As far as I can tell, this was an effort to save a little memory in the case of multiple root commits since it could process some commits before queueing all of them. This involved some helper functions named has_unshown_parent() and handle_tail(). For typical invocations of fast-export, this alternative essentially amounted to a hand-rolled method of reversing the commits -- it was a bunch of work to duplicate the revision traversal machinery's "reverse" option.

This hand-rolled reversing mechanism is actually somewhat difficult to reason about. It takes some time to figure out how it ensures in normal cases that it will actually process all traversed commits (rather than just dropping some and not printing anything for them).

And it turns out there are some cases where the code does drop commits without handling them, and not even printing an error or warning for the user. Due to the has_unshown_parent() checks, some commits could be left in the array at the end of the "while...get_revision()" loop which would be unprocessed. This could be triggered for example with

     git fast-export main -- --first-parent
or non-sensical traversal rules such as
     git fast-export main -- --grep=Merge --invert-grep

While most traversals that don't include all parents should likely trigger errors in fast-export (or at least require being used in combination with --reference-excluded-parents), the --first-parent traversal is at least reasonable and it'd be nice if it didn't just drop commits. It'd also be nice to have a simpler "reverse traversal" mechanism. Use the "reverse" option of the revision traversal machinery to achieve both.

Even for the non-sensical traversal flags like the --grep one above, this would be an improvement. For example, in that case, the code previously would have silently truncated history to only those commits that do not have an ancestor containing "Merge" in their commit message. After this code change, that case would would include all commits without "Merge" in their commit message -- but any commit that previously had a "Merge"-mentioning parent would lose that parent (likely resulting in many new root commits). While the new behavior is still odd, it is at least understandable given that --reference-excluded-parents is not the default.

Quoted 52 lines

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

I've noticed that git fast-export exhibits some odd behavior when passed the --first-parent flag. In the repository I was working on, it would only output the initial commit before exiting. Moreover, adding the --reverse flag causes it to behave differently and instead output all commits in the first parent line that only have one parent. My expectation is more or less that git fast-export should output the same set of commits as git rev-list would output given the same arguments, which matches how it acts when given revision ranges.

It seems like this behavior comes from the fact that has_unshown_parents will never be false for any merge commits encountered when fast-export is called with --first-parent. This causes the while loop to follow the pattern of pushing all commits into the "commits" queue until the initial commit is encountered, at which point it calls handle_tail which falls through on the first merge commit, causing most of the commits to be unhandled.

My impression is that this logic only serves to ensure that parents are processed before their children, so in my patch I've opted to fix the issue by delegating that responsibility to revision.c by adding the reverse flag before performing the revision walk. From what I can see, this should be equivalent to what the previous logic is trying to achieve, but I can also see that there could be risk in these changes.

Changes since v1:

* Moved revs.reverse assignment down to similar assignments below.
* Removed braces around single statement while loop.
* The test now only changes directory inside a sub-shell.
* Applied stylistic feedback on test such as: making redirections be on the form >FILE etc.

There were questions raised about whether it makes sense at all for fast-export to simply accept all git rev-list arguments whether they have an effect or not - in particular for a flag like --reverse. I think I agree that it is questionable behavior, or at least questionably documented, but I also think it is out of scope for this change.

I did consider teaching fast-export to complain if given --reverse, but it seemed inconsistent to me as it will gladly accept every other rev-list argument (for example, "git fast-export HEAD --show-signature --graph" works just fine).

cc Ævar Arnfjörð Bjarmason [email protected]

...

