#16374: ExceptionReporter may re-evaluate error-causing queryset, leading to a
later DatabaseError that masks the original one
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Reporter: aaron | Owner: nobody
Type: Bug | Status: new
Milestone: | Component: Generic
Version: 1.3 | views
Keywords: ExceptionReporter DatabaseError | Severity: Normal
current transaction aborted error queryset | Triage Stage:
Has patch: 0 | Unreviewed
UI/UX: 0 | Easy pickings: 0
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I'm using PostgreSQL and the {{{ TransactionMiddleware }}}, and I'm not
sure if this error occurs with other database engines. Here's the error
that pops up:
{{{
DatabaseError at /blah/
current transaction is aborted, commands ignored until end of transaction
block
Traceback:
File ".../lib/django-1.3/django/core/handlers/base.py" in get_response
178. response = middleware_method(request, response)
File ".../lib/django-1.3/django/contrib/sessions/middleware.py" in
process_response
36. request.session.save()
File ".../lib/django-1.3/django/contrib/sessions/backends/db.py" in save
61. sid = transaction.savepoint(using=using)
File ".../lib/django-1.3/django/db/transaction.py" in savepoint
162. return connection.savepoint()
File ".../lib/django-1.3/django/db/backends/__init__.py" in savepoint
223. self._savepoint(sid)
File ".../lib/django-1.3/django/db/backends/__init__.py" in _savepoint
70. self.cursor().execute(self.ops.savepoint_create_sql(sid))
File ".../lib/django-1.3/django/db/backends/util.py" in execute
34. return self.cursor.execute(sql, params)
File ".../lib/django-1.3/django/db/backends/postgresql_psycopg2/base.py"
in execute
44. return self.cursor.execute(query, args)
}}}
The cause seems to be a {{{ QuerySet }}} that causes an error, for
example, it refers to a table that doesn't exist or is missing a column.
This would be fine, except that there are two lines in
django/views/debug.py around line 117, in {{{
ExceptionReporter.get_traceback_html() }}} (in django 1.3 release version)
that looks like this:
{{{#!python
if 'vars' in frame:
frame['vars'] = [(k, force_escape(pprint(v))) for k, v in
frame['vars']]
}}}
In this case, v is the {{{ QuerySet}}}. Running the pprint template
filter on it evaluates it, and since it caused an error the first time
(and never got {{{ _result_cache }}} filled in) it causes an error the
second time too. This error is caught by the filter, which returns an
error string. However, the running of the SQL query puts the DB in a bad
state, so any SQL commands run after this point also return an error, and
saving the current user's session is one such command. This new error
masks the original one, which is annoying. It's still possible to find
the original error by looking at the database log, but that's defeating
the point of this otherwise beautiful error page.
I'm not sure of the best way to fix this. One way, I guess, is to set a
transaction savepoint before calling this method and roll it back after,
but that seems to be a hack. Maybe we could check for querysets that have
not been evaluated, and leave them unevaluated, printing out something
like {{{ "(Unevaluated QuerySet) %s" % qs.query }}} instead. It seems to
make sense that the error reporting page renderer should not touch the
database at all, since it should be as robust as possible.
I think that ideally {{{ QuerySet.__repr__ }}} wouldn't have any side
effects, but its behaviour has been around for so long and so many people
are used to not needing list(qs) in an interactive shell that this would
not be an easy change to make. I wonder if it would be possible to add a
hook into manage.py shell that evaluates querysets as a substitute?
--
Ticket URL: <https://code.djangoproject.com/ticket/16374>
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