The can-working-tree-updates-be-skipped check has had a long and blemished
history. The update can be skipped iff:
a) The merge is clean
b) The merge matches what was in HEAD (content, mode, pathname)
c) The target path is usable (i.e. not involved in D/F conflict)
Traditionally, we split b into parts:
b1) The merged result matches the content and mode found in HEAD
b2) The merged target path existed in HEAD
Steps a & b1 are easy to check; we have always gotten those right. While
it is easy to overlook step c, this was fixed seven years ago with commit
4ab9a157d0 ("merge_content(): Check whether D/F conflicts are still
present", 2010-09-20). merge-recursive didn't have a readily available
way to directly check step b2, so various approximations were used:
* In commit b2c8c0a762 ("merge-recursive: When we detect we can skip
an update, actually skip it", 2011-02-28), it was noted that although
the code claimed it was skipping the update, it did not actually skip
the update. The code was made to skip it, but used lstat(path, ...)
as an approximation to path-was-tracked-in-index-before-merge.
* In commit 5b448b8530 ("merge-recursive: When we detect we can skip
an update, actually skip it", 2011-08-11), the problem with using
lstat was noted. It was changed to the approximation
path2 && strcmp(path, path2)
which is also wrong. !path2 || strcmp(path, path2) would have been
better, but would have fallen short with directory renames.
* In c5b761fb27 ("merge-recursive: ensure we write updates for
directory-renamed file", 2018-02-14), the problem with the previous
approximation was noted and changed to
was_tracked(path)
That looks close to what we were trying to answer, but was_tracked()
as implemented at the time should have been named is_tracked(); it
returned something different than what we were looking for.
* To make matters more complex, fixing was_tracked() isn't sufficient
because the splitting of b into b1 and b2 is wrong. Consider the
following merge with a rename/add conflict:
side A: modify foo, add unrelated bar
side B: rename foo->bar (but don't modify the mode or contents)
In this case, the three-way merge of original foo, A's foo, and B's
bar will result in a desired pathname of bar with the same
mode/contents that A had for foo. Thus, A had the right mode and
contents for the file, and it had the right pathname present (namely,
bar), but the bar that was present was unrelated to the contents, so
the working tree update was not skippable.
Fix this by introducing a new function:
was_tracked_and_matches(o, path, &mfi.oid, mfi.mode)
and use it to directly check for condition b.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add several tests checking whether updates can be skipped in a merge.
Also add several similar testcases for where updates cannot be skipped in
a merge to make sure that we skip if and only if we should.
In particular:
* Testcase 1a (particularly 1a-check-L) would have pointed out the
problem Linus has been dealing with for year with his merges[1].
* Testcase 2a (particularly 2a-check-L) would have pointed out the
problem with my directory-rename-series before it broke master[2].
* Testcases 3[ab] (particularly 3a-check-L) provide a simpler testcase
than 12b of t6043 making that one easier to understand.
* There are several complementary testcases to make sure we're not just
fixing those particular issues while regressing in the opposite
direction.
* There are also a pair of tests for the special case when a merge
results in a skippable update AND the user has dirty modifications to
the path.
[1] https://public-inbox.org/git/CA+55aFzLZ3UkG5svqZwSnhNk75=fXJRkvU1m_RHBG54NOoaZPA@mail.gmail.com/
[2] https://public-inbox.org/git/xmqqmuya43cs.fsf@gitster-ct.c.googlers.com/
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
