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describe: don't abort too early when searching tags 

When searching the commit graph for tag candidates, `git-describe`
will stop as soon as there is only one active branch left and
it already found an annotated tag as a candidate.

This works well as long as all branches eventually connect back
to a common root, but if the tags are found across branches
with no common ancestor

                  B
                  o----.
                        \
          o-----o---o----x
          A

it can happen that the search on one branch terminates prematurely
because a tag was found on another, independent branch. This scenario
isn't quite as obscure as it sounds, since cloning with a limited
depth often introduces many independent "dead ends" into the commit
graph.

The help text of `git-describe` states pretty clearly that when
describing a commit D, the number appended to the emitted tag X should
correspond to the number of commits found by `git log X..D`.

Thus, this commit modifies the stopping condition to only abort
the search when only one branch is left to search *and* all current
best candidates are descendants from that branch.

For repositories with a single root, this condition is always
true: When the search is reduced to a single active branch, the
current commit must be an ancestor of *all* tag candidates. This
means that in the common case, this change will have no negative
performance impact since the same number of commits as before will
be traversed.

Signed-off-by: Benno Evers <benno@bmevers.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Benno Evers 2020-02-26 18:48:53 +01:00 committed by Junio C Hamano
parent 2d2118b814
commit 30b1c7ad9d
2 changed files with 69 additions and 4 deletions
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22
builtin/describe.c
View File

@ -376,11 +376,25 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
if (!(c->object.flags & t->flag_within))
t->depth++;
}
/* Stop if last remaining path already covered by best candidate(s) */
if (annotated_cnt && !list) {
if (debug)
fprintf(stderr, _("finished search at %s\n"),
oid_to_hex(&c->object.oid));
break;
int best_depth = INT_MAX;
unsigned best_within = 0;
for (cur_match = 0; cur_match < match_cnt; cur_match++) {
struct possible_tag *t = &all_matches[cur_match];
if (t->depth < best_depth) {
best_depth = t->depth;
best_within = t->flag_within;
} else if (t->depth == best_depth) {
best_within |= t->flag_within;
}
}
if ((c->object.flags & best_within) == best_within) {
if (debug)
fprintf(stderr, _("finished search at %s\n"),
oid_to_hex(&c->object.oid));
break;
}
}
while (parents) {
struct commit *p = parents->item;

51
t/t6120-describe.sh
View File

@ -479,4 +479,55 @@ test_expect_success 'name-rev covers all conditions while looking at parents' '
)
'
# B
# o
# \
# o-----o---o----x
# A
#
test_expect_success 'describe commits with disjoint bases' '
git init disjoint1 &&
(
cd disjoint1 &&
echo o >> file && git add file && git commit -m o &&
echo A >> file && git add file && git commit -m A &&
git tag A -a -m A &&
echo o >> file && git add file && git commit -m o &&
git checkout --orphan branch && rm file &&
echo B > file2 && git add file2 && git commit -m B &&
git tag B -a -m B &&
git merge --no-ff --allow-unrelated-histories master -m x &&
check_describe "A-3-*" HEAD
)
'
# B
# o---o---o------------.
# \
# o---o---x
# A
#
test_expect_success 'describe commits with disjoint bases 2' '
git init disjoint2 &&
(
cd disjoint2 &&
echo A >> file && git add file && GIT_COMMITTER_DATE="2020-01-01 18:00" git commit -m A &&
git tag A -a -m A &&
echo o >> file && git add file && GIT_COMMITTER_DATE="2020-01-01 18:01" git commit -m o &&
git checkout --orphan branch &&
echo o >> file2 && git add file2 && GIT_COMMITTER_DATE="2020-01-01 15:00" git commit -m o &&
echo o >> file2 && git add file2 && GIT_COMMITTER_DATE="2020-01-01 15:01" git commit -m o &&
echo B >> file2 && git add file2 && GIT_COMMITTER_DATE="2020-01-01 15:02" git commit -m B &&
git tag B -a -m B &&
git merge --no-ff --allow-unrelated-histories master -m x &&
check_describe "B-3-*" HEAD
)
'
test_done