commit-reach: fix in_merge_bases_many bug

From: Derrick Stolee <[email protected]>

Way back in f9b8908b (commit.c: use generation numbers for in_merge_bases(), 2018-05-01), a heuristic was used to short-circuit the in_merge_bases() walk. This works just fine as long as the caller is checking only two commits, but when there are multiple, there is a possibility that this heuristic is _very wrong_.

Some code moves since then has changed this method to repo_in_merge_bases_many() inside commit-reach.c. The heuristic computes the minimum generation number of the "reference" list, then compares this number to the generation number of the "commit".

In a recent topic, a test was added that used in_merge_bases_many() to test if a commit was reachable from a number of commits pulled from a reflog. However, this highlighted the problem: if any of the reference commits have a smaller generation number than the given commit, then the walk is skipped _even if there exist some with higher generation number_.

This heuristic is wrong! It must check the MAXIMUM generation number of the reference commits, not the MINIMUM.

Fix in_merge_bases_many() with commit-graphs

Johannes alerted me to the difficulties Srinidhi was having with in_merge_bases_many() and commit-graphs. Sorry that I hadn't seen that thread and the issues therein.

After working with Johannes to investigate what was happening, we found a 2-year-old bug in the generation number checks!

Thanks, -Stolee

The fix itself is to swap min_generation with a max_generation in repo_in_merge_bases_many().

Helped-by Johannes Schindelin <[email protected]>

Reported-by Srinidhi Kaushik <[email protected]>

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

 commit-reach.c        |  8 ++++----
 t/helper/test-reach.c |  2 ++
 t/t6600-test-reach.sh | 30 ++++++++++++++++++++++++++++++
 3 files changed, 36 insertions(+), 4 deletions(-)

Quoted 3 lines · Derrick Stolee via GitGitGadget

The fix itself is to swap min_generation with a max_generation in repo_in_merge_bases_many().

Quoted 3 lines

Helped-by Johannes Schindelin <[email protected]>

Reported-by Srinidhi Kaushik <[email protected]>

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

The order of these looked iffy so I rearranged them in chrono order, Srinidhi's series found it broken, and with Dscho's help, fix was produced and you sent it with your sign off.

Quoted 4 lines

 commit-reach.c        |  8 ++++----
 t/helper/test-reach.c |  2 ++
 t/t6600-test-reach.sh | 30 ++++++++++++++++++++++++++++++
 3 files changed, 36 insertions(+), 4 deletions(-)

I've applied this and then rebased Srinidhi's latest on top, with the following to re-enable the commit-graph in the codepath. t5533 and t6600 passes and when the fix in the message I am responding to is reverted, t5533 again fails.

Thanks.

Thanks for double-checking. I also think that dropping the "hide the error" patch is prudent.

Quoted 2 lines · Derrick Stolee

Thanks for double-checking. I also think that dropping the "hide the error" patch is prudent.

Thanks again for a quick and straight-forward fix. As I mentioned elsewhere in the thread, it appears that we invented duplicate API with parallel implementation in get_reachable_subset(), which seems to be a strict superset of in_merge_bases_many(), and that may be what led to an initial and incorrect "get_reachable_subset() is not broken the same way as in_merge_bases_many() so use it instead" response. I haven't paid attention to the quality of implementation of get_reachable_subset() as much as in_merge_bases_many() (e.g. I know of an obvious way to optimize the latter) so far, but it would be wonderful if we can eventually rewrite in_merge_bases_many() as a very thin wrapper() around get_reachable_subset() without any downside.

From: Derrick Stolee <[email protected]>

Way back in f9b8908b (commit.c: use generation numbers for in_merge_bases(), 2018-05-01), a heuristic was used to short-circuit the in_merge_bases() walk. This works just fine as long as the caller is checking only two commits, but when there are multiple, there is a possibility that this heuristic is _very wrong_.

Some code moves since then has changed this method to repo_in_merge_bases_many() inside commit-reach.c. The heuristic computes the minimum generation number of the "reference" list, then compares this number to the generation number of the "commit".

In a recent topic, a test was added that used in_merge_bases_many() to test if a commit was reachable from a number of commits pulled from a reflog. However, this highlighted the problem: if any of the reference commits have a smaller generation number than the given commit, then the walk is skipped _even if there exist some with higher generation number_.

