Re: [PATCH 1/3] revision: complicated pathspecs disable filters

[Taylor Blau <me@ttaylorr.com>]

On Thu, Apr 16, 2020 at 09:26:49AM -0400, Derrick Stolee wrote:
> On 4/15/2020 8:52 PM, Taylor Blau wrote:
> > On Thu, Apr 16, 2020 at 12:18:33AM +0200, Jakub Narębski wrote:
> >> On Wed, 15 Apr 2020 at 20:37, Derrick Stolee <stolee@gmail.com> wrote:
> >> [...]
> >>> -->8--
> >>> From 89beb9598daabb19e3c896bbceeb0fc1b9ccc6ca Mon Sep 17 00:00:00 2001
> >>> From: Derrick Stolee <dstolee@microsoft.com>
> >>> Date: Wed, 15 Apr 2020 18:04:25 +0000
> >>> Subject: [PATCH] bloom: compute all Bloom hashes from lowercase
> >>>
> >>> The changed-path Bloom filters currently hash path strings using
> >>> the exact string for the path. This makes it difficult* to use the
> >>> filters when restricting to case-insensitive pathspecs.
> >>>
> >>> * I say "difficult" because it is possible to generate all 2^n
> >>>   options for the case of a path and test them all, but this is
> >>>   a bad idea and should not be done. "Impossible" is an appropriate
> >>>   alternative.
> >>>
> >>> 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.
> >>>
> >>> TODO: If we decide to move in this direction, then the following
> >>> steps should be done (and some of them should be done anyway):
> >>>
> >>> * 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.
> >>
> >> Why not modify the BDAT chunk to include version of
> >> case folding transformation or other collation algorithm
> >> (other transformation).that is done prior to computing
> >> the Bloom filter key? Though that might be unnecessary
> >> flexibility...
> >
> > If this ends up being something that we want to do, I agree with
> > Stolee's reasoning that this should be a breaking change. If we were,
> > say, several months into having Bloom filters in a release and decided
> > at that point to make the change, then: sure, supporting both by writing
> > a bit in the BDAT chunk makes sense.
> >
> > But, we're many months away from that state yet, and so I don't think
> > the cost of rebuilding what few commit-graphs exist with bloom filters
> > in them today to support both ordinary and lower-cased paths in the
> > filter.
> >
> > Anyway, I'm still not sold on this idea in general (nor do I understand
> > it that others are), so I'll respond in more detail in another part of
> > the thread...
>
> I agree that this is not a good direction to go. I created the patch
> because I was curious how difficult it would be, and it is good to have
> a record of the possible direction. However, it complicates the file
> format and will have unpredictable effects on the entropy (or on the
> performance of history for case-colliding paths).
>
> It is good that we have the capability to extend the filter data in
> the future if we really need to.
>
> I'll make a TODO item for myself to try writing that detailed Bloom
> filter format documentation as a follow-up. In the meantime, I'll try
> to close this out by responding to the feedback we have so far.

Sounds good, and thanks for investigating.

> Thanks,
> -Stolee

Thanks,
Taylor