Quoted 31 lines

builtin/fast-export.c +0 -16

index 8e2caf72819..cb1d6473f12 100644
@@ -107,18 +107,6 @@ static int parse_opt_reencode_mode(const struct option *opt,
 
  static struct decoration idnums;
  static uint32_t last_idnum;
-
-static int has_unshown_parent(struct commit *commit)
-{
-       struct commit_list *parent;
-
-       for (parent = commit->parents; parent; parent = parent->next)
-               if (!(parent->item->object.flags & SHOWN) &&
-                   !(parent->item->object.flags & UNINTERESTING))
-                       return 1;
-       return 0;
-}
-
  struct anonymized_entry {
         struct hashmap_entry hash;
         const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
         return strbuf_detach(&out, NULL);
  }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-                       struct string_list *paths_of_changed_objects)
-{
-       struct commit *commit;

builtin/fast-export.c static char *anonymize_tag(void *data) · line 768

Quoted 10 lines

builtin/fast-export.c +0 -10

-       while (commits->nr) {
-               commit = (struct commit *)object_array_pop(commits);
-               if (has_unshown_parent(commit)) {
-                       /* Queue again, to be handled later */
-                       add_object_array(&commit->object, NULL, commits);
-                       return;
-               }
-               handle_commit(commit, revs, paths_of_changed_objects);
-       }
-}

Deleted code is debugged code! :-)

Quoted 12 lines

builtin/fast-export.c +0 -1

  static void handle_tag(const char *name, struct tag *tag)
  {

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
  int cmd_fast_export(int argc, const char **argv, const char *prefix)
  {
         struct rev_info revs;
-       struct object_array commits = OBJECT_ARRAY_INIT;
         struct commit *commit;
         char *export_filename = NULL,
              *import_filename = NULL,

builtin/fast-export.c int cmd_fast_export(int argc, const char **argv, const char *prefix) · line 1260

Quoted 5 lines

builtin/fast-export.c +2 -0

@@ -1283,18 +1256,13 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
         if (prepare_revision_walk(&revs))
                 die("revision walk setup failed");
+
+       revs.reverse = 1;

I really wanted to see this next to the
rev.topo_order = 1
statement elsewhere, but as you alluded to in the commit message, this part of revision.c makes that unsafe:

"""

} else if (!strcmp(arg, "--reverse")) {
     revs->reverse ^= 1;

"""

However, given that it's unsafe to set revs.reverse=1 earlier, now that I think about it, isn't it also unsafe to set revs.topo_order where it is? Someone could override it by passing --date-order to fast-export. (It's okay if you want to leave fixing that to someone else, just thought I'd mention it while reviewing.)

Quoted 4 lines

builtin/fast-export.c +0 -1

         revs.diffopt.format_callback = show_filemodify;
         revs.diffopt.format_callback_data = &paths_of_changed_objects;
         revs.diffopt.flags.recursive = 1;
-       while ((commit = get_revision(&revs))) {

builtin/fast-export.c int cmd_fast_export(int argc, const char **argv, const char *prefix) · line 1265

Quoted 10 lines

builtin/fast-export.c +2 -8

-               if (has_unshown_parent(commit)) {
-                       add_object_array(&commit->object, NULL, &commits);
-               }
-               else {
-                       handle_commit(commit, &revs, &paths_of_changed_objects);
-                       handle_tail(&commits, &revs, &paths_of_changed_objects);
-               }
-       }
+       while ((commit = get_revision(&revs)))
+               handle_commit(commit, &revs, &paths_of_changed_objects);

Looks good. Nice work on finding this.

Quoted 20 lines

builtin/fast-export.c +0 -0

         handle_tags_and_duplicates(&extra_refs);
         handle_tags_and_duplicates(&tag_refs);

t/t9350-fast-export.sh +9 -0

index 409b48e2442..df1a5b1013a 100755
@@ -750,4 +750,39 @@ test_expect_success 'merge commit gets exported with --import-marks' '
         )
  '
 
+
+test_expect_success 'fast-export --first-parent outputs all revisions output by revision walk' '
+       git init first-parent &&
+       (
+               cd first-parent &&
+               test_commit A &&
+               git checkout -b topic1 &&
+               test_commit B &&
+               git checkout main &&

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 767

Quoted 6 lines

t/t9350-fast-export.sh +6 -0

+               git merge topic1 --no-ff &&
+
+               git checkout -b topic2 &&
+               test_commit C &&
+               git checkout main &&
+               git merge topic2 --no-ff &&

micro nit: I'd really prefer the --no-ff before the "topic1" and "topic2".

