Atomic masks fatal database errors and instead propagates "connection already closed"

[Marti Raudsepp]

I noticed this problem with the new Atomic code in Django 1.6: when the PostgreSQL server raises a FATAL error (meaning that the server closes the connection after the error), then the Atomic block will swallow the original error and propagates a meaningless "InterfaceError: connection already closed" instead. I don't know whether this also affects other database backends.

I find that it's quite annoying to debug problems when the original cause of an error is hidden by subsequent error recovery code. It's not a new problem with Atomic, I believe commit_on_success (and Christophe Pettus's xact) had the same problem.

The easiest way to reproduce this is to terminate your own connection. (In PostgreSQL 9.1 and earlier you need to be superuser to do this):

from django.db import connection
from django.db.transaction import atomic

with atomic():
    cur = connection.cursor()
    cur.execute("select pg_terminate_backend(pg_backend_pid())")
    cur.fetchall()

With current Django 1.6 this raises an error like:

Traceback (most recent call last):
  File "term1.py", line 7, in <module>
    cur.fetchall()
  File "/home/marti/src/django/django/db/transaction.py", line 351, in __exit__
    connection.set_autocommit(True)
  File "/home/marti/src/django/django/db/backends/__init__.py", line 335, in set_autocommit
    self._set_autocommit(autocommit)
  File "/home/marti/src/django/django/db/backends/postgresql_psycopg2/base.py", line 184, in _set_autocommit
    self.connection.autocommit = autocommit
psycopg2.InterfaceError: connection already closed

It tells you that the connection is closed, but not why. And there's a second problem too: a psycopg2 exception is propagated into user code, without using Django's exception wrappers.

To solve this problem, I submitted pull request 1696: ​https://github.com/django/django/issues/1696

1. Changed BaseDatabaseWrapper.set_autocommit to wrap backend-specific errors.
2. Wrapped Atomic.__exit__ in another layer of try/except and re-raising the original exception in case of an InterfaceError.

Now the original exception is properly raised:

django.db.utils.OperationalError: terminating connection due to administrator command

I can also write unit tests if someone can help me figure out how to test this (the example I provided above requires PostgreSQL, with either superuser privs or version 9.2+).

Ran all 'transactions' and 'db_backends' tests on these changes with both SQLite (OK (skipped=42)) and psycopg2 (OK (skipped=1))

[Marti Raudsepp]

Hi, I see you set the "Patch needs improvement" flag, but I can't see a review anywhere. Am I missing something?

[Aymeric Augustin]

The report only covers SQLite and PostgreSQL. We need to check and possibly make changes for MySQL and Oracle.

The diff in atomic is very scary, but maybe that's just a side effect of indenting everything.

[Marti Raudsepp]

Yeah, the diff in atomic is not really scary at all, here's a whitespace-independent diff for 2nd commit:

django/db/transaction.py

@@ -18 +18 @@
 
 from django.db import (
         connections, DEFAULT_DB_ALIAS,
-        DatabaseError, ProgrammingError)
+        DatabaseError, ProgrammingError, InterfaceError)
 from django.utils.decorators import available_attrs
 
 
@@ -312 +312 @@
             connection.in_atomic_block = False
 
         try:
+            try:
                 if exc_type is None and not connection.needs_rollback:
                     if connection.in_atomic_block:
                         # Release savepoint if there is one
@@ -354 +355 @@
                 elif not connection.savepoint_ids and not connection.commit_on_exit:
                     connection.in_atomic_block = False
 
+        except InterfaceError:
+            if exc_type is not None:
+                # We tried to clean up from an exception, but ran into an
+                # InterfaceError, meaning the connection is unusable from now
+                # on. Propagate up the original error instead of masking it
+                # with InterfaceError ("connection already closed")
+                return
+
     def __call__(self, func):
         @wraps(func, assigned=available_attrs(func))
         def inner(*args, **kwargs):

[David Isaacson]

Whatever happened to this? Seeing this happen a lot on production with 1.6 and postgres. Even worse, the connection then doesn't get closed, and so all future requests error as well. So should we also be closing the bad connection... somewhere?

Longer explanation: As it is with this patch applied (to 1.6, I can't seem to get dev to run), django.db.close_old_connections() fails on conn.rollback() (which is called by conn.abort()) with the same InterfaceError. This means that conn.close_if_unusable_or_obsolete() never gets called, and the broken connection is reused for later requests, causing all future requests on that connection to error.

You can replicate this behavior pretty easily as described above, by enabling connection pooling and terminating your own connection in a request, and then trying to make a subsequent request. I suspect this is still the behavior in dev as well, nothing appears to have changed.

My "solution" is to squelch InterfaceError as well as DatabaseError in close_old_connections, but I'm new to the Django internals so I'm not sure if this is best, or if it would be better to just close the connection straight away here in __exit__ before we return, or what. Happy to help sort this but I'm not sure of the correct approach.

[Aymeric Augustin]

​https://github.com/django/django/pull/2468

I believe that patch works for the case when the database closes the connection and also adresses comment 5.

It also prevents further queries until the exit of the outermost atomic block, thus preserving atomicity.

[Aymeric Augustin <aymeric.augustin@…>]

In 3becac84845cee8f12a5fb7f68c87cbaf029c6a0:

Fixed #22321 -- Wrapped exceptions in _set_autocommit.

Refs #21202.

[Aymeric Augustin <aymeric.augustin@…>]

In 8176150850b2e34b2afe1dc107e184eb4c6cd668:

Fixed #21202 -- Maintained atomicity when the server disconnects.

Thanks intgr for the report.

This commit doesn't include a test because I don't know how to emulate a
database disconnection in a cross-database compatible way.

Also simplified a 'backends' test that was constrained by this problem.

[Aymeric Augustin <aymeric.augustin@…>]

In 746cded010c7ba3e2bf2df4cde32dcc9b75cb0d3:

[1.6.x] Fixed #22321 -- Wrapped exceptions in _set_autocommit.

Refs #21202.

Backport of 3becac84 from master

[Aymeric Augustin <aymeric.augustin@…>]

In 1d3d2b9a2458cbf11b7e828fb59b3f2cb73d14fe:

[1.6.x] Fixed #21202 -- Maintained atomicity when the server disconnects.

Thanks intgr for the report.

This commit doesn't include a test because I don't know how to emulate a
database disconnection in a cross-database compatible way.

Also simplified a 'backends' test that was constrained by this problem.

Backport of 81761508 from master

[Aymeric Augustin <aymeric.augustin@…>]

In 5f22bda3827f32289793b90a387d8c77d602a859:

[1.7.x] Fixed #22321 -- Wrapped exceptions in _set_autocommit.

Refs #21202.

Backport of 3becac84 from master

[Aymeric Augustin <aymeric.augustin@…>]

In 2ad0bc132a6f70a128d8e1fec689c6a474ddde89:

[1.7.x] Fixed #21202 -- Maintained atomicity when the server disconnects.

Thanks intgr for the report.

This commit doesn't include a test because I don't know how to emulate a
database disconnection in a cross-database compatible way.

Also simplified a 'backends' test that was constrained by this problem.

Backport of 81761508 from master