Merge pull request #43 from gratipay/fix-race-condition

Try to handle changes to type definitions

postgres/__init__.py

@@ -180,6 +180,7 @@
 from postgres.cursors import SimpleTupleCursor, SimpleNamedTupleCursor
 from postgres.cursors import SimpleDictCursor, SimpleCursorBase
 from postgres.orm import Model
+from psycopg2 import DataError
 from psycopg2.extras import register_composite, CompositeCaster
 from psycopg2.pool import ThreadedConnectionPool as ConnectionPool
 
@@ -824,7 +825,22 @@ def make_DelegatingCaster(postgres):
 
     """
     class DelegatingCaster(CompositeCaster):
+
+        def parse(self, s, curs, retry=True):
+            # Override to protect against race conditions:
+            #   https://github.com/gratipay/postgres.py/issues/26
+
+            try:
+                return super(DelegatingCaster, self).parse(s, curs)
+            except (DataError, ValueError):
+                if not retry:
+                    raise
+                # Re-fetch the type info and retry once
+                self._refetch_type_info(curs)
+                return self.parse(s, curs, False)
+
         def make(self, values):
+            # Override to delegate to the model registry.
             if self.name not in postgres.model_registry:
 
                 # This is probably a bug, not a normal user error. It means
@@ -838,6 +854,12 @@ def make(self, values):
             instance = ModelSubclass(record)
             return instance
 
+        def _refetch_type_info(self, curs):
+            """Given a cursor, update the current object with a fresh type definition.
+            """
+            new_self = self._from_db(self.name, curs)
+            self.__dict__.update(new_self.__dict__)
+
     return DelegatingCaster
 
 

tests.py

@@ -8,7 +8,7 @@
 from postgres.cursors import TooFew, TooMany, SimpleDictCursor
 from postgres.orm import ReadOnly, Model
 from psycopg2 import InterfaceError, ProgrammingError
-from pytest import raises
+from pytest import mark, raises
 
 
 DATABASE_URL = os.environ['DATABASE_URL']
@@ -334,6 +334,32 @@ def test_unregister_unregisters_multiple(self):
         self.db.unregister_model(self.MyModel)
         assert self.db.model_registry == {}
 
+    def test_add_column_doesnt_break_anything(self):
+        self.db.run("ALTER TABLE foo ADD COLUMN boo text")
+        one = self.db.one("SELECT foo.*::foo FROM foo WHERE bar='baz'")
+        assert one.boo is None
+
+    def test_replace_column_different_type(self):
+        self.db.run("CREATE TABLE grok (bar int)")
+        self.db.run("INSERT INTO grok VALUES (0)")
+        class EmptyModel(Model): pass
+        self.db.register_model(EmptyModel, 'grok')
+        # Add a new column then drop the original one
+        self.db.run("ALTER TABLE grok ADD COLUMN biz text NOT NULL DEFAULT 'x'")
+        self.db.run("ALTER TABLE grok DROP COLUMN bar")
+        # The number of columns hasn't changed but the names and types have
+        one = self.db.one("SELECT grok.*::grok FROM grok LIMIT 1")
+        assert one.biz == 'x'
+        assert not hasattr(one, 'bar')
+
+    @mark.xfail(raises=AttributeError)
+    def test_replace_column_same_type_different_name(self):
+        self.db.run("ALTER TABLE foo ADD COLUMN biz text NOT NULL DEFAULT 0")
+        self.db.run("ALTER TABLE foo DROP COLUMN bar")
+        one = self.db.one("SELECT foo.*::foo FROM foo LIMIT 1")
+        assert one.biz == 0
+        assert not hasattr(one, 'bar')
+
 
 # cursor_factory
 # ==============