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 771

Quoted 4 lines

t/t9350-fast-export.sh +4 -0

+
+               test_commit D &&
+
+               git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main >expected &&

I'd much prefer this were changed to

     git log --format="%ad %s" --first-parent >expected &&

because:
* --topo-order is irrelevant when you have a linear history (which
--first-parent gives you)
* --no-commit-header can be tossed by using log instead of rev-list
* %B provides the entire (multi-line) commit message body but you
clearly care about how many commits you saw below and were assuming just one line per commit, so %s is better.
* The format looks weird when inspecting as a human; it's much nicer
with a space between the %ad and %s.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 779

Quoted 8 lines

t/t9350-fast-export.sh +8 -0

+
+               git fast-export main -- --first-parent >first-parent-export &&
+               git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
+
+               git init import &&
+               git -C import fast-import <first-parent-export &&
+
+               git -C import rev-list --format="%ad%B" --topo-order --all --no-commit-header >actual &&

Same simplifications as above here:

    git -C import log --format="%ad %s" --topo-order --all >actual &&

However, is there a reason you're using "--all" instead of "main"? Although main is the only branch, which makes either output the same thing, it'd be easier for folks reading to catch the equivalence if it was clear you were now listing information about the same branch.

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 782

Quoted 3 lines

t/t9350-fast-export.sh +3 -0

+               git -C import rev-list --all >tmp &&
+
+               test_line_count = 4 tmp &&

I don't understand why you need tmp. Why not just use actual on the test_line_count line?

t/t9350-fast-export.sh test_expect_success 'merge commit gets exported with --import-marks' ' · line 784

Quoted 2 lines

t/t9350-fast-export.sh +2 -0

+               test_cmp expected actual &&
+               test_cmp first-parent-export first-parent-reverse-export

This last line would be much nicer to include right after first-parent-reverse-export is created. Or else move the creation of first-parent-reverse-export down to just above this line.

Quoted 4 lines

t/t9350-fast-export.sh +3 -0

+       )
+'
+
  test_done

Quoted 7 lines

However, given that it's unsafe to set revs.reverse=1 earlier, now that I think about it, isn't it also unsafe to set revs.topo_order where it is? Someone could override it by passing --date-order to fast-export. (It's okay if you want to leave fixing that to someone else, just thought I'd mention it while reviewing.)

I couldn't tell you for sure if the topo_order placement is safe. I at least don't see any place where topo_order itself can be toggled off in revision.c. I'm sure there exists at least one rev-list argument which will cause unexpected behaviour, though.

I agree that it would be nice to have the traversal order options be assigned in the same place. I guess we have three options:

1. Put the reverse assignment to the top (together with topo_order),
allowing the user to disable it with --reverse, which will cause odd behaviour.

2. Put the reverse assignment to the top and throw an error if the
user passes the --reverse option.

3. Keep the reverse assignment at the bottom, silently ignoring any
--reverse option.

I don't think any of the three options are particularly good. The first one for obvious reasons. The second seems inconsistent to me as we would only error on --reverse but not any of the other "nonsensical" rev-list args. However, silently ignoring certain arguments does also not make for a good user experience.

I think that it might be a good idea to move up the 'reverse' assignment and then add a paragraph to the man page for git-fast-export explaining that some arguments, in particular the ones that change the ordering of commits and the ones that change how commits are displayed (such as --graph), may have no or unexpected effects.

