Moving Git past SHA-1

Posted Feb 27, 2017 23:04 UTC (Mon) by nix (subscriber, #2304)
In reply to: Moving Git past SHA-1 by Karellen
Parent article: Moving Git past SHA-1

The problem is that when you create a new-hash commit that's a child of an old-hash commit, you have to somehow know that the blobs with new-hash X are equivalent to the parent's blobs with old-hash Y *even though the hashes are different*: i.e. rather than just doing a string compare you have to actually *rehash* under the other hash and then compare that.

Nobody's written that code, so with current code it will assume all the blobs are different and write them out all over again, even though they're all identical. Not even repacking all objects would fix that, because that doesn't rehash anything either...

Linus seems to be happy with this, but having seen the amount of space it would waste forevermore in e.g. the Chromium repo, I'm not so happy :(

Moving Git past SHA-1

Posted Feb 28, 2017 1:07 UTC (Tue) by BenHutchings (subscriber, #37955) [Link] (1 responses)

git doesn't really store each object independently; it eventually packs them up together and applies delta-compression to reduce storage for multiple versions of a file. I would expect that the new-hashed blobs can be delta-compressed down to no more than a few bytes of metadata.

Moving Git past SHA-1

Posted Feb 28, 2017 12:03 UTC (Tue) by nix (subscriber, #2304) [Link]

Hm. Given that the transitions will always happen at a parent/child commit boundary, you're probably right: the delta compression heuristics are partially commit- and partially file-based, so even the smallest window will find and scrunch them into nothing.

OK, all my objections to this scheme are gone then: except for a slightly increased diff cost across the boundary (to, uh, what non-git users pay all the time) it is costless. :)

Moving Git past SHA-1

Posted Feb 28, 2017 3:27 UTC (Tue) by smurf (subscriber, #17840) [Link] (1 responses)

That shouldn't be a problem if you do the hard-linking at the point of transition between old and new hash. You need to mandate that the transitioning commit may not have any other changes. Presto, instant hash correspondence table.

Moving Git past SHA-1

Posted Feb 28, 2017 6:54 UTC (Tue) by johill (subscriber, #25196) [Link]

That only really works - if at all - when your objects are all loose, but they will be in packs so it won't.

Moving Git past SHA-1

Posted Feb 28, 2017 8:39 UTC (Tue) by Karellen (subscriber, #67644) [Link] (7 responses)

Sorry, I wasn't explaining myself clearly.

Rather than have a new-hash commit that's a child of an old-hash commit, maintain two complete sets of commits/history, one for each hash. You can build one history from the other.

e.g. Say you have a repository containing a single file with two revisions. At the moment, you have a repo like (excuse shortened and comically simplified hash values):

.git/
    objects/
        00/000001 -- first revision of file
        00/000002 -- second revision of file
        00/100001 -- tree containing first revision of file
        00/100002 -- tree containing second revision of file
        00/200001 -- commit containing first tree
        00/200002 -- commit containing second tree
    refs/
        heads/
            master -- points to 00200002

Now, replace that with:

.git/
    objects/
        sha1/00/000001 -- first revision of file
        sha1/00/000002 -- second revision of file
        sha1/00/100001 -- sha1-based tree containing first revision of file
        sha1/00/100002 -- sha1-based tree containing second revision of file
        sha1/00/200001 -- sha1-based commit containing first tree
        sha1/00/200002 -- sha1-based commit containing second tree
        sha3/ff/000001 -- first revision of file
        sha3/ff/000002 -- second revision of file
        sha3/ff/100001 -- sha3-based tree containing first revision of file
        sha3/ff/100002 -- sha3-based tree containing second revision of file
        sha3/ff/200001 -- sha3-based commit containing first tree
        sha3/ff/200002 -- sha3-based commit containing second tree
    refs/
        heads/
            master -- points to sha1:00200002

In the above case, sha1/00/000001 and sha3/ff/000001 are hard links to the same file. The blob is identical in each case, it's just linked to multiple times with different names. The same goes for the second revision of the file. Only the tree and commit objects need to be duplicated for each hash, but they're going to be orders of magnitude smaller than the sizes of the blobs. You can build one tree from the other, and even build an entirely new commit history based on a different hash at any time in the future. When you create a new commit, a new commit object will be created for each hash you've configured your repo to use.

