Bug #31890 Partitions: ORDER BY DESC in InnoDB not working.

It's not InnoDB specific bug.
Error is in QUEUE code, about the way we handle queue->max_at_top.
It's either '0' or '-2' and we do '^' operation to get the proper
direction. Though queue->compare() function can return '-2' as
a result of comparison sometimes. So we'll get
queue->compare() ^ queue->max_at_top == 0 (when max_at_top is -2)
and _downheap() function code will go wrong way here:
...
    if (next_index < elements &&
        (queue->compare(queue->first_cmp_arg,
                        queue->root[next_index]+offset_to_key,
                        queue->root[next_index+1]+offset_to_key) ^
         queue->max_at_top) > 0)
      next_index++;
...

Fixed by changing max_at_top to be either 1 or -1, doing
'* max_at_top' to get proper direction.

include/queues.h

@@ -32,7 +32,7 @@ typedef struct st_queue {
   uint elements;
   uint max_elements;
   uint offset_to_key;	/* compare is done on element+offset */
-  int max_at_top;			/* Set if queue_top gives max */
+  int max_at_top;	/* Normally 1, set to -1 if queue_top gives max */
   int  (*compare)(void *, uchar *,uchar *);
   uint auto_extent;
 } QUEUE;
@@ -43,7 +43,7 @@ typedef struct st_queue {
 #define queue_replaced(queue) _downheap(queue,1)
 #define queue_set_cmp_arg(queue, set_arg) (queue)->first_cmp_arg= set_arg
 #define queue_set_max_at_top(queue, set_arg) \
-  (queue)->max_at_top= set_arg ? (-1 ^ 1) : 0
+  (queue)->max_at_top= set_arg ? -1 : 1
 typedef int (*queue_compare)(void *,uchar *, uchar *);
 
 int init_queue(QUEUE *queue,uint max_elements,uint offset_to_key,

mysql-test/r/partition.result

@@ -1296,4 +1296,39 @@ create table t1
 partition by key(s1) partitions 3;
 insert into t1 values (null,null);
 drop table t1;
+CREATE TABLE t1
+(int_column INT, char_column CHAR(5),
+PRIMARY KEY(char_column,int_column))
+PARTITION BY KEY(char_column,int_column)
+PARTITIONS 101;
+INSERT INTO t1 (int_column, char_column) VALUES
+(      39868 ,'zZZRW'),       
+(     545592 ,'zZzSD'),       
+(       4936 ,'zzzsT'),       
+(       9274 ,'ZzZSX'),       
+(     970185 ,'ZZzTN'),       
+(     786036 ,'zZzTO'),       
+(      37240 ,'zZzTv'),       
+(     313801 ,'zzzUM'),       
+(     782427 ,'ZZZva'),       
+(     907955 ,'zZZvP'),       
+(     453491 ,'zzZWV'),       
+(     756594 ,'ZZZXU'),       
+(     718061 ,'ZZzZH');
+SELECT * FROM t1 ORDER BY char_column DESC;
+int_column	char_column
+718061	ZZzZH
+756594	ZZZXU
+453491	zzZWV
+907955	zZZvP
+782427	ZZZva
+313801	zzzUM
+37240	zZzTv
+786036	zZzTO
+970185	ZZzTN
+9274	ZzZSX
+4936	zzzsT
+545592	zZzSD
+39868	zZZRW
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/partition.test

@@ -1549,4 +1549,31 @@ while ($cnt)
 --enable_query_log
 
 drop table t1;
+
+#
+# Bug #31890 Partitions: ORDER BY DESC in InnoDB not working
+#
+
+CREATE TABLE t1
+(int_column INT, char_column CHAR(5),
+PRIMARY KEY(char_column,int_column))
+PARTITION BY KEY(char_column,int_column)
+PARTITIONS 101;
+INSERT INTO t1 (int_column, char_column) VALUES
+(      39868 ,'zZZRW'),       
+(     545592 ,'zZzSD'),       
+(       4936 ,'zzzsT'),       
+(       9274 ,'ZzZSX'),       
+(     970185 ,'ZZzTN'),       
+(     786036 ,'zZzTO'),       
+(      37240 ,'zZzTv'),       
+(     313801 ,'zzzUM'),       
+(     782427 ,'ZZZva'),       
+(     907955 ,'zZZvP'),       
+(     453491 ,'zzZWV'),       
+(     756594 ,'ZZZXU'),       
+(     718061 ,'ZZzZH');       
+SELECT * FROM t1 ORDER BY char_column DESC;
+DROP TABLE t1;
+
 --echo End of 5.1 tests

mysys/queues.c

@@ -61,7 +61,7 @@ int init_queue(QUEUE *queue, uint max_elements, uint offset_to_key,
   queue->first_cmp_arg=first_cmp_arg;
   queue->max_elements=max_elements;
   queue->offset_to_key=offset_to_key;
-  queue->max_at_top= max_at_top ? (-1 ^ 1) : 0;
+  queue_set_max_at_top(queue, max_at_top);
   DBUG_RETURN(0);
 }
 