I've tried writing a snippet in git-fast-export.txt, which I could include in the next version, if you think it seems like a reasonable approach:

Documentation/git-fast-export.txt +0 -0

index 1978dbdc6a..34875ef01d 100644
@@ -157,16 +157,21 @@ by keeping the marks the same across runs.
 [<git-rev-list-args>...]::
        A list of arguments, acceptable to 'git rev-parse' and
        'git rev-list', that specifies the specific objects and references

Quoted 10 lines

Review

Quoted 8 lines

(quoted diff) +8 -0

+
+               git fast-export main -- --first-parent >first-parent-export &&
+               git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
+
+               git init import &&
+               git -C import fast-import <first-parent-export &&
+
+               git -C import rev-list --format="%ad%B" --topo-order --all --no-commit-header >actual &&

Same simplifications as above here:

Quoted 7 lines

    git -C import log --format="%ad %s" --topo-order --all >actual &&

However, is there a reason you're using "--all" instead of "main"? Although main is the only branch, which makes either output the same thing, it'd be easier for folks reading to catch the equivalence if it was clear you were now listing information about the same branch.

I guess the intent is to be completely sure that only four commits were exported, and no other refs made it into the new repository. I don't feel too strongly about it, but I think it is a slightly stronger test than leaving out the '--all'.

Quoted 13 lines · William Sprent

Quoted 7 lines

However, given that it's unsafe to set revs.reverse=1 earlier, now that I think about it, isn't it also unsafe to set revs.topo_order where it is? Someone could override it by passing --date-order to fast-export. (It's okay if you want to leave fixing that to someone else, just thought I'd mention it while reviewing.)

I couldn't tell you for sure if the topo_order placement is safe. I at least don't see any place where topo_order itself can be toggled off in revision.c. I'm sure there exists at least one rev-list argument which will cause unexpected behaviour, though.

Quoted 7 lines

I agree that it would be nice to have the traversal order options be assigned in the same place. I guess we have three options:

1. Put the reverse assignment to the top (together with topo_order),
allowing the user to disable it with --reverse, which will cause odd behaviour.

I'd call it broken rather than merely odd; more on this below.

Quoted 2 lines

2. Put the reverse assignment to the top and throw an error if the
user passes the --reverse option.

Might be a reasonable longer term solution if someone wants to dive into all the non-sensical options and mark them as such. But I agree that it's slightly odd only picking one specific one when we know there's likely a pile of them here.

Quoted 2 lines

3. Keep the reverse assignment at the bottom, silently ignoring any
--reverse option.

"silently ignored" or "dismissed with prejudice"? :-)

Quoted 11 lines

I don't think any of the three options are particularly good. The first one for obvious reasons. The second seems inconsistent to me as we would only error on --reverse but not any of the other "nonsensical" rev-list args. However, silently ignoring certain arguments does also not make for a good user experience.

I think that it might be a good idea to move up the 'reverse' assignment and then add a paragraph to the man page for git-fast-export explaining that some arguments, in particular the ones that change the ordering of commits and the ones that change how commits are displayed (such as --graph), may have no or unexpected effects.

I'd rather choose option #3, like builtin/add.c does with max_count. In part this is because...

Quoted 18 lines

I've tried writing a snippet in git-fast-export.txt, which I could include in the next version, if you think it seems like a reasonable approach:

Documentation/git-fast-export.txt +2 -0

index 1978dbdc6a..34875ef01d 100644
@@ -157,16 +157,21 @@ by keeping the marks the same across runs.
  [<git-rev-list-args>...]::
         A list of arguments, acceptable to 'git rev-parse' and
         'git rev-list', that specifies the specific objects and references
         to export.  For example, `master~10..master` causes the
         current master reference to be exported along with all objects
         added since its 10th ancestor commit and (unless the
         --reference-excluded-parents option is specified) all files
         common to master{tilde}9 and master{tilde}10.
++
+Arguments to `git rev-list` which change the _order_ in which commits are

Documentation/git-fast-export.txt by keeping the marks the same across runs. · line 169

Quoted 3 lines

Documentation/git-fast-export.txt +3 -0

+traversed, such as '--reverse', as well as arguments which control how commits
+are displayed, such as '--graph', may either have no effect or have an
+unexpected effect on which commits are exported.

