Hi Peff,
On Fri, 16 Nov 2018, Jeff King wrote:
> On Thu, Nov 15, 2018 at 09:01:07PM +0100, Johannes Schindelin wrote:
>
> > > It seems like we should be checking that the stale lockfile isn't left,
> > > which is the real problem (the warning is annoying, but is a symptom of
> > > the same thing). I.e., something like:
> > >
> > > test_must_fail git bundle create foobar.bundle master..master &&
> > > test_path_is_missing foobar.bundle.lock
> > >
> > > That would already pass on non-Windows platforms, but that's OK. It will
> > > never give a false failure.
> > >
> > > If you don't mind, can you confirm that the test above fails without
> > > either of the two patches under discussion?
> >
> > This test succeeds with your patch as well as with Gaël's, and fails when
> > neither patch is applied. So you're right, it is the much better test.
>
> Thanks for checking!
>
> > > > Do you want to integrate this test into your patch and run with it, or
> > > > do you want me to shepherd your patch?
> > >
> > > I'll wrap it up with a commit message and a test.
>
> Actually, I realized there's an even simpler way to do this. Here it is.
>
> -- >8 --
> Subject: [PATCH] bundle: dup() output descriptor closer to point-of-use
>
> When writing a bundle to a file, the bundle code actually creates
> "your.bundle.lock" using our lockfile interface. We feed that output
> descriptor to a child git-pack-objects via run-command, which has the
> quirk that it closes the output descriptor in the parent.
>
> To avoid confusing the lockfile code (which still thinks the descriptor
> is valid), we dup() it, and operate on the duplicate.
>
> However, this has a confusing side effect: after the dup() but before we
> call pack-objects, we have _two_ descriptors open to the lockfile. If we
> call die() during that time, the lockfile code will try to clean up the
> partially-written file. It knows to close() the file before unlinking,
> since on some platforms (i.e., Windows) the open file would block the
> deletion. But it doesn't know about the duplicate descriptor. On
> Windows, triggering an error at the right part of the code will result
> in the cleanup failing and the lockfile being left in the filesystem.
>
> We can solve this by moving the dup() much closer to start_command(),
> shrinking the window in which we have the second descriptor open. It's
> easy to place this in such a way that no die() is possible. We could
> still die due to a signal in the exact wrong moment, but we already
> tolerate races there (e.g., a signal could come before we manage to put
> the file on the cleanup list in the first place).
>
> As a bonus, this shields create_bundle() itself from the duplicate-fd
> trick, and we can simplify its error handling (note that the lock
> rollback now happens unconditionally, but that's OK; it's a noop if we
> didn't open the lock in the first place).
>
> The included test uses an empty bundle to cause a failure at the right
> spot in the code, because that's easy to trigger (the other likely
> errors are write() problems like ENOSPC). Note that it would already
> pass on non-Windows systems (because they are happy to unlink an
> already-open file).
Thanks, this is a very nice explanation (and now that I do not feel so
stressed as I did yesterday, I can easily wrap my head around it).
I can confirm that the test fails without the changes to bundle.c, and
succeeds with the changes.
Thank you so much!
Dscho
> Based-on-a-patch-by: Gaël Lhez <gael.l...@gmail.com>
> Signed-off-by: Jeff King <p...@peff.net>
> ---
> bundle.c | 39 ++++++++++++++++++---------------------
> t/t5607-clone-bundle.sh | 6 ++++++
> 2 files changed, 24 insertions(+), 21 deletions(-)
>
> diff --git a/bundle.c b/bundle.c
> index 1ef584b93b..6b0f6d8f10 100644
> --- a/bundle.c
> +++ b/bundle.c
> @@ -243,7 +243,7 @@ static int is_tag_in_date_range(struct object *tag,
> struct rev_info *revs)
> }
>
>
> -/* Write the pack data to bundle_fd, then close it if it is > 1. */
> +/* Write the pack data to bundle_fd */
> static int write_pack_data(int bundle_fd, struct rev_info *revs)
> {
> struct child_process pack_objects = CHILD_PROCESS_INIT;
> @@ -256,6 +256,20 @@ static int write_pack_data(int bundle_fd, struct
> rev_info *revs)
> pack_objects.in = -1;
> pack_objects.out = bundle_fd;
> pack_objects.git_cmd = 1;
> +
> + /*
> + * start_command() will close our descriptor if it's >1. Duplicate it
> + * to avoid surprising the caller.
> + */
> + if (pack_objects.out > 1) {
> + pack_objects.out = dup(pack_objects.out);
> + if (pack_objects.out < 0) {
> + error_errno(_("unable to dup bundle descriptor"));
> + child_process_clear(&pack_objects);
> + return -1;
> + }
> + }
> +
> if (start_command(&pack_objects))
> return error(_("Could not spawn pack-objects"));
>
> @@ -421,21 +435,10 @@ int create_bundle(struct bundle_header *header, const
> char *path,
> bundle_to_stdout = !strcmp(path, "-");
> if (bundle_to_stdout)
> bundle_fd = 1;
> - else {
> + else
> bundle_fd = hold_lock_file_for_update(&lock, path,
> LOCK_DIE_ON_ERROR);
>
> - /*
> - * write_pack_data() will close the fd passed to it,
> - * but commit_lock_file() will also try to close the
> - * lockfile's fd. So make a copy of the file
> - * descriptor to avoid trying to close it twice.
> - */
> - bundle_fd = dup(bundle_fd);
> - if (bundle_fd < 0)
> - die_errno("unable to dup file descriptor");
> - }
> -
> /* write signature */
> write_or_die(bundle_fd, bundle_signature, strlen(bundle_signature));
>
> @@ -463,10 +466,8 @@ int create_bundle(struct bundle_header *header, const
> char *path,
> goto err;
>
> /* write pack */
> - if (write_pack_data(bundle_fd, &revs)) {
> - bundle_fd = -1; /* already closed by the above call */
> + if (write_pack_data(bundle_fd, &revs))
> goto err;
> - }
>
> if (!bundle_to_stdout) {
> if (commit_lock_file(&lock))
> @@ -474,11 +475,7 @@ int create_bundle(struct bundle_header *header, const
> char *path,
> }
> return 0;
> err:
> - if (!bundle_to_stdout) {
> - if (0 <= bundle_fd)
> - close(bundle_fd);
> - rollback_lock_file(&lock);
> - }
> + rollback_lock_file(&lock);
> return -1;
> }
>
> diff --git a/t/t5607-clone-bundle.sh b/t/t5607-clone-bundle.sh
> index 348d9b3bc7..cf39e9e243 100755
> --- a/t/t5607-clone-bundle.sh
> +++ b/t/t5607-clone-bundle.sh
> @@ -71,4 +71,10 @@ test_expect_success 'prerequisites with an empty commit
> message' '
> git bundle verify bundle
> '
>
> +test_expect_success 'failed bundle creation does not leave cruft' '
> + # This fails because the bundle would be empty.
> + test_must_fail git bundle create fail.bundle master..master &&
> + test_path_is_missing fail.bundle.lock
> +'
> +
> test_done
> --
> 2.19.1.1636.gc7a073d580
>
>
