fclose: Fix possible link error.

* lib/fclose.c (rpl_fclose): Invoke _gl_unregister_fd, not
unregister_shadow_fd. Improve comments.
* lib/sockets.c (close_fd_maybe_socket): Add comments. Reported by
Eric Blake.

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/sockets.c b/lib/sockets.c
index 42b8f9ea58..53cb66e468 100644
--- a/lib/sockets.c
+++ b/lib/sockets.c
@@ -37,6 +37,10 @@ close_fd_maybe_socket (const struct fd_hook *remaining_list,
                        gl_close_fn primary,
                        int fd)
 {
+  /* Note about multithread-safety: There is a race condition where, between
+     our calls to closesocket() and the primary close(), some other thread
+     could make system calls that allocate precisely the same HANDLE value
+     as sock; then the primary close() would call CloseHandle() on it.  */
   SOCKET sock;
   WSANETWORKEVENTS ev;