Re: Migrating away from SHA-1?
- From
- H. Peter Anvin <hpa@zytor.com>
- Date
- Apr 13, 2016, 01:38 UTC
- Message-ID
- <570DA311.3000500@zytor.com>
- In-Reply-To
- <xmqqlh4imibd.fsf@gitster.mtv.corp.google.com>
On 04/12/16 18:03, Junio C Hamano wrote:
Show 21 quoted lines
>> >> and so on. Of course trees don't have any space for this; they have a >> fixed-length for the hash part of each record, which is basically: >> >> <mode> <name> NUL <20-byte-sha1> >> >> So we'd probably need a "treev2" object type that gives room for an >> algorithm byte (or we'd have to try to shove it into the mode, but since >> old versions won't know the new algorithm anyway, I don't think it >> solves that much...). Or you can just define for the whole tree object >> (either implicit in its type, or in a header) that it always uses >> algorithm X. > > This will hurt the performance a lot during the transition period as > it no longer will be possible to rely on "most of the time a fine > grained commit changes only a small part of the tree, and we can > cheaply avoid descending into trees that haven't changed because we > can tell that the corresponding tree objects in the pre- and post- > trees have the same object name" optimization. But we cannot avoid > it. >
Not really, because you can point to the algoX hash even for the existing objects.
Perhaps the tree object can add a format descriptor at the beginning; something like:
<invalid mode number> <hash format used>
Show 38 quoted lines
>> Transitioning to that would be something like: >> >> 0. Overhaul all of the git code to handle arbitrary-sized object ids. >> >> 1. Decide on the new algorithm and implement it in git. >> >> 2. Recognize parameterized object ids in commits and tags (designing >> format, implementing the reading side). >> >> 3. Recognize parameterized object ids somehow in trees (designing >> format, implementing the reading side). >> >> 4. Teach the object database to index objects by the new algorithm (or >> possibly both algorithms). >> >> 5. Add a protocol extension so that both sides can decide which >> algorithm is being used when they talk about oids. >> >> 6. Add a config option to write references in objects using the new >> algorithm. >> >> 7. After a while, flip the config option on. Hopefully the readers >> from steps 1-5 have percolated to the masses by then, and it's not >> a horrible flag day. >> >> We're basically on step 0 right now. I'm sure I'm missing some >> subtleties in there, too. > > One subtlety is that 7. "not a flag day" may not be a good thing. > > There has to be a section of a history that spans the transition, > set of commits and trees that have pointers to both kinds of object > names. The narrower such a section of the history, the more > pleasant to use the result of the transition would be. > > Different projects that can have their own flag days at their own > pace is a good thing, so the above observation does not invalidate > your transition plan, though.
I don't think there is any way this can *not* be by repository and somehow require a manual operation in order to preserve the cryptographic integrity. In some ways, the transition point and the transition table becomes a special kind of tag object. There may have to be more than one in the case of commits in multiple trees.