Anyone can refer to a commit by either id (sha1:00200002 /or/ sha3:ff200002), and, provided you have the relevant commit history built in your repo, you can see which commit they're talking about. References to old commits are still valid, because you still have (and can continue to maintain) the old hash history.

A project could say "from date X we will talk to each other about commits using sha3 refs by default", but that won't need to change how your repo actually works under the hood.

Does that explain what I'm trying to get at a bit better?

Moving Git past SHA-1

Posted Feb 28, 2017 11:19 UTC (Tue) by andrewsh (subscriber, #71043) [Link] (5 responses)

How would that work with packs?

Moving Git past SHA-1

Posted Feb 28, 2017 12:06 UTC (Tue) by nix (subscriber, #2304) [Link] (1 responses)

I guess it'll use the db mapping new to old hash values that everyone will need to carry around forever. 250MiB+ for the kernel. Sigh.

Ah well, disk space really is cheap: I've just never internalized it properly. 250MiB still seems like a lot rather than about 1p. (And that price is taken from *enterprise* storage costs. Disks that normal humans not obsessed with RAIDing everything use will be even cheaper.)

Moving Git past SHA-1

Posted Mar 1, 2017 10:39 UTC (Wed) by zenaan (guest, #3778) [Link]

>I guess it'll use the db mapping new to old hash values that everyone will need to carry around forever. 250MiB+ for the kernel. Sigh.

Is 250MiB just the mapping size (i.e. no object data)? Sounds surprising, but perhaps not for the number of years (rather, objects) involved?

Flag day doesn't sound so bad - a repo can simply store some meta-data for it's UTC flag day or "flag moment", with a certain quiescence period (say 15 minutes) during which pulls and pushes are not accepted... thereby ensuring world-sync for the flag moment, yes?

Moving Git past SHA-1

Posted Feb 28, 2017 13:06 UTC (Tue) by Karellen (subscriber, #67644) [Link]

No idea :-)

I have basic familiarity with *some* git internals, but "how packs are formatted" has never been something I've needed to delve into. It's entirely possible that could scuttle the whole idea.

Moving Git past SHA-1

Posted Feb 28, 2017 14:59 UTC (Tue) by garyvdm (subscriber, #82325) [Link] (1 responses)

I think (I'm not 100%) that the object ids are not stored in the pack file, only in the pack index. The pack file can be enumerated, and the object ids can be calculated. Hence it would be possible to have one pack file, and multiple index files for each object id type. Such a repo might look like this:

.git/
    objects/
        pack
            pack-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.idx
            pack-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.idx-sha3
            pack-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.pack

Moving Git past SHA-1

Posted Feb 28, 2017 15:11 UTC (Tue) by nix (subscriber, #2304) [Link]

There *are* object IDs stored in packs, of course -- commit, tag, and tree objects all contain embedded SHA-1 IDs (and, in the new world, new-hash IDs).

But the mapping from SHA-1 IDs to object offsets in each pack is computed at receive time and stored in the index. (This means that every object lookup must check each index in turn, which is why occasionally repacking them into a single big pack without actually recompressing any content at all is still a significant win.)

Moving Git past SHA-1

Posted Feb 28, 2017 22:49 UTC (Tue) by magfr (subscriber, #16052) [Link]

But would this solve the problem at hand?

What happens under this scheme when I add a second blob with the same sha1 hash as one previously in the repository but a different content?

Moving Git past SHA-1

Posted Feb 28, 2017 21:30 UTC (Tue) by jengelh (guest, #33263) [Link] (1 responses)

>The problem is that when you create a new-hash commit that's a child of an old-hash commit, you have to somehow know that the blobs with new-hash X are equivalent to the parent's blobs with old-hash Y *even though the hashes are different*

Not sure how hashes are stored internally, but pretending they are dealt with in their hex form, why not add a character not in the [0-9a-f] alphabet?

if (strlen(hash) == 20 && strchr(hash, ':') == NULL) { method = sha1; } else { (method, hash) = split("f00f....f00f", ":"); } kind of thing.

Something like that worked for shadow(5) too when $2a$ et al support was added.

Moving Git past SHA-1

Posted Mar 1, 2017 0:08 UTC (Wed) by nix (subscriber, #2304) [Link]

Oh you can easily tell which hash they're from -- just check the lengths. What you can't tell (without rehashing) is that specific parent and child blobs denote the same object. (However, as noted elsethread, this is not a problem: the deltifier will spot them anyway, and squash them away to next-to-nothing.)