extmod/uasyncio: Get addr and bind server socket before creating task.

Currently when using uasyncio.start_server() the socket configuration is
done inside a uasyncio.create_task() background function.  If the address
and port are already in use however this throws an OSError which cannot be
cleanly caught behind the create_task().

This commit moves the getaddrinfo and socket binding to the start_server()
function, and only creates the task if that succeeds.  This means that any
OSError from the initial socket configuration is propagated directly up the
call stack, compatible with CPython behaviour.

See #7444.

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

extmod/uasyncio/stream.py

@@ -107,15 +107,7 @@ class Server:
     async def wait_closed(self):
         await self.task
 
-    async def _serve(self, cb, host, port, backlog):
-        import usocket as socket
-
-        ai = socket.getaddrinfo(host, port)[0]  # TODO this is blocking!
-        s = socket.socket()
-        s.setblocking(False)
-        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-        s.bind(ai[-1])
-        s.listen(backlog)
+    async def _serve(self, s, cb):
         # Accept incoming connections
         while True:
             try:
@@ -137,9 +129,20 @@ class Server:
 # Helper function to start a TCP stream server, running as a new task
 # TODO could use an accept-callback on socket read activity instead of creating a task
 async def start_server(cb, host, port, backlog=5):
-    s = Server()
-    s.task = core.create_task(s._serve(cb, host, port, backlog))
-    return s
+    import usocket as socket
+
+    # Create and bind server socket.
+    host = socket.getaddrinfo(host, port)[0]  # TODO this is blocking!
+    s = socket.socket()
+    s.setblocking(False)
+    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    s.bind(host[-1])
+    s.listen(backlog)
+
+    # Create and return server object and task.
+    srv = Server()
+    srv.task = core.create_task(srv._serve(s, cb))
+    return srv
 
 
 ################################################################################

tests/net_hosted/uasyncio_start_server.py

@@ -0,0 +1,31 @@
+# Test basic behaviour of uasyncio.start_server()
+
+try:
+    import uasyncio as asyncio
+except ImportError:
+    try:
+        import asyncio
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+
+async def test():
+    # Test creating 2 servers using the same address
+    print("create server1")
+    server1 = await asyncio.start_server(None, "0.0.0.0", 8000)
+    try:
+        print("create server2")
+        await asyncio.start_server(None, "0.0.0.0", 8000)
+    except OSError as er:
+        print("OSError")
+
+    # Wait for server to close.
+    async with server1:
+        print("sleep")
+        await asyncio.sleep(0)
+
+    print("done")
+
+
+asyncio.run(test())

tests/net_hosted/uasyncio_start_server.py.exp

@@ -0,0 +1,5 @@
+create server1
+create server2
+OSError
+sleep
+done