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@xxxxxxxxx> wrote: >> [...] >>> -->8-- >>> From 89beb9598daabb19e3c896bbceeb0fc1b9ccc6ca Mon Sep 17 00:00:00 2001 >>> From: Derrick Stolee <dstolee@xxxxxxxxxxxxx> >>> 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. Thanks, -Stolee
