extmod/asyncio: Support gather of tasks that finish early.

Adds support to asyncio.gather() for the case that one or more (or all)
sub-tasks finish and/or raise an exception before the gather starts.

Signed-off-by: Damien George <damien@micropython.org>

extmod/asyncio/core.py

@@ -219,6 +219,11 @@ def run_until_complete(main_task=None):
             elif t.state is None:
                 # Task is already finished and nothing await'ed on the task,
                 # so call the exception handler.
+
+                # Save exception raised by the coro for later use.
+                t.data = exc
+
+                # Create exception context and call the exception handler.
                 _exc_context["exception"] = exc
                 _exc_context["future"] = t
                 Loop.call_exception_handler(_exc_context)

extmod/asyncio/funcs.py

@@ -63,9 +63,6 @@ class _Remove:
 
 # async
 def gather(*aws, return_exceptions=False):
-    if not aws:
-        return []
-
     def done(t, er):
         # Sub-task "t" has finished, with exception "er".
         nonlocal state
@@ -86,26 +83,39 @@ def gather(*aws, return_exceptions=False):
         # Gather waiting is done, schedule the main gather task.
         core._task_queue.push(gather_task)
 
+    # Prepare the sub-tasks for the gather.
+    # The `state` variable counts the number of tasks to wait for, and can be negative
+    # if the gather should not run at all (because a task already had an exception).
     ts = [core._promote_to_task(aw) for aw in aws]
+    state = 0
     for i in range(len(ts)):
-        if ts[i].state is not True:
-            # Task is not running, gather not currently supported for this case.
+        if ts[i].state is True:
+            # Task is running, register the callback to call when the task is done.
+            ts[i].state = done
+            state += 1
+        elif not ts[i].state:
+            # Task finished already.
+            if not isinstance(ts[i].data, StopIteration):
+                # Task finished by raising an exception.
+                if not return_exceptions:
+                    # Do not run this gather at all.
+                    state = -len(ts)
+        else:
+            # Task being waited on, gather not currently supported for this case.
             raise RuntimeError("can't gather")
-        # Register the callback to call when the task is done.
-        ts[i].state = done
 
     # Set the state for execution of the gather.
     gather_task = core.cur_task
-    state = len(ts)
     cancel_all = False
 
-    # Wait for the a sub-task to need attention.
-    gather_task.data = _Remove
-    try:
-        yield
-    except core.CancelledError as er:
-        cancel_all = True
-        state = er
+    # Wait for a sub-task to need attention (if there are any to wait for).
+    if state > 0:
+        gather_task.data = _Remove
+        try:
+            yield
+        except core.CancelledError as er:
+            cancel_all = True
+            state = er
 
     # Clean up tasks.
     for i in range(len(ts)):
@@ -118,8 +128,13 @@ def gather(*aws, return_exceptions=False):
             # Sub-task ran to completion, get its return value.
             ts[i] = ts[i].data.value
         else:
-            # Sub-task had an exception with return_exceptions==True, so get its exception.
-            ts[i] = ts[i].data
+            # Sub-task had an exception.
+            if return_exceptions:
+                # Get the sub-task exception to return in the list of return values.
+                ts[i] = ts[i].data
+            elif isinstance(state, int):
+                # Raise the sub-task exception, if there is not already an exception to raise.
+                state = ts[i].data
 
     # Either this gather was cancelled, or one of the sub-tasks raised an exception with
     # return_exceptions==False, so reraise the exception here.

tests/extmod/asyncio_gather_finished_early.py

@@ -0,0 +1,65 @@
+# Test asyncio.gather() when a task is already finished before the gather starts.
+
+try:
+    import asyncio
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+# CPython and MicroPython differ in when they signal (and print) that a task raised an
+# uncaught exception.  So define an empty custom_handler() to suppress this output.
+def custom_handler(loop, context):
+    pass
+
+
+async def task_that_finishes_early(id, event, fail):
+    print("task_that_finishes_early", id)
+    event.set()
+    if fail:
+        raise ValueError("intentional exception", id)
+
+
+async def task_that_runs():
+    for i in range(5):
+        print("task_that_runs", i)
+        await asyncio.sleep(0)
+
+
+async def main(start_task_that_runs, task_fail, return_exceptions):
+    print("== start", start_task_that_runs, task_fail, return_exceptions)
+
+    # Set exception handler to suppress exception output.
+    loop = asyncio.get_event_loop()
+    loop.set_exception_handler(custom_handler)
+
+    # Create tasks.
+    event_a = asyncio.Event()
+    event_b = asyncio.Event()
+    tasks = []
+    if start_task_that_runs:
+        tasks.append(asyncio.create_task(task_that_runs()))
+    tasks.append(asyncio.create_task(task_that_finishes_early("a", event_a, task_fail)))
+    tasks.append(asyncio.create_task(task_that_finishes_early("b", event_b, task_fail)))
+
+    # Make sure task_that_finishes_early() are both done, before calling gather().
+    await event_a.wait()
+    await event_b.wait()
+
+    # Gather the tasks.
+    try:
+        result = "complete", await asyncio.gather(*tasks, return_exceptions=return_exceptions)
+    except Exception as er:
+        result = "exception", er, start_task_that_runs and tasks[0].done()
+
+    # Wait for the final task to finish (if it was started).
+    if start_task_that_runs:
+        await tasks[0]
+
+    # Print results.
+    print(result)
+
+
+# Run the test in the 8 different combinations of its arguments.
+for i in range(8):
+    asyncio.run(main(bool(i & 4), bool(i & 2), bool(i & 1)))