[PATCH 3/3] commit-graph.c: write non-split graphs as read-only

[Taylor Blau <me@ttaylorr.com>]

In the previous commit, Git learned 'hold_lock_file_for_update_mode' to
allow the caller to specify the permission bits used when acquiring a
temporary file.

Use this in the commit-graph machinery for writing a non-split graph to
acquire an opened temporary file with permissions read-only permissions
to match the split behavior. (In the split case, Git uses
'git_mkstemp_mode' for each of the commit-graph layers with permission
bits '0444').

One can notice this discrepancy when moving a non-split graph to be part
of a new chain. This causes a commit-graph chain where all layers have
read-only permission bits, except for the base layer, which is writable
for the current user.

Resolve this discrepancy by using the new
'hold_lock_file_for_update_mode' and passing the desired permission
bits.

Doing so causes some test fallout in t5318 and t6600. In t5318, this
occurs in tests that corrupt a commit-graph file by writing into it. For
these, 'chmod u+w'-ing the file beforehand resolves the issue. The
additional spot in 'corrupt_graph_verify' is necessary because of the
extra 'git commit-graph write' beforehand (which *does* rewrite the
commit-graph file). In t6600, this is caused by copying a read-only
commit-graph file into place and then trying to replace it. For these,
make these files writable.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 commit-graph.c          |  3 ++-
 t/t5318-commit-graph.sh | 11 ++++++++++-
 t/t6600-test-reach.sh   |  2 ++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index f013a84e29..5b5047a7dd 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1388,7 +1388,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 
 		f = hashfd(fd, ctx->graph_name);
 	} else {
-		hold_lock_file_for_update(&lk, ctx->graph_name, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update_mode(&lk, ctx->graph_name,
+					       LOCK_DIE_ON_ERROR, 0444);
 		fd = lk.tempfile->fd;
 		f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 	}
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 9bf920ae17..fb0aae61c3 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -96,6 +96,13 @@ test_expect_success 'write graph' '
 	graph_read_expect "3"
 '
 
+test_expect_success POSIXPERM 'write graph has correct permissions' '
+	test_path_is_file $objdir/info/commit-graph &&
+	echo "-r--r--r--" >expect &&
+	test_modebits $objdir/info/commit-graph >actual &&
+	test_cmp expect actual
+'
+
 graph_git_behavior 'graph exists' full commits/3 commits/1
 
 test_expect_success 'Add more commits' '
@@ -421,7 +428,8 @@ GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
 corrupt_graph_setup() {
 	cd "$TRASH_DIRECTORY/full" &&
 	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
-	cp $objdir/info/commit-graph commit-graph-backup
+	cp $objdir/info/commit-graph commit-graph-backup &&
+	chmod u+w $objdir/info/commit-graph
 }
 
 corrupt_graph_verify() {
@@ -435,6 +443,7 @@ corrupt_graph_verify() {
 	fi &&
 	git status --short &&
 	GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD=true git commit-graph write &&
+	chmod u+w $objdir/info/commit-graph &&
 	git commit-graph verify
 }
 
diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh
index b24d850036..475564bee7 100755
--- a/t/t6600-test-reach.sh
+++ b/t/t6600-test-reach.sh
@@ -51,8 +51,10 @@ test_expect_success 'setup' '
 	done &&
 	git commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-full &&
+	chmod u+w commit-graph-full &&
 	git show-ref -s commit-5-5 | git commit-graph write --stdin-commits &&
 	mv .git/objects/info/commit-graph commit-graph-half &&
+	chmod u+w commit-graph-half &&
 	git config core.commitGraph true
 '
 
-- 
2.26.1.108.gadb95c98e4