On Tue, Jan 07, 2025 at 01:06:20PM +0100, Patrick Steinhardt wrote:

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I think we should perhaps combine forces here. My ideal end-state is to
have the unsafe_hash_algo() stuff land from my earlier series, then have
these two fixes (adjusted to the new world order as above), and finally
the Meson fixes after that.

Does that seem like a plan to you? If so, I can put everything together
and send it out (if you're OK with me forging your s-o-b).

I think the ideal state would be if the hashing function used was stored
as part of `struct git_hash_ctx`. So the flow basically becomes for
example:

    struct git_hash_ctx ctx;
    struct object_id oid;

    git_hash_sha1_init(&ctx);
    git_hash_update(&ctx, data);
    git_hash_final_oid(&oid, &ctx);

Note how the intermediate calls don't need to know which hash function
you used to initialize the `struct git_hash_ctx` -- the structure itself
should remember what it has been initilized with and do the right thing.

I'm not sure I'm following you here. In the stream_blob() function
within fast-import, the problem isn't that we're switching hash
functions mid-stream, but that we're initializing the hashfile_context
structure with the wrong hash function to begin with.

True, but it would have been a non-issue if the hash context itself knew
which hash function to use for updates. Sure, we would've used the slow
variant of SHA1 instead of the fast-but-unsafe one. But that feels like
the lesser evil compared to crashing.

For posterity, Patrick and I used some of our monthly meeting this morning to
spend some time together pairing on this idea.

It ended up being a dead-end, since this approach only protects you
against changing the hash function mid-stream, and not using the
incorrect context type from the union.

That was along the lines of what I was originally thinking, and so I
resurrected my series to introduce 'unsafe_hash_algo()' here:

    https://lore.kernel.org/git/cover.1736363652.git.me@ttaylorr.com/ (local)

I got the impression that Patrick and I are on the same page there as
that being a good path forward, but I'll let him chime in in case I
misunderstood anything.

Thanks,
Taylor