Cygwin: AF_UNIX: rework fixup_after_exec

fhandler_socket_unix::fixup_after_exec incorrectly calls
fhandler_socket_unix::fixup_after_fork with a NULL parent process
handle.  Not only that calling DuplicateHandle with a NULL parent
handle fails, but it's utterly wrong trying to duplicate the handles
at all here.

Rather just set some important values to NULL and reopen the shared
memory region.  Create a fixup_helper method to call common code from
fixup_after_fork and fixup_after_exec.

Add comments to other invocations of fixup_after_fork with NULL
handle to mark them as correct this way.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -1076,6 +1076,7 @@ class fhandler_socket_unix : public fhandler_socket
   void fixup_after_fork (HANDLE parent);
   void fixup_after_exec ();
   void set_close_on_exec (bool val);
+  void fixup_helper ();
 
  public:
   fhandler_socket_unix ();

winsup/cygwin/fhandler_socket_inet.cc

@@ -534,7 +534,7 @@ void
 fhandler_socket_wsock::fixup_after_exec ()
 {
   if (need_fixup_before () && !close_on_exec ())
-    fixup_after_fork (NULL);
+    fixup_after_fork (NULL);	/* No parent handle required. */
 }
 
 int

winsup/cygwin/fhandler_socket_unix.cc

@@ -1149,6 +1149,16 @@ fhandler_socket_unix::set_cred ()
   scred->gid = myself->gid;
 }
 
+void
+fhandler_socket_unix::fixup_helper ()
+{
+  if (shmem_handle)
+    reopen_shmem ();
+  connect_wait_thr = NULL;
+  cwt_termination_evt = NULL;
+  cwt_param = NULL;
+}
+
 /* ========================== public methods ========================= */
 
 void
@@ -1158,20 +1168,15 @@ fhandler_socket_unix::fixup_after_fork (HANDLE parent)
   if (backing_file_handle && backing_file_handle != INVALID_HANDLE_VALUE)
     fork_fixup (parent, backing_file_handle, "backing_file_handle");
   if (shmem_handle)
-    {
     fork_fixup (parent, shmem_handle, "shmem_handle");
-      reopen_shmem ();
+  fixup_helper ();
-    }
-  connect_wait_thr = NULL;
-  cwt_termination_evt = NULL;
-  cwt_param = NULL;
 }
 
 void
 fhandler_socket_unix::fixup_after_exec ()
 {
   if (!close_on_exec ())
-    fixup_after_fork (NULL);
+    fixup_helper ();
 }
 
 void

winsup/cygwin/fhandler_tty.cc

@@ -2974,7 +2974,7 @@ fhandler_pty_slave::fixup_after_exec ()
   reset_switch_to_pcon ();
 
   if (!close_on_exec ())
-    fixup_after_fork (NULL);
+    fixup_after_fork (NULL);	/* No parent handle required. */
   else if (get_pseudo_console ())
     {
       int used = 0;