Fix run_in_subprocess (#299)

1. The `queue.Full` exception is never raised, so expecting it does not
make sense.
An exception that can happen is pickle error, but it's safer to rather
catch all exceptions.
2. When child process puts a message, the parent process does not
process what
remains in the queue. This commit adds a drain mode.
3. React to KeyboardInterrupt on the main process.

src/spdl/dataloader/_iterators.py

@@ -39,7 +39,7 @@
 # Message from worker to the parent
 _MSG_GENERATOR_FAILED = "GENERATOR_FAILED_TO_INITIALIZE"
 _MSG_ITERATION_FINISHED = "ITERATION_FINISHED"
-_MSG_DATA_QUEUE_FULL = "DATA_QUEUE_FULL"
+_MSG_DATA_QUEUE_FAILED = "DATA_QUEUE_FAILED"
 
 
 def _execute_iterator(
@@ -63,7 +63,7 @@ def _execute_iterator(
         else:
             if msg == _MSG_PARENT_REQUEST_STOP:
                 return
-            raise ValueError(f"Unexpected message redeived: {msg}")
+            raise ValueError(f"[INTERNAL ERROR] Unexpected message received: {msg}")
 
         try:
             item = next(gen)
@@ -76,8 +76,8 @@ def _execute_iterator(
 
         try:
             data_queue.put(item)
-        except queue.Full:
-            msg_queue.put(_MSG_DATA_QUEUE_FULL)
+        except Exception:
+            msg_queue.put(_MSG_DATA_QUEUE_FAILED)
             return
 
 
@@ -111,7 +111,11 @@ def run_in_subprocess(
     """
     ctx = mp.get_context(mp_context)
     msg_q = ctx.Queue()
-    data_q = ctx.Queue(maxsize=queue_size)
+    data_q: mp.Queue = ctx.Queue(maxsize=queue_size)
+
+    def _drain() -> Iterator[T]:
+        while not data_q.empty():
+            yield data_q.get_nowait()
 
     process = ctx.Process(
         target=_execute_iterator,
@@ -127,18 +131,21 @@ def run_in_subprocess(
             except queue.Empty:
                 pass
             else:
+                # When a message is found, the child process stopped putting data.
+                yield from _drain()
+
                 if msg == _MSG_ITERATION_FINISHED:
                     return
                 if msg == _MSG_GENERATOR_FAILED:
                     raise RuntimeError(
                         "The worker process quit because the generator failed."
                     )
-                if msg == _MSG_DATA_QUEUE_FULL:
+                if msg == _MSG_DATA_QUEUE_FAILED:
                     raise RuntimeError(
-                        "The worker process quit because the data queue is full for too long."
+                        "The worker process quit because it failed at passing the data."
                     )
 
-                raise ValueError(f"Unexpected message received: {msg}")
+                raise ValueError(f"[INTERNAL ERROR] Unexpected message received: {msg}")
 
             try:
                 yield data_q.get(timeout=1)
@@ -153,12 +160,11 @@ def run_in_subprocess(
                         f"The worker process did not produce any data for {elapsed:.2f} seconds."
                     )
 
-    except Exception:
+    except (Exception, KeyboardInterrupt):
         msg_q.put(_MSG_PARENT_REQUEST_STOP)
         raise
     finally:
-        while not data_q.empty():
-            data_q.get_nowait()
+        yield from _drain()
         process.join(3)
 
         if process.exitcode is None: