extmod/uasyncio: Add StreamReader.readexactly(n) method.

It raises on EOFError instead of an IncompleteReadError (which is what
CPython does).  But the latter is derived from EOFError so code compatible
with MicroPython and CPython can be written by catching EOFError (eg see
included test).

Fixes issue #6156.
Signed-off-by: Damien George <damien@micropython.org>

extmod/uasyncio/stream.py

@@ -30,6 +30,18 @@ class Stream:
         yield core._io_queue.queue_read(self.s)
         return self.s.read(n)
 
+    async def readexactly(self, n):
+        r = b""
+        while n:
+            yield core._io_queue.queue_read(self.s)
+            r2 = self.s.read(n)
+            if r2 is not None:
+                if not len(r2):
+                    raise EOFError
+                r += r2
+                n -= len(r2)
+        return r
+
     async def readline(self):
         l = b""
         while True:

tests/multi_net/uasyncio_tcp_readexactly.py

+# Test uasyncio stream readexactly() method using TCP server/client
+
+try:
+    import uasyncio as asyncio
+except ImportError:
+    try:
+        import asyncio
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+PORT = 8000
+
+
+async def handle_connection(reader, writer):
+    writer.write(b"a")
+    await writer.drain()
+
+    # Split the first 2 bytes up so the client must wait for the second one
+    await asyncio.sleep(0.1)
+
+    writer.write(b"b")
+    await writer.drain()
+
+    writer.write(b"c")
+    await writer.drain()
+
+    writer.write(b"d")
+    await writer.drain()
+
+    print("close")
+    writer.close()
+    await writer.wait_closed()
+
+    print("done")
+    ev.set()
+
+
+async def tcp_server():
+    global ev
+    ev = asyncio.Event()
+    server = await asyncio.start_server(handle_connection, "0.0.0.0", PORT)
+    print("server running")
+    multitest.next()
+    async with server:
+        await asyncio.wait_for(ev.wait(), 10)
+
+
+async def tcp_client():
+    reader, writer = await asyncio.open_connection(IP, PORT)
+    print(await reader.readexactly(2))
+    print(await reader.readexactly(0))
+    print(await reader.readexactly(1))
+    try:
+        print(await reader.readexactly(2))
+    except EOFError as er:
+        print("EOFError")
+    print(await reader.readexactly(0))
+
+
+def instance0():
+    multitest.globals(IP=multitest.get_network_ip())
+    asyncio.run(tcp_server())
+
+
+def instance1():
+    multitest.next()
+    asyncio.run(tcp_client())

tests/multi_net/uasyncio_tcp_readexactly.py.exp

+--- instance0 ---
+server running
+close
+done
+--- instance1 ---
+b'ab'
+b''
+b'c'
+EOFError
+b''