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>

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)))