This heuristic is wrong! It must check the MAXIMUM generation number of the reference commits, not the MINIMUM.

This highlights a testing gap. t6600-test-reach.sh covers many methods in commit-reach.c, including in_merge_bases() and get_merge_bases_many(), but since these methods either restrict to two input commits or actually look for the full list of merge bases, they don't check this heuristic!

Add a possible input to "test-tool reach" that tests in_merge_bases_many() and add tests to t6600-test-reach.sh that cover this heuristic. This includes cases for the reference commits having generation above and below the generation of the input commit, but also having maximum generation below the generation of the input commit.

The fix itself is to swap min_generation with a max_generation in repo_in_merge_bases_many().

Helped-by Johannes Schindelin <[email protected]>

Reported-by Srinidhi Kaushik <[email protected]>

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

---
Fix in_merge_bases_many() with commit-graphs

Johannes alerted me to the difficulties Srinidhi was having with in_merge_bases_many() and commit-graphs. Sorry that I hadn't seen that thread and the issues therein.

After working with Johannes to investigate what was happening, we found a 2-year-old bug in the generation number checks!

Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-739%2Fderrickstolee%2Fin-merge-bases-many-fix-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-739/derrickstolee/in-merge-bases-many-fix-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/739

 commit-reach.c        |  8 ++++----
 t/helper/test-reach.c |  2 ++
 t/t6600-test-reach.sh | 30 ++++++++++++++++++++++++++++++
 3 files changed, 36 insertions(+), 4 deletions(-)

commit-reach.c +1 -1

index efd5925cbb..50175b159e 100644
@@ -321,7 +321,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 {
 	struct commit_list *bases;
 	int ret = 0, i;
-	uint32_t generation, min_generation = GENERATION_NUMBER_INFINITY;
+	uint32_t generation, max_generation = GENERATION_NUMBER_ZERO;
 
 	if (repo_parse_commit(r, commit))
 		return ret;

@@ -330,12 +330,12 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 			return ret;
 
 		generation = commit_graph_generation(reference[i]);

commit-reach.c +0 -0

 	bases = paint_down_to_common(r, commit,

t/helper/test-reach.c +2 -0

index 14a3655442..cda804ed79 100644
@@ -107,6 +107,8 @@ int cmd__reach(int ac, const char **av)
 		printf("%s(A,B):%d\n", av[1], ref_newer(&oid_A, &oid_B));
 	else if (!strcmp(av[1], "in_merge_bases"))
 		printf("%s(A,B):%d\n", av[1], in_merge_bases(A, B));
+	else if (!strcmp(av[1], "in_merge_bases_many"))
+		printf("%s(A,X):%d\n", av[1], in_merge_bases_many(A, X_nr, X_array));
 	else if (!strcmp(av[1], "is_descendant_of"))
 		printf("%s(A,X):%d\n", av[1], repo_is_descendant_of(r, A, X));
 	else if (!strcmp(av[1], "get_merge_bases_many")) {

t/t6600-test-reach.sh +30 -0

index 475564bee7..f807276337 100755
@@ -110,6 +110,36 @@ test_expect_success 'in_merge_bases:miss' '
 	test_three_modes in_merge_bases
 '
 
+test_expect_success 'in_merge_bases_many:hit' '
+	cat >input <<-\EOF &&
+	A:commit-6-8
+	X:commit-6-9
+	X:commit-5-7
+	EOF
+	echo "in_merge_bases_many(A,X):1" >expect &&
+	test_three_modes in_merge_bases_many
+'
+
+test_expect_success 'in_merge_bases_many:miss' '
+	cat >input <<-\EOF &&
+	A:commit-6-8
+	X:commit-7-7
+	X:commit-8-6
+	EOF
+	echo "in_merge_bases_many(A,X):0" >expect &&
+	test_three_modes in_merge_bases_many
+'
+
+test_expect_success 'in_merge_bases_many:miss-heuristic' '
+	cat >input <<-\EOF &&
+	A:commit-6-8
+	X:commit-7-5
+	X:commit-6-6
+	EOF
+	echo "in_merge_bases_many(A,X):0" >expect &&
+	test_three_modes in_merge_bases_many
+'
+
 test_expect_success 'is_descendant_of:hit' '
 	cat >input <<-\EOF &&
 	A:commit-5-7

base-commit: 47ae905ffb98cc4d4fd90083da6bc8dab55d9ecc

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