After your patch, --reverse won't have an unexpected effect on _which_ commits are exported, it would instead have an unexpected effect on _how_ commits are exported (turning _every_ commit into a root commit). I'd rather just go with your option #3.

Quoted 18 lines

Quoted 17 lines

Review

Quoted 8 lines

(quoted diff) +8 -0

+
+               git fast-export main -- --first-parent >first-parent-export &&
+               git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
+
+               git init import &&
+               git -C import fast-import <first-parent-export &&
+
+               git -C import rev-list --format="%ad%B" --topo-order --all --no-commit-header >actual &&

Same simplifications as above here:

     git -C import log --format="%ad %s" --topo-order --all >actual &&

However, is there a reason you're using "--all" instead of "main"? Although main is the only branch, which makes either output the same thing, it'd be easier for folks reading to catch the equivalence if it was clear you were now listing information about the same branch.

Quoted 4 lines

I guess the intent is to be completely sure that only four commits were exported, and no other refs made it into the new repository. I don't feel too strongly about it, but I think it is a slightly stronger test than leaving out the '--all'.

Fair enough, '--all' works for me with that explanation.

From: William Sprent <[email protected]>

The revision traversal machinery typically processes and returns all children before any parent. fast-export needs to operate in the reverse fashion, handling parents before any of their children in order to build up the history starting from the root commit(s). This would be a clear case where we could just use the revision traversal machinery's "reverse" option to achieve this desired affect.

However, this wasn't what the code did. It added its own array for queuing. The obvious hand-rolled solution would be to just push all the commits into the array and then traverse afterwards, but it didn't quite do that either. It instead attempted to process anything it could as soon as it could, and once it could, check whether it could process anything that had been queued. As far as I can tell, this was an effort to save a little memory in the case of multiple root commits since it could process some commits before queueing all of them. This involved some helper functions named has_unshown_parent() and handle_tail(). For typical invocations of fast-export, this alternative essentially amounted to a hand-rolled method of reversing the commits -- it was a bunch of work to duplicate the revision traversal machinery's "reverse" option.

This hand-rolled reversing mechanism is actually somewhat difficult to reason about. It takes some time to figure out how it ensures in normal cases that it will actually process all traversed commits (rather than just dropping some and not printing anything for them).

And it turns out there are some cases where the code does drop commits without handling them, and not even printing an error or warning for the user. Due to the has_unshown_parent() checks, some commits could be left in the array at the end of the "while...get_revision()" loop which would be unprocessed. This could be triggered for example with

    git fast-export main -- --first-parent
or non-sensical traversal rules such as
    git fast-export main -- --grep=Merge --invert-grep

While most traversals that don't include all parents should likely trigger errors in fast-export (or at least require being used in combination with --reference-excluded-parents), the --first-parent traversal is at least reasonable and it'd be nice if it didn't just drop commits. It'd also be nice for future readers of the code to have a simpler "reverse traversal" mechanism. Use the "reverse" option of the revision traversal machinery to achieve both.

Even for the non-sensical traversal flags like the --grep one above, this would be an improvement. For example, in that case, the code previously would have silently truncated history to only those commits that do not have an ancestor containing "Merge" in their commit message. After this code change, that case would include all commits without "Merge" in their commit message -- but any commit that previously had a "Merge"-mentioning parent would lose that parent
(likely resulting in many new root commits). While the new behavior is still odd, it is at least understandable given that --reference-excluded-parents is not the default.

Signed-off-by William Sprent <[email protected]>

---
fast-export: fix surprising behavior with --first-parent

Hi,

Here's a revised version of my previously submitted patch regarding fast-export and how it reacts to the --first-parent flag.

Changes since v2:

* Changed commit message to include the detailed description of the problem area as suggested by Elijah. I went back and forth with my self about whether the message needs some "lead in", but it ended up getting long without adding much.

* Cleaned up test such that the comparison with the --reverse output is just below where it is generated.
* In test, moved all usages of --no-ff to just after 'git merge' .
* In test, replaced long 'git rev-list' incantation with simpler 'git log' format.

