MDEV-4566 : Failing DBUG_ASSERT() in SELECT SLEEP(), with threadpool.

This bug only happens with long sleep()s ( > 5 sec), and in debug version.

Analysis: 
The assertion is caused by nested thd_wait_begin() calls, which is not an expected condition.
-  "outer" thd_wait_begin()) , in Item_func_sleep::val_int()
-  "inner" thd_wait_begin() in Interruptible_wait::wait(). This function periodically checks whether connection is still valid, via THD::is_connection(), which ends up  calling vio_io_wait()  with timeout parameter set to  0.


Fix is not to call thd wait callback in vio_io_wait(), if timeout parameter is 0. There is no "waiting" in this case.

mysql-test/r/pool_of_threads.result

@@ -2157,10 +2157,10 @@ Warnings:
 Warning	1052	Column 'kundentyp' in group statement is ambiguous
 drop table t1;
 SET optimizer_switch=@save_optimizer_switch;
-SELECT sleep(5);
+SELECT sleep(5.5);
 SELECT sleep(5);
 # -- Success: more than --thread_pool_max_threads normal connections not possible
-sleep(5)
+sleep(5.5)
 0
 sleep(5)
 0

mysql-test/t/pool_of_threads.test

@@ -17,7 +17,10 @@ SET optimizer_switch=@save_optimizer_switch;
 # First set two connections running, and check that extra connection
 # on normal port fails due to--thread-pool-max_threads=2
 connection default;
-send SELECT sleep(5);
+
+# Sleep for slightly longer than 5 sec to trigger MDEV-4566 
+# (abort in interruptible wait connection check)
+send SELECT sleep(5.5); 
 --sleep  1 
 
 connect(con2,localhost,root,,);

vio/viosocket.c

@@ -42,21 +42,21 @@ static void (*before_io_wait)(void)= 0;
 static void (*after_io_wait)(void)= 0;
 
 /* Wait callback macros (both performance schema and threadpool */
-#define START_SOCKET_WAIT(locker, state_ptr, sock, which) \
+#define START_SOCKET_WAIT(locker, state_ptr, sock, which, timeout) \
 do                                                        \
 {                                                         \
   MYSQL_START_SOCKET_WAIT(locker, state_ptr, sock,        \
                             which, 0);                    \
-  if (before_io_wait)                                     \
+  if (timeout && before_io_wait)                          \
     before_io_wait();                                     \
 } while(0)
 
 
-#define END_SOCKET_WAIT(locker)                          \
+#define END_SOCKET_WAIT(locker,timeout)                  \
 do                                                       \
 {                                                        \
   MYSQL_END_SOCKET_WAIT(locker, 0);                      \
-  if (after_io_wait)                                     \
+  if (timeout && after_io_wait)                          \
     after_io_wait();                                     \
 } while(0)
 
@@ -930,11 +930,11 @@ int vio_io_wait(Vio *vio, enum enum_vio_io_event event, int timeout)
   if (timeout != 0 && vio->async_context && vio->async_context->active)
   {
     START_SOCKET_WAIT(locker, &state, vio->mysql_socket,
-                            PSI_SOCKET_SELECT);
+                            PSI_SOCKET_SELECT, timeout);
     ret= my_io_wait_async(vio->async_context, event, timeout);
     if (ret == 0)
       errno= SOCKET_ETIMEDOUT;
-    END_SOCKET_WAIT(locker);
+    END_SOCKET_WAIT(locker,timeout);
     DBUG_RETURN(ret);
   }
 
@@ -959,7 +959,7 @@ int vio_io_wait(Vio *vio, enum enum_vio_io_event event, int timeout)
     break;
   }
 
-  START_SOCKET_WAIT(locker, &state, vio->mysql_socket, PSI_SOCKET_SELECT);
+  START_SOCKET_WAIT(locker, &state, vio->mysql_socket, PSI_SOCKET_SELECT, timeout);
   /*
     Wait for the I/O event and return early in case of
     error or timeout.
@@ -982,7 +982,7 @@ int vio_io_wait(Vio *vio, enum enum_vio_io_event event, int timeout)
     break;
   }
 
-  END_SOCKET_WAIT(locker);
+  END_SOCKET_WAIT(locker, timeout);
   DBUG_RETURN(ret);
 }
 
@@ -1004,11 +1004,11 @@ int vio_io_wait(Vio *vio, enum enum_vio_io_event event, int timeout)
   if (timeout != 0 && vio->async_context && vio->async_context->active)
   {
     START_SOCKET_WAIT(locker, &state, vio->mysql_socket,
-                            PSI_SOCKET_SELECT);
+                            PSI_SOCKET_SELECT, timeout);
     ret= my_io_wait_async(vio->async_context, event, timeout);
     if (ret == 0)
       WSASetLastError(SOCKET_ETIMEDOUT);
-    END_SOCKET_WAIT(locker);
+    END_SOCKET_WAIT(locker, timeout);
     DBUG_RETURN(ret);
   }
 
@@ -1039,12 +1039,12 @@ int vio_io_wait(Vio *vio, enum enum_vio_io_event event, int timeout)
     break;
   }
 
-  START_SOCKET_WAIT(locker, &state, vio->mysql_socket, PSI_SOCKET_SELECT);
+  START_SOCKET_WAIT(locker, &state, vio->mysql_socket, PSI_SOCKET_SELECT, timeout);
 
   /* The first argument is ignored on Windows. */
   ret= select(0, &readfds, &writefds, &exceptfds, (timeout >= 0) ? &tm : NULL);
 
-  END_SOCKET_WAIT(locker);
+  END_SOCKET_WAIT(locker, timeout);
 
   /* Set error code to indicate a timeout error. */
   if (ret == 0)