BUG#21051: RESET QUERY CACHE very slow when query_cache_type=0

There were two problems: RESET QUERY CACHE took a long time to complete
and other threads were blocked during this time.

The patch does three things:
  1 fixes a bug with improper use of test-lock-test_again technique.
      AKA Double-Checked Locking is applicable here only in few places.
  2 Somewhat improves performance of RESET QUERY CACHE.
      Do my_hash_reset() instead of deleting elements one by one.  Note
      however that the slowdown also happens when inserting into sorted
      list of free blocks, should be rewritten using balanced tree.
  3 Makes RESET QUERY CACHE non-blocking.
      The patch adjusts the locking protocol of the query cache in the
      following way: it introduces a flag flush_in_progress, which is
      set when Query_cache::flush_cache() is in progress.  This call
      sets the flag on enter, and then releases the lock.  Every other
      call is able to acquire the lock, but does nothing if
      flush_in_progress is set (as if the query cache is disabled).
      The only exception is the concurrent calls to
      Query_cache::flush_cache(), that are blocked until the flush is
      over.  When leaving Query_cache::flush_cache(), the lock is
      acquired and the flag is reset, and one thread waiting on
      Query_cache::flush_cache() (if any) is notified that it may
      proceed.


include/mysql_com.h:
  Add comment for NET::query_cache_query.
sql/net_serv.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query if query cache is used.
  Do not access net->query_cache_query without a lock.
sql/sql_cache.cc:
  Fix bug with accessing query_cache_size, Query_cache_query::wri and
  thd->net.query_cache_query before acquiring the lock---leave
  double-check locking only in safe places.
  Wherever we check that cache is usable (query_cache_size > 0) we now
  also check that flush_in_progress is false, i.e. we are not in the
  middle of cache flush.
  Add Query_cache::not_in_flush_or_wait() method and use it in
  Query_cache::flush_cache(), so that threads doing cache flush will
  wait it to finish, while other threads will bypass the cache as if
  it is disabled.
  Extract Query_cache::free_query_internal() from Query_cache::free_query(),
  which does not removes elements from the hash, and use it together with
  my_hash_reset() in Query_cache::flush_cache().
sql/sql_cache.h:
  Add declarations for new members and methods.
  Make is_cacheable() a static method.
  Add query_cache_init_query() function.
sql/sql_class.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query.

sql/sql_cache.h

@@ -195,7 +195,6 @@ extern "C"
   byte *query_cache_table_get_key(const byte *record, uint *length,
 				  my_bool not_used);
 }
-void query_cache_insert(NET *thd, const char *packet, ulong length);
 extern "C" void query_cache_invalidate_by_MyISAM_filename(const char* filename);
 
 
@@ -241,6 +240,12 @@ public:
   ulong free_memory, queries_in_cache, hits, inserts, refused,
     free_memory_blocks, total_blocks, lowmem_prunes;
 
+private:
+  pthread_cond_t COND_flush_finished;
+  bool flush_in_progress;
+
+  void free_query_internal(Query_cache_block *point);
+
 protected:
   /*
     The following mutex is locked when searching or changing global
@@ -249,6 +254,12 @@ protected:
     LOCK SEQUENCE (to prevent deadlocks):
       1. structure_guard_mutex
       2. query block (for operation inside query (query block/results))
+
+    Thread doing cache flush releases the mutex once it sets
+    flush_in_progress flag, so other threads may bypass the cache as
+    if it is disabled, not waiting for reset to finish.  The exception
+    is other threads that were going to do cache flush---they'll wait
+    till the end of a flush operation.
   */
   pthread_mutex_t structure_guard_mutex;
   byte *cache;					// cache memory
@@ -358,6 +369,7 @@ protected:
     If query is cacheable return number tables in query
     (query without tables not cached)
   */
+  static
   TABLE_COUNTER_TYPE is_cacheable(THD *thd, uint32 query_len, char *query,
 				  LEX *lex, TABLE_LIST *tables_used,
 				  uint8 *tables_type);
@@ -410,6 +422,7 @@ protected:
 
   void destroy();
 
+  friend void query_cache_init_query(NET *net);
   friend void query_cache_insert(NET *net, const char *packet, ulong length);
   friend void query_cache_end_of_result(THD *thd);
   friend void query_cache_abort(NET *net);
@@ -435,6 +448,8 @@ protected:
 
 extern Query_cache query_cache;
 extern TYPELIB query_cache_type_typelib;
+void query_cache_init_query(NET *net);
+void query_cache_insert(NET *net, const char *packet, ulong length);
 void query_cache_end_of_result(THD *thd);
 void query_cache_abort(NET *net);