MDEV-246 - Aborted_clients incremented during ordinary connection close

The problem was increment of aborted_threads variable due to thd->killed which was set when threadpool connection was terminated .  The fix is not to set thd->killed anymore, there is no real reason for doing it..

Added a test that checks that status variable aborted_clients does not grow for ordinary disconnects, and that successful KILL increments this variable.

mysql-test/r/aborted_clients.result

@@ -0,0 +1,10 @@
+FLUSH STATUS;
+SELECT VARIABLE_VALUE FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME='aborted_clients';
+VARIABLE_VALUE
+0
+KILL CONNECTION_ID();
+ERROR 70100: Connection was killed
+SELECT VARIABLE_VALUE FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME='aborted_clients';
+VARIABLE_VALUE
+1
+FLUSH STATUS;

mysql-test/t/aborted_clients.test

@@ -0,0 +1,26 @@
+# Test case for MDEV-246, lp:992983
+# Check that ordinary connect/disconnect does not increase aborted_clients
+# status variable, but KILL connection does
+
+-- source include/not_embedded.inc
+
+FLUSH STATUS;
+# Connect/Disconnect look that aborted_clients stays 0
+connect (con1,localhost,root,,);
+disconnect con1;
+connection default;
+# Check that there is 0 aborted clients so far
+SELECT VARIABLE_VALUE FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME='aborted_clients';
+
+# Kill a connection, check that aborted_clients is incremented
+connect(con2,localhost,root,,); 
+--disable_reconnect
+--error ER_CONNECTION_KILLED
+KILL CONNECTION_ID();
+disconnect con2;
+connection default;
+
+# aborted clients must be 1 now
+SELECT VARIABLE_VALUE FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME='aborted_clients';
+FLUSH STATUS;
+

sql/threadpool_common.cc

@@ -167,7 +167,6 @@ void threadpool_remove_connection(THD *thd)
   worker_context.save();
 
   thread_attach(thd);
-  thd->killed= KILL_CONNECTION;
   thd->net.reading_or_writing= 0;
 
   end_connection(thd);

sql/threadpool_win.cc

@@ -573,6 +573,10 @@ static VOID CALLBACK io_completion_callback(PTP_CALLBACK_INSTANCE instance,
   }
 
   connection_t *connection = (connection_t*)context;
+
+  if (io_result != ERROR_SUCCESS)
+    goto error;
+
   THD *thd= connection->thd;
   ulonglong old_timeout = connection->timeout;
   connection->timeout = ULONGLONG_MAX;