When doing combined diffs, we have two possible code paths:
- a slower one which independently diffs against each parent, applies
any filters, and then intersects the resulting paths
- a faster one which walks all trees simultaneously
When the diff options specify that we must do certain filters, like
pickaxe, then we always use the slow path, since the pickaxe code only
knows how to handle filepairs, not the n-parent entries generated for
combined diffs.
But there are two problems with the slow path:
1. It's slow. Running:
git rev-list HEAD | git diff-tree --stdin -r -c
in git.git takes ~3s on my machine. But adding "--find-object" to
that increases it to ~6s, even though find-object itself should
incur only a few extra oid comparisons. On linux.git, it's even
worse: 35s versus 215s.
2. It doesn't catch all cases where a particular path is interesting.
Consider a merge with parent blobs X and Y for a particular path,
and end result Z. That should be interesting according to "-c",
because the result doesn't match either parent. And it should be
interesting even with "--find-object=X", because "X" went away in
the merge.
But because we perform each pairwise diff independently, this
confuses the intersection code. The change from X to Z is still
interesting according to --find-object. But in the other parent we
went from Y to Z, so the diff appears empty! That causes the
intersection code to think that parent didn't change the path, and
thus it's not interesting for "-c".
This patch fixes both by implementing --find-object for the multitree
code. It's a bit unfortunate that we have to duplicate some logic from
diffcore-pickaxe, but this is the best we can do for now. In an ideal
world, all of the diffcore code would stop thinking about filepairs and
start thinking about n-parent sets, and we could use the multitree walk
with all of it.
Until then, there are some leftover warts:
- other pickaxe operations, like -S or -G, still suffer from both
problems. These would be hard to adapt because they rely on having
a diff_filespec() for each path to look at content. And we'd need to
define what an n-way "change" means in each case (probably easy for
"-S", which can compare counts, but not so clear for -G, which is
about grepping diffs).
- other options besides --find-object may cause us to use the slow
pairwise path, in which case we'll go back to producing a different
(wrong) answer for the X/Y/Z case above.
We may be able to hack around these, but I think the ultimate solution
will be a larger rewrite of the diffcore code. For now, this patch
improves one specific case but leaves the rest.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Sometimes users are given a hash of an object and they want to
identify it further (ex.: Use verify-pack to find the largest blobs,
but what are these? or [1])
One might be tempted to extend git-describe to also work with blobs,
such that `git describe <blob-id>` gives a description as
'<commit-ish>:<path>'. This was implemented at [2]; as seen by the sheer
number of responses (>110), it turns out this is tricky to get right.
The hard part to get right is picking the correct 'commit-ish' as that
could be the commit that (re-)introduced the blob or the blob that
removed the blob; the blob could exist in different branches.
Junio hinted at a different approach of solving this problem, which this
patch implements. Teach the diff machinery another flag for restricting
the information to what is shown. For example:
$ ./git log --oneline --find-object=v2.0.0:Makefile
b2feb64309 Revert the whole "ask curl-config" topic for now
47fbfded53 i18n: only extract comments marked with "TRANSLATORS:"
we observe that the Makefile as shipped with 2.0 was appeared in
v1.9.2-471-g47fbfded53 and in v2.0.0-rc1-5-gb2feb6430b. The
reason why these commits both occur prior to v2.0.0 are evil
merges that are not found using this new mechanism.
[1] https://stackoverflow.com/questions/223678/which-commit-has-this-blob
[2] https://public-inbox.org/git/20171028004419.10139-1-sbeller@google.com/
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
