dup() the input fd for fast-import used for remote helpers

When a remote helper exposes the "import" capability, stdout of the
helper is sent to stdin of a new fast-import process. This is done by
setting the corresponding child_process's in field to the value of the
out field of the helper child_process.

The child_process API is defined to close the file descriptors it's
given when calling start_command. This means when start_command is
called for the fast-import process, its input fd (the output fd of the
helper), is closed.

But when the transport helper is later destroyed, in disconnect_helper,
its input and output are closed, which means close() is called with
an invalid fd (since it was already closed as per above). Or worse, with
a valid fd owned by something else (since fd numbers can be reused).

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -421,7 +421,7 @@ static int get_importer(struct transport *transport, struct child_process *fasti
 	struct helper_data *data = transport->data;
 	int cat_blob_fd, code;
 	child_process_init(fastimport);
-	fastimport->in = helper->out;
+	fastimport->in = xdup(helper->out);
 	argv_array_push(&fastimport->args, "fast-import");
 	argv_array_push(&fastimport->args, debug ? "--stats" : "--quiet");