MDEV-8030 - Apc_target::disable() locks mutex twice

Moved Apc_target::destroy(), Apc_target::enable() and Apc_targe::disable()
definitions to my_apc.h so that they can be inlined.

Apc_targe::disable() now calls Apc_target::process_apc_requests() only if
there're APC requests. This saves one pthread_mutex_lock() call.

Overhead change:
Apc_target::disable              0.04% -> out of radar
Apc_target::enable               0.03% -> out of radar
Apc_target::process_apc_requests 0.02% -> out of radar
pthread_mutex_lock               0.43% -> 0.42%
pthread_mutex_unlock             0.26% -> 0.25%

sql/my_apc.cc

@@ -41,45 +41,6 @@ void Apc_target::init(mysql_mutex_t *target_mutex)
 }
 
 
-/* 
-  Destroy the target. The target must be disabled when this call is made.
-*/
-void Apc_target::destroy()
-{
-  DBUG_ASSERT(!enabled);
-}
-
-
-/* 
-  Enter ther state where the target is available for serving APC requests
-*/
-void Apc_target::enable()
-{
-  /* Ok to do without getting/releasing the mutex: */
-  enabled++;
-}
-
-
-/* 
-  Make the target unavailable for serving APC requests. 
-  
-  @note
-    This call will serve all requests that were already enqueued
-*/
-
-void Apc_target::disable()
-{
-  bool process= FALSE;
-  DBUG_ASSERT(enabled);
-  mysql_mutex_lock(LOCK_thd_data_ptr);
-  if (!(--enabled))
-    process= TRUE;
-  mysql_mutex_unlock(LOCK_thd_data_ptr);
-  if (process)
-    process_apc_requests();
-}
-
-
 /* [internal] Put request qe into the request list */
 
 void Apc_target::enqueue_request(Call_request *qe)

sql/my_apc.h

@@ -50,10 +50,29 @@ public:
   ~Apc_target() { DBUG_ASSERT(!enabled && !apc_calls);}
 
   void init(mysql_mutex_t *target_mutex);
-  void destroy();
-  void enable();
-  void disable();
-  
+
+  /* Destroy the target. The target must be disabled when this call is made. */
+  void destroy() { DBUG_ASSERT(!enabled); }
+
+  /* Enter ther state where the target is available for serving APC requests */
+  void enable() { enabled++; }
+
+  /*
+    Make the target unavailable for serving APC requests.
+
+    @note
+      This call will serve all requests that were already enqueued
+  */
+  void disable()
+  {
+    DBUG_ASSERT(enabled);
+    mysql_mutex_lock(LOCK_thd_data_ptr);
+    bool process= !--enabled && have_apc_requests();
+    mysql_mutex_unlock(LOCK_thd_data_ptr);
+    if (unlikely(process))
+      process_apc_requests();
+  }
+
   void process_apc_requests();
   /* 
     A lightweight function, intended to be used in frequent checks like this: