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From: Taylor Blau <me@ttaylorr.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Derrick Stolee <stolee@gmail.com>,
	Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org, me@ttaylorr.com, jnareb@gmail.com,
	garimasigit@gmail.com, Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH 1/3] revision: complicated pathspecs disable filters
Date: Wed, 15 Apr 2020 18:56:40 -0600	[thread overview]
Message-ID: <20200416005640.GC36156@syl.local> (raw)
In-Reply-To: <xmqq8siw5tlv.fsf@gitster.c.googlers.com>

On Wed, Apr 15, 2020 at 02:25:48PM -0700, Junio C Hamano wrote:
> Derrick Stolee <stolee@gmail.com> writes:
>
> > THIS IS A BREAKING CHANGE. Commit-graph files with changed-path
> > Bloom filters computed by a previous commit will not be compatible
> > with the filters computed in this commit, nor will we get correct
> > results when testing across these incompatible versions. Normally,
> > this would be a completely unacceptable change, but the filters
> > have not been released and hence are still possible to update
> > before release.
>
> Sure, it hasn't even hit 'next' yet.
>
> But I think we are both sort-of leaning towards concluding that it
> does not help all that much.  So I think it is OK.

Yeah, I think that the only thing that it is potentially helping is the
:(icase) magic. In a repository that doesn't have colliding paths in
case-insensitive filesystems (i.e., having both 'foo' and 'FOO' in the
same tree), this doesn't seem to be obviously hurting anything.

But, it is at least semi-harmful to repositories that do have such
trees, since we now can't answer "probably yes" or "definitely not" for
colliding paths unless both produce the same fingerprint. That seems
like a clear downside.

Now, how common is that against people who would benefit from
changed-path Bloom filters in their commit-graphs? I don't know one way
or the other. But, the upside seems to be minimal compared to the
potential cost, so I think that it may be better to just leave this one
alone.

> > TODO: If we decide to move in this direction, then the following
> > steps should be done (and some of them should be done anyway):
>
> Even if we decide not to do this "downcase before hashing" thing, we
> should document how exactly we compute, I think.
>
> And if we decide do change our mind later, it is not the end of the
> world.  We should be able to use a different chunk type to store the
> filters computed over downcased paths.
>
> > * We need to document the Bloom filter format to specify exactly
> >   how we compute the filter data. The details should be careful
> >   enough that someone can reproduce the exact file format without
> >   looking at the C code.
> >
> > * That document would include the tolower() transformation that is
> >   being done here.
>
> As the tree-diff comparison done internally while running "git
> blame" does not take an end-user specified pathspec in any
> meaningful way, this does not matter in practice, but there is
> another possible complication we would want to consider when we
> extend the support to "git log" and friends---negative pathspecs
> (e.g. "git log ':(exclude)COPYING'").  A commit that cannot possibly
> have touched the COPYING file would be eligible for output without
> actually running tree-diff between it and its parent (as long as the
> trees of the two commits are different, we know it must be shown).
>
> Thanks.

Thanks,
Taylor

  reply	other threads:[~2020-04-16  0:57 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-11  1:02 [PATCH 0/3] Integrate changed-path Bloom filters with 'git blame' Derrick Stolee via GitGitGadget
2020-04-11  1:02 ` [PATCH 1/3] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-04-11 21:40   ` Junio C Hamano
2020-04-13 11:49     ` Derrick Stolee
2020-04-14 18:25       ` Junio C Hamano
2020-04-15 13:27         ` Derrick Stolee
2020-04-15 18:37           ` Derrick Stolee
2020-04-15 19:32             ` Junio C Hamano
2020-04-15 19:39               ` Junio C Hamano
2020-04-15 21:25             ` Junio C Hamano
2020-04-16  0:56               ` Taylor Blau [this message]
2020-04-15 22:18             ` Jakub Narębski
2020-04-16  0:52               ` Taylor Blau
2020-04-16 13:26                 ` Derrick Stolee
2020-04-16 16:33                   ` Taylor Blau
2020-04-16 18:02                     ` Junio C Hamano
2020-04-12 22:22   ` Taylor Blau
2020-04-12 22:30     ` Junio C Hamano
2020-04-13  0:07       ` Taylor Blau
2020-04-13 11:54         ` Derrick Stolee
2020-04-11  1:03 ` [PATCH 2/3] commit: write commit-graph with bloom filters Derrick Stolee via GitGitGadget
2020-04-11 21:57   ` Junio C Hamano
2020-04-12 20:51     ` Taylor Blau
2020-04-13 12:08       ` Derrick Stolee
2020-04-13 22:11         ` Junio C Hamano
2020-04-11  1:03 ` [PATCH 3/3] blame: use changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-04-11 22:03   ` Junio C Hamano
2020-04-12  7:39     ` Eric Sunshine
2020-04-11 21:30 ` [PATCH 0/3] Integrate changed-path Bloom filters with 'git blame' Junio C Hamano
2020-04-13 14:45 ` [PATCH v2 0/4] " Derrick Stolee via GitGitGadget
2020-04-13 14:45   ` [PATCH v2 1/4] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-04-13 16:09     ` Taylor Blau
2020-04-13 22:18       ` Junio C Hamano
2020-04-13 14:45   ` [PATCH v2 2/4] commit: write commit-graph with Bloom filters Derrick Stolee via GitGitGadget
2020-04-13 16:12     ` Taylor Blau
2020-04-13 22:21       ` Junio C Hamano
2020-04-14 15:04         ` Derrick Stolee
2020-04-14 17:26           ` Junio C Hamano
2020-04-14 17:40             ` Derrick Stolee
2020-04-15  0:17               ` Taylor Blau
2020-04-13 14:45   ` [PATCH v2 3/4] commit-graph: write commit-graph in more tests Derrick Stolee via GitGitGadget
2020-04-13 14:45   ` [PATCH v2 4/4] blame: use changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-04-13 16:21   ` [PATCH v2 0/4] Integrate changed-path Bloom filters with 'git blame' Taylor Blau
2020-04-16 20:14   ` [PATCH v3 0/3] " Derrick Stolee via GitGitGadget
2020-04-16 20:14     ` [PATCH v3 1/3] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-06-07 20:33       ` SZEDER Gábor
2020-04-16 20:14     ` [PATCH v3 2/3] tests: write commit-graph with Bloom filters Derrick Stolee via GitGitGadget
2020-04-16 20:14     ` [PATCH v3 3/3] blame: use changed-path " Derrick Stolee via GitGitGadget

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