bundle: avoid closing file descriptor twice

Already when introduced in c7a8a16 (Add bundle transport,
2007-09-10), the `bundle` transport had a bug where it would open a file
descriptor to the bundle file and then close it _twice_: First, the file
descriptor (`data->fd`) is passed to `unbundle()`, which would use it as
the `stdin` of the `index-pack` process, which as a consequence would
close it via `start_command()`. However, `data->fd` would still hold the
numerical value of the file descriptor, and `close_bundle()` would see
that and happily close it again.

This seems not to have caused too many problems in almost two decades,
but I encountered a situation today where it _does_ cause problems: In
i686 variants of Git for Windows, it seems that file descriptors are
reused quickly after they have been closed.

In the particular scenario I faced, `git fetch <bundle> <ref>` gets the
same file descriptor value when opening the bundle file and importing
its embedded packfile (which implicitly closes the file descriptor) and
then when opening a pack file in `fetch_and_consume_refs()` while
looking up an object's header.

Later on, after the bundle has been imported (and the `close_bundle()`
function erroneously closes the file descriptor that has _already_ been
closed when using it as `stdin` for `git index-pack`), the same file
descriptor value has now been reused via `use_pack()`. Now, when either
the recursive fetch (which defaults to "on", unfortunately) or a
commit-graph update needs to `mmap()` the packfile, it fails due to a
now-invalid file descriptor that _should_ point to the pack file but
doesn't anymore.

To fix that, let's invalidate `data->fd` after calling `unbundle()`.
That way, `close_bundle()` does not close a file descriptor that may
have been reused for something different. While at it, document that
`unbundle()` closes the file descriptor, and ensure that it also does
that when failing to verify the bundle.

Luckily, this bug does not affect the bundle URI feature, it only
affects the `git fetch <bundle>` code path.

Note that this patch does not _completely_ clarifies who is responsible
to close that file descriptor, as `run_command()` may fail _without_
closing `cmd->in`. Addressing this issue thoroughly, however, would
require a rather thorough re-design of the `start_command()` and
`finish_command()` functionality to make it a lot less murky who is
responsible for what file descriptors.

At least this here patch is relatively easy to reason about, and
addresses a hard failure (`fatal: mmap: could not determine filesize`)
at the expense of leaking a file descriptor under very rare
circumstances in which `git fetch` would error out anyway.

Signed-off-by: Johannes Schindelin <[email protected]>

bundle.c

@@ -607,8 +607,10 @@ int unbundle(struct repository *r, struct bundle_header *header,
 	if (!opts)
 		opts = &opts_fallback;
 
-	if (verify_bundle(r, header, opts->flags))
+	if (verify_bundle(r, header, opts->flags)) {
+		close(bundle_fd);
 		return -1;
+	}
 
 	strvec_pushl(&ip.args, "index-pack", "--fix-thin", "--stdin", NULL);
 

bundle.h

@@ -62,6 +62,8 @@ struct unbundle_opts {
  *
  * Before unbundling, this method will call verify_bundle() with 'flags'
  * provided in 'opts'.
+ *
+ * Note that the `bundle_fd` will be closed as part of the operation.
  */
 int unbundle(struct repository *r, struct bundle_header *header,
 	     int bundle_fd, struct strvec *extra_index_pack_args,

transport.c

@@ -207,6 +207,7 @@ static int fetch_refs_from_bundle(struct transport *transport,
 
 	ret = unbundle(the_repository, &data->header, data->fd,
 		       &extra_index_pack_args, &opts);
+	data->fd = -1; /* `unbundle()` closes the file descriptor */
 	transport->hash_algo = data->header.hash_algo;
 
 	strvec_clear(&extra_index_pack_args);