@@ -137,7 +137,7 @@ int reinit_queue(QUEUE *queue, uint max_elements, uint offset_to_key,
   queue->compare=compare;
   queue->first_cmp_arg=first_cmp_arg;
   queue->offset_to_key=offset_to_key;
-  queue->max_at_top= max_at_top ? (-1 ^ 1) : 0;
+  queue_set_max_at_top(queue, max_at_top);
   resize_queue(queue, max_elements);
   DBUG_RETURN(0);
 }
@@ -208,16 +208,14 @@ void delete_queue(QUEUE *queue)
 void queue_insert(register QUEUE *queue, uchar *element)
 {
   reg2 uint idx, next;
-  int cmp;
   DBUG_ASSERT(queue->elements < queue->max_elements);
   queue->root[0]= element;
   idx= ++queue->elements;
   /* max_at_top swaps the comparison if we want to order by desc */
-  while ((cmp= queue->compare(queue->first_cmp_arg,
+  while ((queue->compare(queue->first_cmp_arg,
                          element + queue->offset_to_key,
                          queue->root[(next= idx >> 1)] +
-                              queue->offset_to_key)) &&
-         (cmp ^ queue->max_at_top) < 0)
+                         queue->offset_to_key) * queue->max_at_top) < 0)
   {
     queue->root[idx]= queue->root[next];
     idx= next;
@@ -287,19 +285,17 @@ void _downheap(register QUEUE *queue, uint idx)
 
   while (idx <= half_queue)
   {
-    int cmp;
     next_index=idx+idx;
     if (next_index < elements &&
 	(queue->compare(queue->first_cmp_arg,
 			queue->root[next_index]+offset_to_key,
-			queue->root[next_index+1]+offset_to_key) ^
+			queue->root[next_index+1]+offset_to_key) *
 	 queue->max_at_top) > 0)
       next_index++;
     if (first && 
-        (((cmp=queue->compare(queue->first_cmp_arg,
+        (((queue->compare(queue->first_cmp_arg,
                           queue->root[next_index]+offset_to_key,
-                              element+offset_to_key)) == 0) ||
-	 ((cmp ^ queue->max_at_top) > 0)))
+                          element+offset_to_key) * queue->max_at_top) >= 0)))
     {
       queue->root[idx]= element;
       return;
@@ -314,7 +310,7 @@ void _downheap(register QUEUE *queue, uint idx)
   {
     if ((queue->compare(queue->first_cmp_arg,
                        queue->root[next_index]+offset_to_key,
-                       element+offset_to_key) ^
+                       element+offset_to_key) *
          queue->max_at_top) < 0)
       break;
     queue->root[idx]=queue->root[next_index];
@@ -334,7 +330,6 @@ void _downheap(register QUEUE *queue, uint idx)
 {
   uchar *element;
   uint elements,half_queue,next_index,offset_to_key;
-  int cmp;
 
   offset_to_key=queue->offset_to_key;
   element=queue->root[idx];
@@ -346,13 +341,12 @@ void _downheap(register QUEUE *queue, uint idx)
     if (next_index < elements &&
 	(queue->compare(queue->first_cmp_arg,
 			queue->root[next_index]+offset_to_key,
-			queue->root[next_index+1]+offset_to_key) ^
+			queue->root[next_index+1]+offset_to_key) *
 	 queue->max_at_top) > 0)
       next_index++;
-    if ((cmp=queue->compare(queue->first_cmp_arg,
+    if ((queue->compare(queue->first_cmp_arg,
                         queue->root[next_index]+offset_to_key,
-			    element+offset_to_key)) == 0 ||
-	(cmp ^ queue->max_at_top) > 0)
+                        element+offset_to_key) * queue->max_at_top) >= 0)
       break;
     queue->root[idx]=queue->root[next_index];
     idx=next_index;