On Fri, Jun 2, 2017 at 7:54 PM, Jonathan Nieder <jrnie...@gmail.com> wrote: > Hi Dscho, > > Johannes Schindelin wrote: >> On Thu, 1 Jun 2017, Stefan Beller wrote: > >>> We had a discussion off list how much of the test suite is in bad shape, >>> and "$ git grep ^index" points out a lot of places as well. >> >> Maybe we should call out a specific month (or even a longer period) during >> which we try to push toward that new hash function, and focus more on >> those tasks (and on critical bug fixes, if any) than anything else. > > Thanks for offering. ;-) > > Here's a rough list of some useful tasks, in no particular order: > > 1. bc/object-id: This patch series continues, eliminating assumptions > about the size of object ids by encapsulating them in a struct. > One straightforward way to find code that still needs to be > converted is to grep for "sha" --- often the conversion patches > change function and variable names to refer to oid_ where they used > to use sha1_, making the stragglers easier to spot. > > 2. Hard-coded object ids in tests: As Stefan hinted, many tests beyond > t00* make assumptions about the exact values of object ids. That's > bad for maintainability for other reasons beyond the hash function > transition, too. > > It should be possible to suss them out by patching git's sha1 > routine to use the ones-complement of sha1 (~sha1) instead and > seeing which tests fail.
I just hacked this up locally. While a lot of stuff fails, it's not exactly an out of control garbage fire and could be churned through by someone interested. A lot of it's just lazy sha1 hardcoding for no good reason where we could use a tag, or e.g. test_cmp on ls-tree output without the test really caring about the hash. Things that really need to test the sha1 could be guarded by some new prereq. Both of my attempts to fuzz SHA1DCInit though resulted in some segfaults, I tried both changing "0x" to "~0x" in the ihv assignment, and just calling SHA1DCUpdate(ctx, "x", 1) at the end of the function, not sure why that's happening. If someone knows offhand what I'm doing wrong here I might be interested in going through this if I don't have to sift through the segfaults. I.e. what's an easy hack to make all of git pretend that "foo" hashes to "xfoo", "" to "x" etc. > 4. When choosing a hash function, people may argue about performance. > It would be useful for run some benchmarks for git (running > the test suite, t/perf tests, etc) using a variety of hash > functions as input to such a discussion. To the extent that such benchmarks matter, it seems prudent to heavily weigh them in favor of whatever seems to be likely to be the more common hash function going forward, since those are likely to get faster through future hardware acceleration. E.g. Intel announced Goldmont last year which according to one SHA-1 implementation improved from 9.5 cycles per byte to 2.7 cpb[1]. They only have acceleration for SHA-1 and SHA-256[2] 1. https://github.com/weidai11/cryptopp/issues/139#issuecomment-264283385 2. https://en.wikipedia.org/wiki/Goldmont
