Bug report - git rev-list --exclude-first-parent-only [SEC=UNOFFICIAL]

Bug report - git rev-list --exclude-first-parent-only [SEC=UNOFFICIAL]

[Michael Hore]

I believe I have found a bug -

My repo has a commit structure like

R2
|\
| F
|/
R1

i.e.
 - there is a merge commit R2 with parents R1 and F
 - the parent of F is R1

I ran "git rev-list --exclude-first-parent-only F ^R2"

it gave the expected result: "F"

I ran "git rev-list --exclude-first-parent-only F R1 ^R2"

I expected the same result, but I got an unexpected result - nothing at all

Suspected cause - I had a look at the code, and it looks like process_parents() in revision.c, when processing uninteresting flags, will skip the 1st parent and mark the 2nd parent as uninteresting if the 1st parent is already SEEN, even with the flag exclude-first-parent-only. I think maybe explicitly selecting R1 on the command line causes it to be marked SEEN before ^R2 is processed, thus resulting in F being marked uninteresting.

[System Info]
git version:
git version 2.54.0.windows.1
cpu: x86_64
built from commit: 2b8a3ab140826ac423c2845ef81d4c6ac4f7bf3c
sizeof-long: 4
sizeof-size_t: 8
shell-path: D:/git-sdk-64-build-installers/usr/bin/sh
rust: disabled
feature: fsmonitor--daemon
gettext: enabled
SHA-1: SHA1_DC
SHA-256: SHA256_BLK
default-ref-format: files
default-hash: sha1


[Enabled Hooks]

Regards,
Michael

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Re: Bug report - git rev-list --exclude-first-parent-only [SEC=UNOFFICIAL]

[Junio C Hamano]

Michael Hore <Michael.Hore@asic.gov.au> writes:

> I believe I have found a bug -
>
> My repo has a commit structure like
>
> R2
> |\
> | F
> |/
> R1
>
> i.e.
>  - there is a merge commit R2 with parents R1 and F
>  - the parent of F is R1

IOW, R2 is a useless merge that could have been a simple
fast-forward directly to F.

> I ran "git rev-list --exclude-first-parent-only F ^R2"
>
> it gave the expected result: "F"
>
> I ran "git rev-list --exclude-first-parent-only F R1 ^R2"
>
> I expected the same result, but I got an unexpected result - nothing at all

This seems to have come from 9d505b7b49 (git-rev-list: add
--exclude-first-parent-only flag, 2022-01-11).  I do not know if the
original author is still around, but it would have been nicer to ask
for input from them (cc'ed).

A fix could be something along this line, but I've never used this
feature even once (I instead use Michael Haggerty's exellent "git
when-merged" thing), so I may very well be breaking _other_ use
cases this feature was originally intended for without knowing.

The patched part is inside a huge "while (parent)" loop.  The idea
is to break out before the loop goes on to smudge later parents when
we are in the "smudge only first parent as uninteresting, without
contaminating the history leading to other parents" mode.

 revision.c                   | 10 ++++++++--
 t/t6012-rev-list-simplify.sh | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git c/revision.c w/revision.c
index e91d7e1f11..1f50d42a7a 100644
--- c/revision.c
+++ w/revision.c
@@ -1151,12 +1151,18 @@ static int process_parents(struct rev_info *revs, struct commit *commit,
 			if (p)
 				p->object.flags |= UNINTERESTING |
 						   CHILD_VISITED;
-			if (repo_parse_commit_gently(revs->repo, p, 1) < 0)
+			if (repo_parse_commit_gently(revs->repo, p, 1) < 0) {
+				if (revs->exclude_first_parent_only)
+					break;
 				continue;
+			}
 			if (p->parents)
 				mark_parents_uninteresting(revs, p);
-			if (p->object.flags & SEEN)
+			if (p->object.flags & SEEN) {
+				if (revs->exclude_first_parent_only)
+					break;
 				continue;
+			}
 			p->object.flags |= (SEEN | NOT_USER_GIVEN);
 			if (queue)
 				prio_queue_put(queue, p);
diff --git c/t/t6012-rev-list-simplify.sh w/t/t6012-rev-list-simplify.sh
index 4cecb6224c..2284bbba12 100755
--- c/t/t6012-rev-list-simplify.sh
+++ w/t/t6012-rev-list-simplify.sh
@@ -285,4 +285,22 @@ test_expect_success 'log --graph --simplify-merges --show-pulls' '
 	test_cmp expect actual
 '
 
+test_expect_success 'exclude-first-parent-only with parent already seen' '
+	git checkout --orphan test-seen &&
+	git rm -rf . &&
+	test_commit r1 &&
+	git checkout -b branch-f &&
+	test_commit f &&
+	git checkout test-seen &&
+	git merge --no-ff --no-edit -m r2 branch-f &&
+	git tag r2 &&
+
+	git rev-list --exclude-first-parent-only f ^r2 >actual &&
+	git rev-parse f >expect &&
+	test_cmp expect actual &&
+
+	git rev-list --exclude-first-parent-only f r1 ^r2 >actual2 &&
+	test_cmp expect actual2
+'
+
 test_done