Try to improve tls disconnect behavior where there are networking issues

ChangeLog

@@ -1,3 +1,11 @@
+2021-12-07 Richard Frith-Macdonald <rfm@gnu.org>
+
+	* Source/GSTLS.m: (-disconnect:) try once if we are closing the TCP
+	connection, for up to 10 seconds if trying to keep it available.
+	* Source/NSFileHandle.m: (-sslDisconnect) operate in non-blocking
+	mode so that network issues should not cause a disconnect attempt
+	to hang. This should behave better on heavily loaded systems.
+
 2021-11-23 Richard Frith-Macdonald <rfm@gnu.org>
 
 	* configure.ac: check for zlib before bfd

Source/GSTLS.m

@@ -1592,26 +1592,36 @@ retrieve_callback(gnutls_session_t session,
 
   if (YES == active || YES == handshake)
     {
+      int	result;
+
       active = NO;
       handshake = NO;
       if (NO == reusable)
         {
-          gnutls_bye(session, GNUTLS_SHUT_WR);
+	  /* Since the connection is not reusable, we need only try once.
+	   */
+          result = gnutls_bye(session, GNUTLS_SHUT_WR);
         }
       else
         {
-          int   result;
+	  NSTimeInterval	start;
 
+	  /* Attempting to do a clean shutdown on a reusable connection,
+	   * so we keep retrying for a little while to let the other end
+	   * close down cleanly.
+	   */
+	  start = [NSDate timeIntervalSinceReferenceDate];
           do
             {
               result = gnutls_bye(session, GNUTLS_SHUT_RDWR);
             }
-          while (GNUTLS_E_AGAIN == result || GNUTLS_E_INTERRUPTED == result);
-          if (result < 0)
-            {
-              ok = NO;
-            }
+          while ((GNUTLS_E_AGAIN == result || GNUTLS_E_INTERRUPTED == result)
+	    && ([NSDate timeIntervalSinceReferenceDate] - start) < 10.0);
         }
+      if (result < 0)
+	{
+	  ok = NO;
+	}
     }
   if (YES == setup)
     {

Source/NSFileHandle.m

@@ -1034,7 +1034,14 @@ GSTLSHandlePush(gnutls_transport_ptr_t handle, const void *buffer, size_t len)
 
 - (void) sslDisconnect
 {
-  [self setNonBlocking: NO];
+  /* When disconnecting, since the TCP/IP connection is not going to be
+   * re-used, we can use non-blocking I/O and abandon the cleanup in the
+   * TLS layer if the I/O cannot complete immediately.
+   * We don't want to block because a network issue (or a failure at the
+   * remote end) could tie up this thread until the 5 minute TCP/IP
+   * keepalive expires.
+   */
+  [self setNonBlocking: YES];
   [session disconnect: NO];
 }