MDEV-18662 ib_wqueue_t has a data race

ib_wqueue_is_empty(): protect ib_list_is_empty() call

Closes #1202

storage/innobase/include/ut0wqueue.h

 /*****************************************************************************
 
 Copyright (c) 2006, 2009, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation. All Rights Reserved.
+Copyright (c) 2017, 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -67,15 +67,10 @@ ib_wqueue_add(
 	mem_heap_t*	heap);	/*!< in: memory heap to use for allocating the
 				list node */
 
-/********************************************************************
-Check if queue is empty. */
-
-ibool
-ib_wqueue_is_empty(
-/*===============*/
-					/* out: TRUE if queue empty
-					else FALSE */
-	const ib_wqueue_t*      wq);    /* in: work queue */
+/** Check if queue is empty.
+@param wq wait queue
+@return whether the queue is empty */
+bool ib_wqueue_is_empty(ib_wqueue_t* wq);
 
 /****************************************************************//**
 Wait for a work item to appear in the queue.

storage/innobase/ut/ut0wqueue.cc

 /*****************************************************************************
 
 Copyright (c) 2006, 2011, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -193,17 +194,15 @@ ib_wqueue_nowait(
 	return (node ? node->data : NULL);
 }
 
-/********************************************************************
-Check if queue is empty. */
-
-ibool
-ib_wqueue_is_empty(
-/*===============*/
-					/* out: TRUE if queue empty
-					else FALSE */
-	const ib_wqueue_t*	wq)	/* in: work queue */
+/** Check if queue is empty.
+@param wq wait queue
+@return whether the queue is empty */
+bool ib_wqueue_is_empty(ib_wqueue_t* wq)
 {
-	return(ib_list_is_empty(wq->items));
+	mutex_enter(&wq->mutex);
+	bool is_empty = ib_list_is_empty(wq->items);
+	mutex_exit(&wq->mutex);
+	return is_empty;
 }
 
 /********************************************************************

storage/xtradb/include/ut0wqueue.h

 /*****************************************************************************
 
 Copyright (c) 2006, 2009, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation. All Rights Reserved.
+Copyright (c) 2017, 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -67,15 +67,10 @@ ib_wqueue_add(
 	mem_heap_t*	heap);	/*!< in: memory heap to use for allocating the
 				list node */
 
-/********************************************************************
-Check if queue is empty. */
-
-ibool
-ib_wqueue_is_empty(
-/*===============*/
-					/* out: TRUE if queue empty
-					else FALSE */
-	const ib_wqueue_t*      wq);    /* in: work queue */
+/** Check if queue is empty.
+@param wq wait queue
+@return whether the queue is empty */
+bool ib_wqueue_is_empty(ib_wqueue_t* wq);
 
 /****************************************************************//**
 Wait for a work item to appear in the queue.

storage/xtradb/ut/ut0wqueue.cc

 /*****************************************************************************
 
 Copyright (c) 2006, 2011, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -193,17 +194,15 @@ ib_wqueue_nowait(
 	return (node ? node->data : NULL);
 }
 
-/********************************************************************
-Check if queue is empty. */
-
-ibool
-ib_wqueue_is_empty(
-/*===============*/
-					/* out: TRUE if queue empty
-					else FALSE */
-	const ib_wqueue_t*	wq)	/* in: work queue */
+/** Check if queue is empty.
+@param wq wait queue
+@return whether the queue is empty */
+bool ib_wqueue_is_empty(ib_wqueue_t* wq)
 {
-	return(ib_list_is_empty(wq->items));
+	mutex_enter(&wq->mutex);
+	bool is_empty = ib_list_is_empty(wq->items);
+	mutex_exit(&wq->mutex);
+	return is_empty;
 }
 
 /********************************************************************