[PATCH v2 3/4] commit-graph.c: don't assume that stat() succeeds

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

Fix code added in 8d84097f965 (commit-graph: expire commit-graph files, 2019-06-18) to check the return value of the stat() system call. Not doing so caused us to use uninitialized memory in the "Bloom generation is limited by --max-new-filters" test in t4216-log-bloom.sh:

	+ rm -f trace.event
	+ pwd
	+ GIT_TRACE2_EVENT=[...]/t/trash directory.t4216-log-bloom/limits/trace.event git commit-graph write --reachable --split=replace --changed-paths --max-new-filters=2
	==24835== Syscall param utimensat(times[0].tv_sec) points to uninitialised byte(s)
	==24835==    at 0x499E65A: __utimensat64_helper (utimensat.c:34)
	==24835==    by 0x4999142: utime (utime.c:36)
	==24835==    by 0x552BE0: mark_commit_graphs (commit-graph.c:2213)
	==24835==    by 0x550822: write_commit_graph (commit-graph.c:2424)
	==24835==    by 0x54E3A0: write_commit_graph_reachable (commit-graph.c:1681)
	==24835==    by 0x4374BB: graph_write (commit-graph.c:269)
	==24835==    by 0x436F7D: cmd_commit_graph (commit-graph.c:326)
	==24835==    by 0x407B9A: run_builtin (git.c:465)
	==24835==    by 0x406651: handle_builtin (git.c:719)
	==24835==    by 0x407575: run_argv (git.c:786)
	==24835==    by 0x406410: cmd_main (git.c:917)
	==24835==    by 0x511F09: main (common-main.c:56)
	==24835==  Address 0x1ffeffde70 is on thread 1's stack
	==24835==  in frame #1, created by utime (utime.c:25)
	==24835==  Uninitialised value was created by a stack allocation
	==24835==    at 0x552B50: mark_commit_graphs (commit-graph.c:2201)
	==24835==
	[...]
	error: last command exited with $?=126
	not ok 137 - Bloom generation is limited by --max-new-filters

This would happen as we stat'd the non-existing ".git/objects/info/commit-graph" file. Let's fix mark_commit_graphs() to check the stat()'s return value, and while we're at it fix another case added in the same commit to do the same.

The caller in expire_commit_graphs() would have been less likely to run into this, as it's operating on files it just got from readdir(), but it could still happen due to a race with e.g. a concurrent "rm -rf" of the commit-graph files.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 commit-graph.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)