Optimised fcntl() when accepting connections

Removed FD_CLOEXEC setting: already done by mysql_socket_accept().

Moved O_NONBLOCK setting out of accepting loop.

Removed blocking acceptance attempt, which was dead code. It was supposed
to be executed before the last iteration, however it was actually executed
during the last iteration. And it is not correct to block on one socket
leaving other sockets unattended anyway.

Gives ~5% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

sql/mysqld.cc

@@ -6301,10 +6301,6 @@ void handle_accepted_socket(MYSQL_SOCKET new_sock, MYSQL_SOCKET sock)
   CONNECT *connect;
   bool is_unix_sock;
 
-#ifdef FD_CLOEXEC
-  (void) fcntl(mysql_socket_getfd(new_sock), F_SETFD, FD_CLOEXEC);
-#endif
-
 #ifdef HAVE_LIBWRAP
   {
     if (mysql_socket_getfd(sock) == mysql_socket_getfd(base_ip_sock) ||
@@ -6387,16 +6383,29 @@ void handle_accepted_socket(MYSQL_SOCKET new_sock, MYSQL_SOCKET sock)
 }
 
 #ifndef _WIN32
+static void set_non_blocking_if_supported(MYSQL_SOCKET sock)
+{
+#if !defined(NO_FCNTL_NONBLOCK)
+  if (!(test_flags & TEST_BLOCKING))
+  {
+    int flags= fcntl(mysql_socket_getfd(sock), F_GETFL, 0);
+#if defined(O_NONBLOCK)
+    fcntl(mysql_socket_getfd(sock), F_SETFL, flags | O_NONBLOCK);
+#elif defined(O_NDELAY)
+    fcntl(mysql_socket_getfd(sock), F_SETFL, flags | O_NDELAY);
+#endif
+  }
+#endif
+}
+
+
 void handle_connections_sockets()
 {
   MYSQL_SOCKET sock= mysql_socket_invalid();
   MYSQL_SOCKET new_sock= mysql_socket_invalid();
   uint error_count=0;
   struct sockaddr_storage cAddr;
-  int ip_flags __attribute__((unused))=0;
-  int socket_flags __attribute__((unused))= 0;
-  int extra_ip_flags __attribute__((unused))=0;
-  int flags=0,retval;
+  int retval;
 #ifdef HAVE_POLL
   int socket_count= 0;
   struct pollfd fds[3]; // for ip_sock, unix_sock and extra_ip_sock
@@ -6418,16 +6427,16 @@ void handle_connections_sockets()
   if (mysql_socket_getfd(base_ip_sock) != INVALID_SOCKET)
   {
     setup_fds(base_ip_sock);
-    ip_flags = fcntl(mysql_socket_getfd(base_ip_sock), F_GETFL, 0);
+    set_non_blocking_if_supported(base_ip_sock);
   }
   if (mysql_socket_getfd(extra_ip_sock) != INVALID_SOCKET)
   {
     setup_fds(extra_ip_sock);
-    extra_ip_flags = fcntl(mysql_socket_getfd(extra_ip_sock), F_GETFL, 0);
+    set_non_blocking_if_supported(extra_ip_sock);
   }
 #ifdef HAVE_SYS_UN_H
   setup_fds(unix_sock);
-  socket_flags=fcntl(mysql_socket_getfd(unix_sock), F_GETFL, 0);
+  set_non_blocking_if_supported(unix_sock);
 #endif
 
   sd_notify(0, "READY=1\n"
@@ -6469,39 +6478,19 @@ void handle_connections_sockets()
       if (fds[i].revents & POLLIN)
       {
         sock= pfs_fds[i];
-        flags= fcntl(mysql_socket_getfd(sock), F_GETFL, 0);
         break;
       }
     }
 #else  // HAVE_POLL
     if (FD_ISSET(mysql_socket_getfd(base_ip_sock),&readFDs))
-    {
       sock=  base_ip_sock;
-      flags= ip_flags;
-    }
     else
     if (FD_ISSET(mysql_socket_getfd(extra_ip_sock),&readFDs))
-    {
       sock=  extra_ip_sock;
-      flags= extra_ip_flags;
-    }
     else
-    {
       sock = unix_sock;
-      flags= socket_flags;
-    }
 #endif // HAVE_POLL
 
-#if !defined(NO_FCNTL_NONBLOCK)
-    if (!(test_flags & TEST_BLOCKING))
-    {
-#if defined(O_NONBLOCK)
-      fcntl(mysql_socket_getfd(sock), F_SETFL, flags | O_NONBLOCK);
-#elif defined(O_NDELAY)
-      fcntl(mysql_socket_getfd(sock), F_SETFL, flags | O_NDELAY);
-#endif
-    }
-#endif /* NO_FCNTL_NONBLOCK */
     for (uint retry=0; retry < MAX_ACCEPT_RETRY; retry++)
     {
       size_socket length= sizeof(struct sockaddr_storage);
@@ -6511,16 +6500,6 @@ void handle_connections_sockets()
       if (mysql_socket_getfd(new_sock) != INVALID_SOCKET ||
 	  (socket_errno != SOCKET_EINTR && socket_errno != SOCKET_EAGAIN))
 	break;
-#if !defined(NO_FCNTL_NONBLOCK)
-      if (!(test_flags & TEST_BLOCKING))
-      {
-	if (retry == MAX_ACCEPT_RETRY - 1)
-        {
-          // Try without O_NONBLOCK
-	  fcntl(mysql_socket_getfd(sock), F_SETFL, flags);
-        }
-      }
-#endif
     }
 
     if (mysql_socket_getfd(new_sock) == INVALID_SOCKET)
@@ -6537,10 +6516,6 @@ void handle_connections_sockets()
 	sleep(1);				// Give other threads some time
       continue;
     }
-#if !defined(NO_FCNTL_NONBLOCK)
-	if (!(test_flags & TEST_BLOCKING))
-		fcntl(mysql_socket_getfd(sock), F_SETFL, flags);
-#endif
     handle_accepted_socket(new_sock, sock);
   }
   sd_notify(0, "STOPPING=1\n"