Guard transaction methods against underlying connection release

Similarly to other connection-dependent objects, transaction methods
should not be called once the underlying connection has been released to
the pool.

Also, add a special handling for the case of asynchronous generator
finalization, in which case it's OK for `Transaction.__aexit__()` to be
called _after_ `Pool.release()`, since we cannot control when the
finalization task would execute.

Fixes: #232.

asyncpg/connection.py

@@ -998,6 +998,19 @@ async def reset(self, *, timeout=None):
         self._listeners.clear()
         self._log_listeners.clear()
         reset_query = self._get_reset_query()
+
+        if self._protocol.is_in_transaction() or self._top_xact is not None:
+            if self._top_xact is None or not self._top_xact._managed:
+                # Managed transactions are guaranteed to __aexit__
+                # correctly.
+                self._loop.call_exception_handler({
+                    'message': 'Resetting connection with an '
+                               'active transaction {!r}'.format(self)
+                })
+
+            self._top_xact = None
+            reset_query = 'ROLLBACK;\n' + reset_query
+
         if reset_query:
             await self.execute(reset_query, timeout=timeout)
 
@@ -1152,13 +1165,6 @@ def _get_reset_query(self):
         caps = self._server_caps
 
         _reset_query = []
-        if self._protocol.is_in_transaction() or self._top_xact is not None:
-            self._loop.call_exception_handler({
-                'message': 'Resetting connection with an '
-                           'active transaction {!r}'.format(self)
-            })
-            self._top_xact = None
-            _reset_query.append('ROLLBACK;')
         if caps.advisory_locks:
             _reset_query.append('SELECT pg_advisory_unlock_all();')
         if caps.sql_close_all:

asyncpg/connresource.py

@@ -36,3 +36,9 @@ def _check_conn_validity(self, meth_name):
                 'cannot call {}.{}(): '
                 'the underlying connection has been released back '
                 'to the pool'.format(self.__class__.__name__, meth_name))
+
+        if self._connection.is_closed():
+            raise exceptions.InterfaceError(
+                'cannot call {}.{}(): '
+                'the underlying connection is closed'.format(
+                    self.__class__.__name__, meth_name))

asyncpg/pool.py

@@ -72,13 +72,14 @@ def __getattr__(self, attr):
         # Proxy all unresolved attributes to the wrapped Connection object.
         return getattr(self._con, attr)
 
-    def _detach(self):
+    def _detach(self) -> connection.Connection:
         if self._con is None:
             raise exceptions.InterfaceError(
                 'cannot detach PoolConnectionProxy: already detached')
 
         con, self._con = self._con, None
         con._set_proxy(None)
+        return con
 
     def __repr__(self):
         if self._con is None:
@@ -179,8 +180,6 @@ async def release(self, timeout):
         self._in_use = False
         self._timeout = None
 
-        self._con._on_release()
-
         if self._con.is_closed():
             self._con = None
 
@@ -508,7 +507,8 @@ async def _release_impl(ch: PoolConnectionHolder, timeout: float):
             # Already released, do nothing.
             return
 
-        connection._detach()
+        con = connection._detach()
+        con._on_release()
 
         if timeout is None:
             timeout = connection._holder._timeout

asyncpg/transaction.py

@@ -7,6 +7,7 @@
 
 import enum
 
+from . import connresource
 from . import exceptions as apg_errors
 
 
@@ -21,7 +22,7 @@ class TransactionState(enum.Enum):
 ISOLATION_LEVELS = {'read_committed', 'serializable', 'repeatable_read'}
 
 
-class Transaction:
+class Transaction(connresource.ConnectionResource):
     """Represents a transaction or savepoint block.
 
     Transactions are created by calling the
@@ -33,6 +34,8 @@ class Transaction:
                  '_state', '_nested', '_id', '_managed')
 
     def __init__(self, connection, isolation, readonly, deferrable):
+        super().__init__(connection)
+
         if isolation not in ISOLATION_LEVELS:
             raise ValueError(
                 'isolation is expected to be either of {}, '
@@ -49,7 +52,6 @@ def __init__(self, connection, isolation, readonly, deferrable):
                     '"deferrable" is only supported for '
                     'serializable readonly transactions')
 
-        self._connection = connection
         self._isolation = isolation
         self._readonly = readonly
         self._deferrable = deferrable
@@ -66,6 +68,22 @@ async def __aenter__(self):
         await self.start()
 
     async def __aexit__(self, extype, ex, tb):
+        try:
+            self._check_conn_validity('__aexit__')
+        except apg_errors.InterfaceError:
+            if extype is GeneratorExit:
+                # When a PoolAcquireContext is being exited, and there
+                # is an open transaction in an async generator that has
+                # not been iterated fully, there is a possibility that
+                # Pool.release() would race with this __aexit__(), since
+                # both would be in concurrent tasks.  In such case we
+                # yield to Pool.release() to do the ROLLBACK for us.
+                # See https://github.com/MagicStack/asyncpg/issues/232
+                # for an example.
+                return
+            else:
+                raise
+
         try:
             if extype is not None:
                 await self.__rollback()
@@ -74,6 +92,7 @@ async def __aexit__(self, extype, ex, tb):
         finally:
             self._managed = False
 
