refs/reftable: reload locked stack when preparing transaction

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 While it may happen that the stack is out-of-date at this point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that an out-of-date stack will be reloaded after having been locked.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>

refs/reftable-backend.c

@@ -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");

t/t0610-reftable-basics.sh

@@ -450,6 +450,37 @@ test_expect_success 'ref transaction: retry acquiring tables.list lock' '
 	)
 '
 
+test_expect_success 'ref transaction: many concurrent writers' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		# Set a high timeout such that a busy CI machine will not abort
+		# early. 10 seconds should hopefully be ample of time to make
+		# this non-flaky.
+		git config set reftable.lockTimeout 10000 &&
+		test_commit --no-tag initial &&
+
+		head=$(git rev-parse HEAD) &&
+		for i in $(test_seq 100)
+		do
+			printf "%s commit\trefs/heads/branch-%s\n" "$head" "$i" ||
+			return 1
+		done >expect &&
+		printf "%s commit\trefs/heads/main\n" "$head" >>expect &&
+
+		for i in $(test_seq 100)
+		do
+			{ git update-ref refs/heads/branch-$i HEAD& } ||
+			return 1
+		done &&
+
+		wait &&
+		git for-each-ref --sort=v:refname >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'pack-refs: compacts tables' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&