Changes since v1:

* Moved revs.reverse assignment down to similar assignments below.
* Removed braces around single statement while loop.
* The test now only changes directory inside a sub-shell.
* Applied stylistic feedback on test such as: making redirections be on the form >FILE etc.

There were questions raised about whether it makes sense at all for fast-export to simply accept all git rev-list arguments whether they have an effect or not - in particular for a flag like --reverse. I think I agree that it is questionable behavior, or at least questionably documented, but I also think it is out of scope for this change.

I did consider teaching fast-export to complain if given --reverse, but it seemed inconsistent to me as it will gladly accept every other rev-list argument (for example, "git fast-export HEAD --show-signature --graph" works just fine).

Original Message: I've noticed that git fast-export exhibits some odd behavior when passed the --first-parent flag. In the repository I was working on, it would only output the initial commit before exiting. Moreover, adding the --reverse flag causes it to behave differently and instead output all commits in the first parent line that only have one parent. My expectation is more or less that git fast-export should output the same set of commits as git rev-list would output given the same arguments, which matches how it acts when given revision ranges.

It seems like this behavior comes from the fact that has_unshown_parents will never be false for any merge commits encountered when fast-export is called with --first-parent. This causes the while loop to follow the pattern of pushing all commits into the "commits" queue until the initial commit is encountered, at which point it calls handle_tail which falls through on the first merge commit, causing most of the commits to be unhandled.

My impression is that this logic only serves to ensure that parents are processed before their children, so in my patch I've opted to fix the issue by delegating that responsibility to revision.c by adding the reverse flag before performing the revision walk. From what I can see, this should be equivalent to what the previous logic is trying to achieve, but I can also see that there could be risk in these changes.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1084%2Fwilliams-unity%2Ffast-export%2Ffirst-parent-issues-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1084/williams-unity/fast-export/first-parent-issues-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1084

Range-diff vs v2:

1: af552f37dbe ! 1: 312255260b1 fast-export: fix surprising behavior with --first-parent
@@ Metadata
## Commit message ##
fast-export: fix surprising behavior with --first-parent

- When invoking git-fast-export with the --first-parent flag on a branch
- with merges, fast-export would early-out on processing the first merge
- on the branch. If combined with --reverse, fast-export would instead
- output all single parent commits on the branch.
+ The revision traversal machinery typically processes and returns all
+ children before any parent. fast-export needs to operate in the
+ reverse fashion, handling parents before any of their children in
+ order to build up the history starting from the root commit(s). This
+ would be a clear case where we could just use the revision traversal
+ machinery's "reverse" option to achieve this desired affect.

- This commit makes fast-export output the same commits as rev-list
- --first-parent, and makes --reverse not have an effect on which commits
- are output.
+ However, this wasn't what the code did. It added its own array for
+ queuing. The obvious hand-rolled solution would be to just push all
+ the commits into the array and then traverse afterwards, but it didn't
+ quite do that either. It instead attempted to process anything it
+ could as soon as it could, and once it could, check whether it could
+ process anything that had been queued. As far as I can tell, this was
+ an effort to save a little memory in the case of multiple root commits
+ since it could process some commits before queueing all of them. This
+ involved some helper functions named has_unshown_parent() and
+ handle_tail(). For typical invocations of fast-export, this
+ alternative essentially amounted to a hand-rolled method of reversing
+ the commits -- it was a bunch of work to duplicate the revision
+ traversal machinery's "reverse" option.

