[PATCH v6 0/1] refs: report old values to transaction hooks

[Maciej Ciemborowicz <maciej.ciemborowicz@gmail.com>]

This follows up on the reference-transaction bug report at [1].

The reference-transaction hook currently reports an all-zero old object ID
when a caller queues an unconditional update. As a result, batched branch,
tag, and remote-ref deletions report zero for both the old and new values.

Earlier versions changed refs_delete_refs() and its callers to pass expected
old OIDs. As Patrick pointed out, that makes previously unconditional
deletions conditional and adds complexity at the wrong layer.

Resolve the old value in the common transaction hook layer instead. Keep it
separate from the caller-supplied old_oid and old_target so it cannot
constrain the update. Resolve it before the "preparing" hook and refresh it
after the backend locks the references, allowing later phases to report the
value protected by those locks. Do this work only when a
reference-transaction hook exists.

Changes since v5:

 * Replace the three-patch caller-specific approach with one transaction
   layer change.
 * Leave refs_delete_refs(), ref_transaction_delete(), and all command call
   sites unchanged.
 * Preserve unconditional deletion semantics and test a concurrent update
   from the "preparing" hook.
 * Report observed old values for all unconditional transactions, including
   symbolic ref targets.
 * Document that the unlocked value reported in "preparing" may differ from
   later phases if the reference changes before it is locked.

The full test suite passes (1060 files, 34660 tests). The focused
reference-transaction tests also pass with SHA-1 and SHA-256 using both the
files and reftable backends.

[1] https://lore.kernel.org/git/CACQ=SRGTTdQ+dHXhN6F52dBv5KxZBRfk_Em2fvmEmGJDoB6oTg@mail.gmail.com/

Maciej Ciemborowicz (1):
  refs: report old values to transaction hooks

 Documentation/githooks.adoc      | 17 +++++----
 refs.c                           | 54 ++++++++++++++++++++++++---
 refs/refs-internal.h             |  8 ++++
 t/t1416-ref-transaction-hooks.sh | 64 +++++++++++++++++++++++++++++++-
 4 files changed, 128 insertions(+), 15 deletions(-)

Range-diff against v5:
1:  9b76cc2c40 < -:  ---------- refs: allow callers to supply old OIDs for batch deletion
2:  6a8401c448 < -:  ---------- branch, tag: retain old OIDs in batched deletions
3:  541da44c37 < -:  ---------- fetch, remote: retain old OIDs when pruning refs
-:  ---------- > 1:  2af3eeadd1 refs: report old values to transaction hooks
-- 
2.39.3 (Apple Git-146)