+    @connresource.guarded
     async def start(self):
         """Enter the transaction or savepoint block."""
         self.__check_state_base('start')
@@ -183,13 +202,15 @@ async def __rollback(self):
         else:
             self._state = TransactionState.ROLLEDBACK
 
+    @connresource.guarded
     async def commit(self):
         """Exit the transaction or savepoint block and commit changes."""
         if self._managed:
             raise apg_errors.InterfaceError(
                 'cannot manually commit from within an `async with` block')
         await self.__commit()
 
+    @connresource.guarded
     async def rollback(self):
         """Exit the transaction or savepoint block and rollback changes."""
         if self._managed:

tests/test_pool.py

@@ -11,6 +11,8 @@
 import os
 import platform
 import random
+import sys
+import textwrap
 import time
 import unittest
 
@@ -195,6 +197,7 @@ async def test_pool_11(self):
             self.assertIn(repr(con._con), repr(con))  # Test __repr__.
 
             ps = await con.prepare('SELECT 1')
+            txn = con.transaction()
             async with con.transaction():
                 cur = await con.cursor('SELECT 1')
                 ps_cur = await ps.cursor()
@@ -233,6 +236,14 @@ async def test_pool_11(self):
 
                 c.forward(1)
 
+        for meth in ('start', 'commit', 'rollback'):
+            with self.assertRaisesRegex(
+                    asyncpg.InterfaceError,
+                    r'cannot call Transaction\.{meth}.*released '
+                    r'back to the pool'.format(meth=meth)):
+
+                getattr(txn, meth)()
+
         await pool.close()
 
     async def test_pool_12(self):
@@ -661,6 +672,75 @@ async def test_pool_handles_inactive_connection_errors(self):
         await con.close()
         await pool.close()
 
+    @unittest.skipIf(sys.version_info[:2] < (3, 6), 'no asyncgen support')
+    async def test_pool_handles_transaction_exit_in_asyncgen_1(self):
+        pool = await self.create_pool(database='postgres',
+                                      min_size=1, max_size=1)
+
+        locals_ = {}
+        exec(textwrap.dedent('''\
+            async def iterate(con):
+                async with con.transaction():
+                    for record in await con.fetch("SELECT 1"):
+                        yield record
+        '''), globals(), locals_)
+        iterate = locals_['iterate']
+
+        class MyException(Exception):
+            pass
+
+        with self.assertRaises(MyException):
+            async with pool.acquire() as con:
+                async for _ in iterate(con):  # noqa
+                    raise MyException()
+
+    @unittest.skipIf(sys.version_info[:2] < (3, 6), 'no asyncgen support')
+    async def test_pool_handles_transaction_exit_in_asyncgen_2(self):
+        pool = await self.create_pool(database='postgres',
+                                      min_size=1, max_size=1)
+
+        locals_ = {}
+        exec(textwrap.dedent('''\
+            async def iterate(con):
+                async with con.transaction():
+                    for record in await con.fetch("SELECT 1"):
+                        yield record
+        '''), globals(), locals_)
+        iterate = locals_['iterate']
+
+        class MyException(Exception):
+            pass
+
+        with self.assertRaises(MyException):
+            async with pool.acquire() as con:
+                iterator = iterate(con)
+                async for _ in iterator:  # noqa
+                    raise MyException()
+
+            del iterator
+
+    @unittest.skipIf(sys.version_info[:2] < (3, 6), 'no asyncgen support')
+    async def test_pool_handles_asyncgen_finalization(self):
+        pool = await self.create_pool(database='postgres',
+                                      min_size=1, max_size=1)
+
+        locals_ = {}
+        exec(textwrap.dedent('''\
+            async def iterate(con):
+                for record in await con.fetch("SELECT 1"):
+                    yield record
+        '''), globals(), locals_)
+        iterate = locals_['iterate']
+
+        class MyException(Exception):
+            pass
+
+        with self.assertRaises(MyException):
+            async with pool.acquire() as con:
+                async with con.transaction():
+                    async for _ in iterate(con):  # noqa
+                        raise MyException()
+
 
 @unittest.skipIf(os.environ.get('PGHOST'), 'using remote cluster for testing')
 class TestHotStandby(tb.ConnectedTestCase):