Bug #45058 init_available_charsets uses double checked locking

      
As documented in the bug report, the double checked locking
pattern has inherent issues, and cannot guarantee correct
initialization.

This patch replaces the logic in init_available_charsets()
with the use of pthread_once(3). A wrapper function,
my_pthread_once(), is introduced and is used in lieu of direct
calls to init_available_charsets(). Related defines
MY_PTHREAD_ONCE_* are also introduced.

For the Windows platform, the implementation in lp:sysbench is
ported. For single-thread use, a simple define calls the
function and sets the pthread_once control variable.

Charset initialization is modified to use my_pthread_once().

1 files changed, 32 insertions, 0 deletions

diff --git a/mysys/my_winthread.c b/mysys/my_winthread.c
index e94369bec32..ef2a20c2ddc 100644
--- a/mysys/my_winthread.c
+++ b/mysys/my_winthread.c
@@ -137,4 +137,36 @@ int win_pthread_setspecific(void *a,void *b,uint length)
   return 0;
 }
 
+
+/*
+ One time initialization. For simplicity, we assume initializer thread
+ does not exit within init_routine().
+*/
+int my_pthread_once(my_pthread_once_t *once_control, 
+    void (*init_routine)(void))
+{
+  LONG state= InterlockedCompareExchange(once_control, MY_PTHREAD_ONCE_INPROGRESS,
+                                          MY_PTHREAD_ONCE_INIT);
+  switch(state)
+  {
+  case MY_PTHREAD_ONCE_INIT:
+    /* This is initializer thread */
+    (*init_routine)();
+    *once_control= MY_PTHREAD_ONCE_DONE;
+    break;
+
+  case MY_PTHREAD_ONCE_INPROGRESS:
+    /* init_routine in progress. Wait for its completion */
+    while(*once_control == MY_PTHREAD_ONCE_INPROGRESS)
+    {
+      Sleep(1);
+    }
+    break;
+  case MY_PTHREAD_ONCE_DONE:
+    /* Nothing to do */
+    break;
+  }
+  return 0;
+}
+
 #endif