ZMySQLDA: automatically switch isolation if the first query contains SELECT.

product/ZMySQLDA/connectionAdd.dtml

@@ -107,7 +107,9 @@
       !<em>isolation_level</em> at the begining of the connection string
       will set the transaction isolation level in each transaction. The
       value should be one of REPEATABLE-READ, READ-COMMITTED,
-      READ-UNCOMMITTED or SERIALIZABLE.
+      READ-UNCOMMITTED or SERIALIZABLE. If not specified, REPEATABLE-READ
+      is used for if the first query in a Zope transaction is SELECT query,
+      otherwise READ-COMMITTED is used.
     </dd>
     <dd>
       Transactions are highly recommended. Using a named lock in

product/ZMySQLDA/db.py

@@ -303,7 +303,7 @@ class DB(TM):
         FIELD_TYPE.TINY: "i", FIELD_TYPE.YEAR: "i",
         }
 
-    _p_oid=_p_changed=_registered=None
+    _p_oid=_p_changed=_registered=_current_isolation_level=None
 
     def __del__(self):
       if self.db is not None:
@@ -440,7 +440,6 @@ class DB(TM):
 
     def query(self, query_string, max_rows=1000):
         """Execute 'query_string' and return at most 'max_rows'."""
-        self._use_TM and self._register()
         desc = None
         if isinstance(query_string, six.text_type):
             query_string = query_string.encode('utf-8')
@@ -449,6 +448,17 @@ class DB(TM):
         # Unfortunately, MySQLdb does not want to be graceful.
         if query_string[-1:] == b';':
           query_string = query_string[:-1]
+        if self._use_TM and not self._registered:
+            if self._isolation_level:
+                self._current_isolation_level = self._isolation_level
+            else:
+                for qs in query_string.split(b'\0'):
+                    if match_select(qs.strip()):
+                        self._current_isolation_level = 'REPEATABLE-READ'
+                        break
+                else:
+                    self._current_isolation_level = 'READ-COMMITTED'
+            self._register()
         for qs in query_string.split(b'\0'):
             qs = qs.strip()
             if qs:
@@ -493,8 +503,8 @@ class DB(TM):
         try:
             self._transaction_begun = True
             if self._transactions:
-                if self._isolation_level:
-                    self._query("SET TRANSACTION ISOLATION LEVEL %s" % self._isolation_level.replace('-', ' '))
+                if self._current_isolation_level:
+                    self._query("SET TRANSACTION ISOLATION LEVEL %s" % self._current_isolation_level.replace('-', ' '))
                 self._query("BEGIN", allow_reconnect=True)
             if self._mysql_lock:
                 self._query("SELECT GET_LOCK('%s',0)" % self._mysql_lock, allow_reconnect=not self._transactions)