- The fix involves removing logic within fast-export which was responsible
- for ensuring that parents are processed before their children, which was
- what was exiting early due to missing second parents. This is replaced
- by setting 'reverse = 1' before revision walking, which, in conjuction
- with topo_order, allows for delegating the ordering of commits to
- revision.c. The reverse flag is set after parsing rev-list arguments to
- avoid having it disabled.
+ This hand-rolled reversing mechanism is actually somewhat difficult to
+ reason about. It takes some time to figure out how it ensures in
+ normal cases that it will actually process all traversed commits
+ (rather than just dropping some and not printing anything for them).
+
+ And it turns out there are some cases where the code does drop commits
+ without handling them, and not even printing an error or warning for
+ the user. Due to the has_unshown_parent() checks, some commits could
+ be left in the array at the end of the "while...get_revision()" loop
+ which would be unprocessed. This could be triggered for example with
+ git fast-export main -- --first-parent
+ or non-sensical traversal rules such as
+ git fast-export main -- --grep=Merge --invert-grep
+
+ While most traversals that don't include all parents should likely
+ trigger errors in fast-export (or at least require being used in
+ combination with --reference-excluded-parents), the --first-parent
+ traversal is at least reasonable and it'd be nice if it didn't just drop
+ commits. It'd also be nice for future readers of the code to have a
+ simpler "reverse traversal" mechanism. Use the "reverse" option of the
+ revision traversal machinery to achieve both.
+
+ Even for the non-sensical traversal flags like the --grep one above,
+ this would be an improvement. For example, in that case, the code
+ previously would have silently truncated history to only those commits
+ that do not have an ancestor containing "Merge" in their commit message.
+ After this code change, that case would include all commits without
+ "Merge" in their commit message -- but any commit that previously had a
+ "Merge"-mentioning parent would lose that parent
+ (likely resulting in many new root commits). While the new behavior is
+ still odd, it is at least understandable given that
+ --reference-excluded-parents is not the default.

Signed-off-by William Sprent <[email protected]>

@@ t/t9350-fast-export.sh: test_expect_success 'merge commit gets exported with --i
+ git checkout -b topic1 &&
+ test_commit B &&
+ git checkout main &&
-+ git merge topic1 --no-ff &&
++ git merge --no-ff topic1 &&
+
+ git checkout -b topic2 &&
+ test_commit C &&
+ git checkout main &&
-+ git merge topic2 --no-ff &&
++ git merge --no-ff topic2 &&
+
+ test_commit D &&
+
-+ git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main >expected &&
-+
+ git fast-export main -- --first-parent >first-parent-export &&
+ git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
++ test_cmp first-parent-export first-parent-reverse-export &&
+
+ git init import &&
+ git -C import fast-import <first-parent-export &&
+
-+ git -C import rev-list --format="%ad%B" --topo-order --all --no-commit-header >actual &&
-+ git -C import rev-list --all >tmp &&
-+
-+ test_line_count = 4 tmp &&
++ git log --format="%ad %s" --first-parent main >expected &&
++ git -C import log --format="%ad %s" --all >actual &&
+ test_cmp expected actual &&
-+ test_cmp first-parent-export first-parent-reverse-export
++ test_line_count = 4 actual
+ )
+'
+

 builtin/fast-export.c  | 40 ++++------------------------------------
 t/t9350-fast-export.sh | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 36 deletions(-)

builtin/fast-export.c +4 -36

index 8e2caf72819..cb1d6473f12 100644
@@ -107,18 +107,6 @@ static int parse_opt_reencode_mode(const struct option *opt,
 
 static struct decoration idnums;
 static uint32_t last_idnum;
-
-static int has_unshown_parent(struct commit *commit)
-{
-       struct commit_list *parent;
-
-       for (parent = commit->parents; parent; parent = parent->next)
-               if (!(parent->item->object.flags & SHOWN) &&
-                   !(parent->item->object.flags & UNINTERESTING))
-                       return 1;
-       return 0;
-}
-
 struct anonymized_entry {
        struct hashmap_entry hash;
        const char *anon;

@@ -752,20 +740,6 @@ static char *anonymize_tag(void *data)
        return strbuf_detach(&out, NULL);
 }
 
