[PATCH v2] fast-export: fix surprising behavior with --first-parent

From: William Sprent <williams@unity3d.com>

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 <williams@unity3d.com>
---
    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 avarab@gmail.com

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1084%2Fwilliams-unity%2Ffast-export%2Ffirst-parent-issues-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1084/williams-unity/fast-export/first-parent-issues-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1084

Range-diff vs v1:

 1:  e2824193a8f ! 1:  af552f37dbe fast-export: fix surprising behavior with --first-parent
     @@ builtin/fast-export.c: static int parse_opt_anonymize_map(const struct option *o
       	     *import_filename = NULL,
      @@ builtin/fast-export.c: 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.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))) {
     +-	while ((commit = get_revision(&revs))) {
      -		if (has_unshown_parent(commit)) {
      -			add_object_array(&commit->object, NULL, &commits);
      -		}
     @@ builtin/fast-export.c: int cmd_fast_export(int argc, const char **argv, const ch
      -			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 ##
      @@ t/t9350-fast-export.sh: test_expect_success 'merge commit gets exported with --import-marks' '
     @@ t/t9350-fast-export.sh: test_expect_success 'merge commit gets exported with --i
      +
      +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 &&
     ++	(
     ++		cd first-parent &&
     ++		test_commit A &&
     ++		git checkout -b topic1 &&
     ++		test_commit B &&
     ++		git checkout main &&
     ++		git merge topic1 --no-ff &&
     ++
     ++		git checkout -b topic2 &&
     ++		test_commit C &&
     ++		git checkout main &&
     ++		git merge topic2 --no-ff &&
     ++
     ++		test_commit D &&
      +
     -+	git checkout -b topic2 &&
     -+	test_commit file3 file3.txt &&
     -+	git checkout main &&
     -+	git merge topic2 --no-ff &&
     ++		git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main >expected &&
      +
     -+	test_commit branch-head &&
     ++		git fast-export main -- --first-parent >first-parent-export &&
     ++		git fast-export main -- --first-parent --reverse >first-parent-reverse-export &&
      +
     -+	git rev-list --format="%ad%B" --first-parent --topo-order --no-commit-header main > expected &&
     ++		git init import &&
     ++		git -C import fast-import <first-parent-export &&
      +
     -+	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 -C import rev-list --format="%ad%B" --topo-order --all --no-commit-header >actual &&
     ++		git -C import rev-list --all >tmp &&
      +
     -+	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_line_count = 4 tmp &&
     ++		test_cmp expected actual &&
     ++		test_cmp first-parent-export first-parent-reverse-export
     ++	)
      +'
      +
       test_done

 builtin/fast-export.c  | 40 ++++------------------------------------
 t/t9350-fast-export.sh | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 36 deletions(-)