examples/network: Add example of HTTPS client using non-blocking socket.

Non-blocking SSL streams can be difficult to get right, so provide a
working example, of a HTTPS client.

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

examples/network/https_client_nonblocking.py

@@ -0,0 +1,75 @@
+# Example of a HTTPS client working with non-blocking sockets.
+#
+# Non-blocking SSL streams works differently in MicroPython compared to CPython.  In
+# CPython a write to an SSLSocket may raise ssl.SSLWantReadError.  In MicroPython an
+# SSLSocket behaves like a normal socket/stream and can be polled for reading/writing.
+
+from errno import EINPROGRESS
+import select
+import socket
+import ssl
+
+
+def connect_nonblocking(sock, addr):
+    sock.setblocking(False)
+    try:
+        sock.connect(addr)
+    except OSError as er:
+        if er.errno != EINPROGRESS:
+            raise er
+
+
+def write_nonblocking(poller, sock, data):
+    poller.register(sock, select.POLLOUT)
+    while data:
+        poller.poll()
+        n = sock.write(data)
+        print("Wrote:", n)
+        if n is not None:
+            data = data[n:]
+
+
+def read_nonblocking(poller, sock, n):
+    poller.register(sock, select.POLLIN)
+    poller.poll()
+    data = sock.read(n)
+    print("Read:", len(data))
+    return data
+
+
+def main(url):
+    # Split the given URL into components.
+    proto, _, host, path = url.split(b"/", 3)
+    assert proto == b"https:"
+
+    # Note: this getaddrinfo() call is blocking!
+    ai = socket.getaddrinfo(host, 443)[0]
+    addr = ai[-1]
+    print("Connect address:", addr)
+
+    # Create a TCP socket and connect to the server in non-blocking mode.
+    sock = socket.socket(ai[0], ai[1], ai[2])
+    connect_nonblocking(sock, addr)
+
+    # Wrap the TCP socket in an SSL stream in non-blocking mode.
+    ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+    sock = ctx.wrap_socket(sock, server_hostname=host, do_handshake_on_connect=False)
+    sock.setblocking(False)
+
+    # Create an object to poll the SSL stream for readability/writability.
+    poller = select.poll()
+
+    # Send the HTTP request on the SSL stream.
+    request = b"GET /%s HTTP/1.0\r\nHost: %s\r\n\r\n" % (path, host)
+    write_nonblocking(poller, sock, request)
+
+    # Receive the HTTP response from the SSL stream.
+    response = read_nonblocking(poller, sock, 1000)
+    for line in response.split(b"\n"):
+        print(line)
+
+    # Close the SSL stream.  This will also close the underlying TCP socket.
+    sock.close()
+
+
+main(b"https://micropython.org/ks/test.html")