[PATCH v4 0/8] hash: introduce unsafe_hash_algo(), drop unsafe_ variants

[Taylor Blau <me@ttaylorr.com>]

(This series is based on 14650065b7 (RelNotes/2.48.0: fix typos etc.,
2025-01-07)).

Here is a hopefully final version of my series to harden the unsafe hash
algorithm changes added in v2.47.0. The only difference from last time
is that hash_algo_by_ptr() now returns GIT_HASH_UNKNOWN for NULL and
unsafe variants, which is a strict improvement from both v3 of this
series and the status-quo on master.

As usual, a (small) range-diff is included below for convenience, and
the original cover letter is as follows:

------------

This series implements an idea discussed in [2] which suggests that we
introduce a way to access a wrapped version of a 'struct git_hash_algo'
which represents the unsafe variant of that algorithm, rather than
having individual unsafe_ functions (like unsafe_init_fn() versus
init_fn(), etc.).

This approach is relatively straightforward to implement, and removes a
significant deficiency in the original implementation of
unsafe/non-cryptographic hash functions by making it impossible to
switch between safe- and unsafe variants of hash functions. It also
cleans up the sha1-unsafe test helper's implementation by removing a
large number of "if (unsafe)"-style conditionals.

The series is laid out as follows:

  * The first two patches prepare the hashfile API for the upcoming
    change:

      csum-file: store the hash algorithm as a struct field
      csum-file.c: extract algop from hashfile_checksum_valid()

  * The next patch implements the new 'unsafe_hash_algo()' function at
    the heart of this series' approach:

      hash.h: introduce `unsafe_hash_algo()`

  * The next two patches convert existing callers to use the new
    'unsafe_hash_algo()' function, instead of switching between safe and
    unsafe_ variants of individual functions:

      csum-file.c: use unsafe_hash_algo()
      t/helper/test-hash.c: use unsafe_hash_algo()

  * The final patch drops the unsafe_ function variants following all
    callers being converted to use the new pattern:

      hash.h: drop unsafe_ function variants

Thanks in advance for your review!

[1]: https://lore.kernel.org/git/20241230-pks-meson-sha1-unsafe-v1-0-efb276e171f5@pks.im/
[2]: https://lore.kernel.org/git/20241107013915.GA961214@coredump.intra.peff.net/

Taylor Blau (8):
  t/helper/test-tool: implement sha1-unsafe helper
  csum-file: store the hash algorithm as a struct field
  csum-file.c: extract algop from hashfile_checksum_valid()
  hash.h: introduce `unsafe_hash_algo()`
  csum-file.c: use unsafe_hash_algo()
  t/helper/test-hash.c: use unsafe_hash_algo()
  csum-file: introduce hashfile_checkpoint_init()
  hash.h: drop unsafe_ function variants

 builtin/fast-import.c  |  2 +-
 bulk-checkin.c         |  9 ++++++---
 csum-file.c            | 40 +++++++++++++++++++++++++---------------
 csum-file.h            |  2 ++
 hash.h                 | 28 ++++++++++++----------------
 object-file.c          | 41 ++++++++++++++++++++++++++---------------
 t/helper/test-hash.c   |  4 +++-
 t/helper/test-sha1.c   |  7 ++++++-
 t/helper/test-sha1.sh  | 38 ++++++++++++++++++++++----------------
 t/helper/test-sha256.c |  2 +-
 t/helper/test-tool.c   |  1 +
 t/helper/test-tool.h   |  3 ++-
 12 files changed, 107 insertions(+), 70 deletions(-)

Range-diff against v3:
1:  ae6b8c75294 = 1:  b64ae238248 t/helper/test-tool: implement sha1-unsafe helper
2:  2b79c76e471 = 2:  d03f503682f csum-file: store the hash algorithm as a struct field
3:  d7deb3f338e = 3:  73554c3b881 csum-file.c: extract algop from hashfile_checksum_valid()
4:  b6b2bb2714f ! 4:  ae01f1f4274 hash.h: introduce `unsafe_hash_algo()`
    @@ hash.h: int hash_algo_by_length(int len);
     +	size_t i;
     +	for (i = 0; i < GIT_HASH_NALGOS; i++) {
     +		const struct git_hash_algo *algop = &hash_algos[i];
    -+		if (p == algop || (algop->unsafe && p == algop->unsafe))
    ++		if (p == algop)
     +			return i;
     +	}
     +	return GIT_HASH_UNKNOWN;
5:  ca67de80971 = 5:  64a850c77ae csum-file.c: use unsafe_hash_algo()
6:  21b175b07ff = 6:  3dcccccf752 t/helper/test-hash.c: use unsafe_hash_algo()
7:  850d4f407db = 7:  da97157c4a1 csum-file: introduce hashfile_checkpoint_init()
8:  0c4d006e6e8 = 8:  0ba27182b5e hash.h: drop unsafe_ function variants

base-commit: 14650065b76b28d3cfa9453356ac5669b19e706e
-- 
2.48.0.rc2.35.gd215225db14