-static void handle_tail(struct object_array *commits, struct rev_info *revs,
-                       struct string_list *paths_of_changed_objects)
-{
-       struct commit *commit;
-       while (commits->nr) {
-               commit = (struct commit *)object_array_pop(commits);
-               if (has_unshown_parent(commit)) {
-                       /* Queue again, to be handled later */
-                       add_object_array(&commit->object, NULL, commits);
-                       return;
-               }
-               handle_commit(commit, revs, paths_of_changed_objects);
-       }
-}
 
 static void handle_tag(const char *name, struct tag *tag)
 {

@@ -1185,7 +1159,6 @@ static int parse_opt_anonymize_map(const struct option *opt,
 int cmd_fast_export(int argc, const char **argv, const char *prefix)
 {
        struct rev_info revs;
-       struct object_array commits = OBJECT_ARRAY_INIT;
        struct commit *commit;
        char *export_filename = NULL,
             *import_filename = NULL,

@@ -1283,18 +1256,13 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
        if (prepare_revision_walk(&revs))
                die("revision walk setup failed");
+
+       revs.reverse = 1;
        revs.diffopt.format_callback = show_filemodify;
        revs.diffopt.format_callback_data = &paths_of_changed_objects;
        revs.diffopt.flags.recursive = 1;
-       while ((commit = get_revision(&revs))) {
-               if (has_unshown_parent(commit)) {
-                       add_object_array(&commit->object, NULL, &commits);
-               }
-               else {
-                       handle_commit(commit, &revs, &paths_of_changed_objects);
-                       handle_tail(&commits, &revs, &paths_of_changed_objects);
-               }
-       }
+       while ((commit = get_revision(&revs)))
+               handle_commit(commit, &revs, &paths_of_changed_objects);
 
        handle_tags_and_duplicates(&extra_refs);
        handle_tags_and_duplicates(&tag_refs);

t/t9350-fast-export.sh +32 -0

index 409b48e2442..7b7a18dd2c1 100755
@@ -750,4 +750,36 @@ test_expect_success 'merge commit gets exported with --import-marks' '
        )
 '
 
+
+test_expect_success 'fast-export --first-parent outputs all revisions output by revision walk' '
+       git init first-parent &&
+       (
+               cd first-parent &&
+               test_commit A &&
+               git checkout -b topic1 &&
+               test_commit B &&
+               git checkout main &&
+               git merge --no-ff topic1 &&
+
+               git checkout -b topic2 &&
+               test_commit C &&
+               git checkout main &&
+               git merge --no-ff topic2 &&
+
+               test_commit D &&
+
+               git fast-export main -- --first-parent >first-parent-export &&
+               git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
+               test_cmp first-parent-export first-parent-reverse-export &&
+
+               git init import &&
+               git -C import fast-import <first-parent-export &&
+
+               git log --format="%ad %s" --first-parent main >expected &&
+               git -C import log --format="%ad %s" --all >actual &&
+               test_cmp expected actual &&
+               test_line_count = 4 actual
+       )
+'
+
 test_done

base-commit: cd3e606211bb1cf8bc57f7d76bab98cc17a150bc

Quoted 4 lines

* Changed commit message to include the detailed description of the problem area as suggested by Elijah. I went back and forth with my self about whether the message needs some "lead in", but it ended up getting long without adding much.

If it feels weird quoting someone else so extensively and just using their words for the commit message, we've used a "Commit-message-by:" trailer in the past; that could be useful here.

This version looks good to me:

Reviewed-by Elijah Newren <[email protected]>

Quoted 8 lines · Elijah Newren

Quoted 4 lines

* Changed commit message to include the detailed description of the problem area as suggested by Elijah. I went back and forth with my self about whether the message needs some "lead in", but it ended up getting long without adding much.

If it feels weird quoting someone else so extensively and just using their words for the commit message, we've used a "Commit-message-by:" trailer in the past; that could be useful here.

I do not mind tweaking the copy I already have with "Helped-by" myself, while ...

Quoted 3 lines

This version looks good to me:

Reviewed-by Elijah Newren <[email protected]>

... adding this one before merging the result to 'next'.

Thanks, both.