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2020-09-30combine-diff: handle --find-object in multitree code pathLibravatar Jeff King1-0/+55
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>
2018-01-04diffcore: add a pickaxe option to find a specific blobLibravatar Stefan Beller1-0/+68
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>