Fixed BUG#51763 Can't delete rows from MEMORY table with HASH key

mysql-test/suite/heap/heap.result:
  Added test case
mysql-test/suite/heap/heap.test:
  Added test case
storage/heap/hp_delete.c:
  Fixed that we don't change order of keys for the current key when we delete them from the hash table.
  Fixed that 'current_hash_ptr' is correct after heap_delete_key().
  Don't "reset current_hash_ptr" on delete; This will improve time a lot for delete of rows when not all rows matches the search criteria.

mysql-test/suite/heap/heap.result

@@ -790,3 +790,22 @@ select data_length,index_length from information_schema.tables where table_schem
 data_length	index_length
 81024	121024
 drop table t1;
+CREATE TABLE t1 (
+id int(11) NOT NULL AUTO_INCREMENT,
+color enum('GREEN', 'WHITE')  DEFAULT NULL,
+ts int,
+PRIMARY KEY (id),
+KEY color (color) USING HASH
+) ENGINE=MEMORY DEFAULT CHARSET=utf8;
+INSERT INTO t1 VALUES("1","GREEN",1);
+INSERT INTO t1 VALUES("2","GREEN",1);
+INSERT INTO t1 VALUES("3","GREEN",1);
+INSERT INTO t1 VALUES("4","GREEN",1);
+INSERT INTO t1 VALUES("5","GREEN",1);
+INSERT INTO t1 VALUES("6","GREEN",1);
+DELETE FROM t1 WHERE id = 1;
+INSERT INTO t1 VALUES("7","GREEN", 2);
+DELETE FROM t1 WHERE ts = 1 AND color = 'GREEN';
+SELECT * from t1 WHERE ts = 1 AND color = 'GREEN';
+id	color	ts
+DROP TABLE t1;

mysql-test/suite/heap/heap.test

@@ -537,3 +537,27 @@ insert into t1 select rand(100000000) from t1;
 --replace_result 40512 81024 60512 121024
 select data_length,index_length from information_schema.tables where table_schema="test" and table_name="t1";
 drop table t1;
+
+#
+# BUG#51763 Can't delete rows from MEMORY table with HASH key
+#
+
+CREATE TABLE t1 (
+  id int(11) NOT NULL AUTO_INCREMENT,
+  color enum('GREEN', 'WHITE')  DEFAULT NULL,
+  ts int,
+  PRIMARY KEY (id),
+  KEY color (color) USING HASH
+) ENGINE=MEMORY DEFAULT CHARSET=utf8;
+
+INSERT INTO t1 VALUES("1","GREEN",1);
+INSERT INTO t1 VALUES("2","GREEN",1);
+INSERT INTO t1 VALUES("3","GREEN",1);
+INSERT INTO t1 VALUES("4","GREEN",1);
+INSERT INTO t1 VALUES("5","GREEN",1);
+INSERT INTO t1 VALUES("6","GREEN",1);
+DELETE FROM t1 WHERE id = 1;
+INSERT INTO t1 VALUES("7","GREEN", 2);
+DELETE FROM t1 WHERE ts = 1 AND color = 'GREEN';
+SELECT * from t1 WHERE ts = 1 AND color = 'GREEN';
+DROP TABLE t1;

storage/heap/hp_delete.c

@@ -48,7 +48,6 @@ int heap_delete(HP_INFO *info, const uchar *record)
   pos[share->reclength]=0;		/* Record deleted */
   share->deleted++;
   share->key_version++;
-  info->current_hash_ptr=0;
 #if !defined(DBUG_OFF) && defined(EXTRA_HEAP_DEBUG)
   DBUG_EXECUTE("check_heap",heap_check_heap(info, 0););
 #endif
@@ -182,21 +181,50 @@ int hp_delete_key(HP_INFO *info, register HP_KEYDEF *keyinfo,
   }
   pos2= hp_mask(lastpos_hashnr, blength, share->records + 1);
   if (pos2 == hp_mask(pos_hashnr, blength, share->records + 1))
-  {					/* Identical key-positions */
+  {
+    /* lastpos and the row in the main bucket entry (pos) has the same hash */ 
     if (pos2 != share->records)
     {
-      empty[0]=lastpos[0];
+      /*
+        The bucket entry was not deleted. Copy lastpos over the
+        deleted entry and update previous link to point to it.
+      */
+      empty[0]= lastpos[0];
       hp_movelink(lastpos, pos, empty);
+      if (last_ptr == lastpos)
+      {
+        /*
+          We moved the row that info->current_hash_ptr points to.
+          Update info->current_hash_ptr to point to the new position.
+        */
+        info->current_hash_ptr= empty;
+      }
       DBUG_RETURN(0);
     }
-    pos3= pos;				/* Link pos->next after lastpos */
-  }
-  else
-  {
-    pos3= 0;				/* Different positions merge */
-    keyinfo->hash_buckets--;
+    /*
+      Shrinking the hash table deleted the main bucket entry for this hash.
+      In this case the last entry was the first key in the key chain.
+      We move things around so that we keep the original key order to ensure
+      that heap_rnext() works.
+      
+      - Move the row at the main bucket entry to the empty spot.
+      - Move the last entry first in the new chain.
+      - Link in the first element of the hash.
+    */
+    empty[0]= pos[0];
+    pos[0]= lastpos[0];
+    hp_movelink(pos, pos, empty);
+
+    /* Update current_hash_ptr if the entry moved */
+    if (last_ptr == lastpos)
+      info->current_hash_ptr= pos;
+    else if (last_ptr == pos)
+      info->current_hash_ptr= empty;
+    DBUG_RETURN(0);
   }
 
+  pos3= 0;				/* Different positions merge */
+  keyinfo->hash_buckets--;
   empty[0]=lastpos[0];
   hp_movelink(pos3, empty, pos->next_key);
   pos->next_key=empty;