MDEV-8617: Multiple galera tests failures with --ps-protocol

In galera cluster, when myisam replication is enabled
(wsrep_replicate_myisam=ON), DML statements are replicated
in open_tables(). However, in case of prepared statements,
for an INSERT, open_tables() gets invoked twice. Once for
COM_STMT_PREPARE (to validate and prepare INSERT) and later
for COM_STMT_EXECUTE. As a result, the command gets replicated
twice. Same happens for REPLACE, UPDATE and DELETE commands.
Fixed by adding a check to not replicate during 'prepare'
phase. Also changed the order of conditions to make it more
efficient. Lastly, in order to support wsrep_dirty_reads, made
changes to allow COM_STMT_XXX commands to continue past initial
check even when wsrep is not ready.

sql/sql_base.cc

@@ -5226,20 +5226,22 @@ bool open_tables(THD *thd, TABLE_LIST **start, uint *counter, uint flags,
     }
   }
 #ifdef WITH_WSREP
-  if ((thd->lex->sql_command== SQLCOM_INSERT         ||
-       thd->lex->sql_command== SQLCOM_INSERT_SELECT  ||
-       thd->lex->sql_command== SQLCOM_REPLACE        ||
-       thd->lex->sql_command== SQLCOM_REPLACE_SELECT ||
-       thd->lex->sql_command== SQLCOM_UPDATE         ||
-       thd->lex->sql_command== SQLCOM_UPDATE_MULTI   ||
-       thd->lex->sql_command== SQLCOM_LOAD           ||
-       thd->lex->sql_command== SQLCOM_DELETE)        &&
-      wsrep_replicate_myisam                         &&
-      (*start)                                       &&
-      (*start)->table && (*start)->table->file->ht->db_type == DB_TYPE_MYISAM)
-    {
-      WSREP_TO_ISOLATION_BEGIN(NULL, NULL, (*start));
-    }
+  if (wsrep_replicate_myisam                               &&
+      (*start)                                             &&
+      (*start)->table                                      &&
+      (*start)->table->file->ht->db_type == DB_TYPE_MYISAM &&
+      thd->get_command() != COM_STMT_PREPARE               &&
+      ((thd->lex->sql_command == SQLCOM_INSERT             ||
+        thd->lex->sql_command == SQLCOM_INSERT_SELECT      ||
+        thd->lex->sql_command == SQLCOM_REPLACE            ||
+        thd->lex->sql_command == SQLCOM_REPLACE_SELECT     ||
+        thd->lex->sql_command == SQLCOM_UPDATE             ||
+        thd->lex->sql_command == SQLCOM_UPDATE_MULTI       ||
+        thd->lex->sql_command == SQLCOM_LOAD               ||
+        thd->lex->sql_command == SQLCOM_DELETE)))
+  {
+    WSREP_TO_ISOLATION_BEGIN(NULL, NULL, (*start));
+  }
  error:
 #endif
 

sql/sql_class.h

@@ -3076,6 +3076,8 @@ class THD :public Statement,
 public:
   /** Overloaded to guard query/query_length fields */
   virtual void set_statement(Statement *stmt);
+  inline enum enum_server_command get_command() const
+  { return command; }
 
   /**
     Assign a new value to thd->query and thd->query_id and mysys_var.

sql/sql_parse.cc

@@ -862,6 +862,11 @@ bool do_command(THD *thd)
         command != COM_SLEEP        &&
         command != COM_STATISTICS   &&
         command != COM_TIME         &&
+        command != COM_STMT_PREPARE &&
+        command != COM_STMT_SEND_LONG_DATA &&
+        command != COM_STMT_EXECUTE &&
+        command != COM_STMT_RESET   &&
+        command != COM_STMT_CLOSE   &&
         command != COM_END
     ) {
       my_message(ER_UNKNOWN_COM_ERROR,