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shallow: verify shallow file after taking lock

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Before writing the shallow file, we stat() the existing file
to make sure it has not been updated since our operation
began. However, we do not do so under a lock, so there is a
possible race:

  1. Process A takes the lock.

  2. Process B calls check_shallow_file_for_update and finds
     no update.

  3. Process A commits the lockfile.

  4. Process B takes the lock, then overwrite's process A's
     changes.

We can fix this by doing our check while we hold the lock.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2014-03-14 23:47:06 -04:00 committed by Junio C Hamano
parent 0179c945fc
commit 7839632167
1 changed files with 2 additions and 2 deletions
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4
shallow.c
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@ -263,9 +263,9 @@ void setup_alternate_shallow(struct lock_file *shallow_lock,
struct strbuf sb = STRBUF_INIT;
int fd;
check_shallow_file_for_update();
fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
LOCK_DIE_ON_ERROR);
check_shallow_file_for_update();
if (write_shallow_commits(&sb, 0, extra)) {
if (write_in_full(fd, sb.buf, sb.len) != sb.len)
die_errno("failed to write to %s",
@ -310,9 +310,9 @@ void prune_shallow(int show_only)
strbuf_release(&sb);
return;
}
check_shallow_file_for_update();
fd = hold_lock_file_for_update(&shallow_lock, git_path("shallow"),
LOCK_DIE_ON_ERROR);
check_shallow_file_for_update();
if (write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY)) {
if (write_in_full(fd, sb.buf, sb.len) != sb.len)
die_errno("failed to write to %s",