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refs/reftable: reload locked stack when preparing transaction

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When starting a reftable transaction we lock all stacks we are about to
modify. While it may happen that the stack is out-of-date at this point
in time we don't really care: transactional updates encode the expected
state of a certain reference, so all that we really want to verify is
that the _current_ value matches that expected state.

Pass `REFTABLE_STACK_NEW_ADDITION_RELOAD` when locking the stack such
that an out-of-date stack will be reloaded after having been locked.
This change is safe because all verifications of the expected state
happen after this step anyway.

Add a testcase that verifies that many writers are now able to write to
the stack concurrently without failures and with a deterministic end
result.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Patrick Steinhardt
2024-09-24 07:33:08 +02:00
committed by Junio C Hamano
parent 80e7342ea8
commit 6241ce2170
2 changed files with 33 additions and 1 deletions
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3
refs/reftable-backend.c
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@ -770,7 +770,8 @@ static int prepare_transaction_update(struct write_transaction_table_arg **out,
if (ret)
return ret;
ret = reftable_stack_new_addition(&addition, stack, 0);
ret = reftable_stack_new_addition(&addition, stack,
REFTABLE_STACK_NEW_ADDITION_RELOAD);
if (ret) {
if (ret == REFTABLE_LOCK_ERROR)
strbuf_addstr(err, "cannot lock references");