Fixed problem shutting down threaded doc mapper

Synchronization problem with queues. The input reader queue is getting
shut down while the processor is waiting for a document, which was
causing an exception to be raised in the worker process while shutting
down.

This is not a problem: we just have to catch the exception and ignore
it. The stopped flag is set by this point, so we let the process exit
naturally when this happens.

src/python/pimlico/core/modules/map/threaded.py

@@ -58,6 +58,12 @@ def run(self):
                     except Empty:
                         # Don't worry if the queue is empty: just keep waiting for more until we're shut down
                         pass
+                    except IOError as e:
+                        if e.args[0] == "handle is closed":
+                            # The queue was closed while we were waiting
+                            # Stopped should have been set by now: we continue and check that
+                            continue
+                        raise
                     else:
                         input_buffer.append(tuple([archive, filename] + docs))
                     if len(input_buffer) >= self.docs_per_batch or self.no_more_inputs.is_set():