Don't silently fail when unpickling.

When a pickled job string can't be unpickled because some required
module isn't loadable, this leads to an `UnpickleError` in the worker
(not in the horse).

Currently we just assume "garbage" in the job's data field, and silently
ignore it.

This is bad.

Really bad.

Because it avoids the normal exception handling mechanism that RQ has.

Historically, this "feature" was introduced to ignore any invalid pickle
data ("bad strings") on queues, and go on. However, we must assume data
inside `job.data` to be valid pickle data.

While an invalid _format_ of pickle data (e.g. the string "blablah"
isn't valid) leads to unpickle errors, unpickling errors will also occur
when the job can't be validly constructed in memory for other reasons,
like being unable to load a specific class.

Django is a good example of this: try submitting jobs that use
`django.conf.settings` while the `DJANGO_SETTINGS_MODULE` env var isn't
set. Currently, RQ workers will drop these jobs and dismiss them like
any non-valid pickle data. You won't be notified.

This patch changes RQ's behaviour to never ignore invalid string data on
any queue and _always_ handle these errors explicitly (but without
bringing the main loop down, of course).

rq/job.py

@@ -160,6 +160,15 @@ class Job(object):
         job.refresh()
         return job
 
+    @classmethod
+    def safe_fetch(cls, id, connection=None):
+        """Fetches a persisted job from its corresponding Redis key, but does
+        not instantiate it, making it impossible to get UnpickleErrors.
+        """
+        job = cls(id, connection=connection)
+        job.refresh(safe=True)
+        return job
+
     def __init__(self, id=None, connection=None):
         self.connection = resolve_connection(connection)
         self._id = id
@@ -240,7 +249,7 @@ class Job(object):
 
 
     # Persistence
-    def refresh(self):  # noqa
+    def refresh(self, safe=False):  # noqa
         """Overwrite the current instance's properties with the values in the
         corresponding Redis key.
 
@@ -257,7 +266,12 @@ class Job(object):
             else:
                 return times.to_universal(date_str)
 
-        self._func_name, self._instance, self._args, self._kwargs = unpickle(obj.get('data'))  # noqa
+        self.data = obj.get('data')
+        try:
+            self._func_name, self._instance, self._args, self._kwargs = unpickle(self.data)
+        except UnpickleError:
+            if not safe:
+                raise
         self.created_at = to_date(obj.get('created_at'))
         self.origin = obj.get('origin')
         self.description = obj.get('description')

rq/queue.py

@@ -73,7 +73,7 @@ class Queue(object):
         """Returns a list of all (valid) jobs in the queue."""
         def safe_fetch(job_id):
             try:
-                job = Job.fetch(job_id, connection=self.connection)
+                job = Job.safe_fetch(job_id, connection=self.connection)
             except NoSuchJobError:
                 return None
             except UnpickleError:

rq/worker.py

@@ -19,7 +19,7 @@ except ImportError:
     from logging import Logger
 from .queue import Queue, get_failed_queue
 from .connections import get_current_connection
-from .job import Status
+from .job import Job, Status
 from .utils import make_colorizer
 from .exceptions import NoQueueError, UnpickleError
 from .timeouts import death_penalty_after
@@ -309,13 +309,8 @@ class Worker(object):
                 except StopRequested:
                     break
                 except UnpickleError as e:
-                    msg = '*** Ignoring unpickleable data on %s.' % \
-                            green(e.queue.name)
-                    self.log.warning(msg)
-                    self.log.debug('Data follows:')
-                    self.log.debug(e.raw_data)
-                    self.log.debug('End of unreadable data.')
-                    self.failed_queue.push_job_id(e.job_id)
+                    job = Job.safe_fetch(e.job_id)
+                    self.handle_exception(job, *sys.exc_info())
                     continue
 
                 self.state = 'busy'