webassembly/asyncio: Fix ThenableEvent to handle rejected thenables.

Prior to this fix, if a JavaScript thenable/Promise that was part of an
asyncio chain was rejected it would be ignored because the Python-side
`ThenableEvent` did not register a handler for the rejection.

That's fixed by this commit, and a corresponding test added.

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

ports/webassembly/asyncio/core.py

@@ -79,9 +79,8 @@ class TopLevelCoro:
 
 class ThenableEvent:
     def __init__(self, thenable):
-        self.result = None  # Result of the thenable
         self.waiting = None  # Task waiting on completion of this thenable
-        thenable.then(self.set)
+        thenable.then(self.set, self.cancel)
 
     def set(self, value=None):
         # Thenable/Promise is fulfilled, set result and schedule any waiting task.
@@ -90,6 +89,15 @@ class ThenableEvent:
             _task_queue.push(self.waiting)
             self.waiting = None
 
+    def cancel(self, value=None):
+        # Thenable/Promise is rejected, set error and schedule any waiting task.
+        self.error = jsffi.JsException(
+            value, getattr(value, "name", None), getattr(value, "message", None)
+        )
+        if self.waiting:
+            _task_queue.push(self.waiting)
+            self.waiting = None
+
     def remove(self, task):
         self.waiting = None
 
@@ -101,7 +109,9 @@ class ThenableEvent:
         cur_task.data = self
         # Wait for the thenable to fulfill.
         yield
-        # Return the result of the thenable.
+        # Raise the error, or return the result, of the thenable.
+        if hasattr(self, "error"):
+            raise self.error
         return self.result
 
 

tests/ports/webassembly/asyncio_top_level_await.mjs

@@ -117,6 +117,10 @@ async def main():
 
 # Test top-level waiting on a coro that catches.
 await main()
+
+# Test top-level waiting on a task that catches.
+t = asyncio.create_task(main())
+await t
 `);
 
 console.log("finished");

tests/ports/webassembly/asyncio_top_level_await.mjs.exp

@@ -22,4 +22,6 @@ jsFail
 caught exception: <class 'JsException'> <class 'JsProxy'> ('Error', 'jsFail')
 jsFail
 caught exception: <class 'JsException'> <class 'JsProxy'> ('Error', 'jsFail')
+jsFail
+caught exception: <class 'JsException'> <class 'JsProxy'> ('Error', 'jsFail')
 finished