From ce54672f9b017adf60d15bc7174994b63cb29d3a Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Tue, 20 Dec 2022 15:52:14 +0100 Subject: [PATCH] refs: fix corruption by not correctly syncing packed-refs to disk At GitLab we have recently received a report where a repository was left with a corrupted `packed-refs` file after the node hard-crashed even though `core.fsync=reference` was set. This is something that in theory should not happen if we correctly did the atomic-rename dance to: 1. Write the data into a temporary file. 2. Synchronize the temporary file to disk. 3. Rename the temporary file into place. So if we crash in the middle of writing the `packed-refs` file we should only ever see either the old or the new state of the file. And while we do the dance when writing the `packed-refs` file, there is indeed one gotcha: we use a `FILE *` stream to write the temporary file, but don't flush it before synchronizing it to disk. As a consequence any data that is still buffered will not get synchronized and a crash of the machine may cause corruption. Fix this bug by flushing the file stream before we fsync. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano --- refs/packed-backend.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/refs/packed-backend.c b/refs/packed-backend.c index 9d704ccd3e..62b5644596 100644 --- a/refs/packed-backend.c +++ b/refs/packed-backend.c @@ -1262,7 +1262,8 @@ static int write_with_updates(struct packed_ref_store *refs, goto error; } - if (fsync_component(FSYNC_COMPONENT_REFERENCE, get_tempfile_fd(refs->tempfile)) || + if (fflush(out) || + fsync_component(FSYNC_COMPONENT_REFERENCE, get_tempfile_fd(refs->tempfile)) || close_tempfile_gently(refs->tempfile)) { strbuf_addf(err, "error closing file %s: %s", get_tempfile_path(refs->tempfile),