Bug#42649 THR_LOCK_charset global mutex abused by InnoDB

The patch was originally proposed by Mikael and reviewed by Bar.

mysys/charset.c

@@ -494,21 +494,27 @@ static CHARSET_INFO *get_internal_charset(uint cs_number, myf flags)
 {
   char  buf[FN_REFLEN];
   CHARSET_INFO *cs;
+
+  if ((cs= all_charsets[cs_number]))
+  {
+    if (cs->state & MY_CS_READY)  /* if CS is already initialized */
+        return cs;
+
     /*
       To make things thread safe we are not allowing other threads to interfere
       while we may changing the cs_info_table
     */
     pthread_mutex_lock(&THR_LOCK_charset);
-  if ((cs= all_charsets[cs_number]))
-  {
-    if (!(cs->state & MY_CS_COMPILED) && !(cs->state & MY_CS_LOADED))
+
+    if (!(cs->state & (MY_CS_COMPILED|MY_CS_LOADED))) /* if CS is not in memory */
     {
       strxmov(get_charsets_dir(buf), cs->csname, ".xml", NullS);
       my_read_charset_file(buf,flags);
     }
-    cs= (cs->state & MY_CS_AVAILABLE) ? cs : NULL;
-  }
-  if (cs && !(cs->state & MY_CS_READY))
+
+    if (cs->state & MY_CS_AVAILABLE)
+    {
+      if (!(cs->state & MY_CS_READY))
       {
         if ((cs->cset->init && cs->cset->init(cs, cs_alloc)) ||
             (cs->coll->init && cs->coll->init(cs, cs_alloc)))
@@ -516,7 +522,12 @@ static CHARSET_INFO *get_internal_charset(uint cs_number, myf flags)
         else
           cs->state|= MY_CS_READY;
       }
+    }
+    else
+      cs= NULL;
+
     pthread_mutex_unlock(&THR_LOCK_charset);
+  }
   return cs;
 }