[PATCH] sparse-checkout: use fdopen_lock_file() instead of xfdopen()

When updating sparse patterns, we open a lock_file to write out the new data. The lock_file struct holds the file descriptor, but we call fdopen() to get a stdio handle to do the actual write.

After we finish writing, we fflush() so that all of the data is on disk, and then call commit_lock_file() which closes the descriptor. But we never fclose() the stdio handle, leaking it.

The obvious solution seems like it would be to just call fclose(). But when? If we do it before commit_lock_file(), then the lock_file code is left thinking it owns the now-closed file descriptor, and will do an extra close() on the descriptor. But if we do it before, we have the opposite problem: the lock_file code will close the descriptor, and fclose() will do the extra close().

We can handle this correctly by using fdopen_lock_file(). That leaves ownership of the stdio handle with the lock_file, which knows not to double-close it.

We do have to adjust the code a bit:

  - we have to handle errors ourselves; we can just die(), since that's
    what xfdopen() would have done (and we can even provide a more
    specific error message).

  - we no longer need to call fflush(); committing the lock-file
    auto-closes it, which will now do the flush for us. As a bonus, this
    will actually check that the flush was successful before renaming
    the file into place. Let's likewise report when committing the lock
    fails (rather than quietly returning success from the command).

  - we can get rid of the local "fd" variable, since we never look at it
    ourselves now

Signed-off-by: Jeff King <peff@peff.net>
---
Curiously, this is not reported as a leak by LSan, and it is included in
the "reachable" set by valgrind. I imagine this is because libc has to
maintain a list of open writable handles, since fflush(NULL) is supposed
to flush all of them. I peeked at other fdopen(..., "w") calls to see if
there were other similar spots, but didn't notice any.

I found this because I was building git on an Android system, and they have an "fdsan" that complains when the underlying descriptor of a handle is closed. It flagged some other spots, too, but the rest that I looked at involved the tempfile atexit() handler closing the descriptors manually (we don't care about flushing there, as our goal is to just prevent the "can't delete an open file" problem on Windows).

 builtin/sparse-checkout.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)