Fixed bug #30201.

Recommit to 5.1.22.
Killing a SELECT query with KILL QUERY or KILL CONNECTION
causes a server crash if the query cache is enabled.

Normal evaluation of a query may be interrupted by the
KILL QUERY/CONNECTION statement, in this case the mysql_execute_command
function returns TRUE, and the thd->killed flag has true value.
In this case the result of the query may
be cached incompletely (omitting call to query_cache_insert inside
the net_real_write function), and next call to query_cache_end_of_result
may lead to server crash.
Thus, the query_cache_end_of_result function has been modified to abort
query cache in the case of killed thread.

sql/sql_cache.cc

@@ -749,6 +749,12 @@ void query_cache_end_of_result(THD *thd)
   if (thd->net.query_cache_query == 0)
     DBUG_VOID_RETURN;
 
+  if (thd->killed)
+  {
+    query_cache_abort(&thd->net);
+    DBUG_VOID_RETURN;
+  }
+
 #ifdef EMBEDDED_LIBRARY
   query_cache_insert(&thd->net, (char*)thd, 
                      emb_count_querycache_size(thd));
@@ -775,28 +781,31 @@ void query_cache_end_of_result(THD *thd)
     DUMP(&query_cache);
     BLOCK_LOCK_WR(query_block);
     Query_cache_query *header= query_block->query();
-    Query_cache_block *last_result_block= header->result()->prev;
-    ulong allign_size= ALIGN_SIZE(last_result_block->used);
-    ulong len= max(query_cache.min_allocation_unit, allign_size);
-    if (last_result_block->length >= query_cache.min_allocation_unit + len)
-      query_cache.split_block(last_result_block,len);
+    Query_cache_block *last_result_block;
+    ulong allign_size;
+    ulong len;
 
-#ifndef DBUG_OFF
     if (header->result() == 0)
     {
-      DBUG_PRINT("error", ("end of data whith no result. query '%s'",
-                           header->query()));
-      query_cache.wreck(__LINE__, "");
-
+      DBUG_PRINT("error", ("End of data with no result blocks; "
+                           "Query '%s' removed from cache.", header->query()));
       /*
-        We do not need call of BLOCK_UNLOCK_WR(query_block); here because
-        query_cache.wreck() switched query cache off but left content
-        untouched for investigation (it is debugging method).
+        Extra safety: empty result should not happen in the normal call
+        to this function. In the release version that query should be ignored
+        and removed from QC.
       */
+      DBUG_ASSERT(0);
+      query_cache.free_query(query_block);
       STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
       DBUG_VOID_RETURN;
     }
-#endif
+
+    last_result_block= header->result()->prev;
+    allign_size= ALIGN_SIZE(last_result_block->used);
+    len= max(query_cache.min_allocation_unit, allign_size);
+    if (last_result_block->length >= query_cache.min_allocation_unit + len)
+      query_cache.split_block(last_result_block,len);
+
     header->found_rows(current_thd->limit_found_rows);
     header->result()->type= Query_cache_block::RESULT;