unix/modsocket: Set file descriptor to -1 on close.

After s.close(), s.fileno() now returns -1, matching CPython behavior.
Some code relies on this compatibility, as it allows checking whether
a socket is closed by testing its fileno() value. This change ensures
better interoperability with existing Python code and libraries.

Signed-off-by: Mike Wang <mikewang000000@gmail.com>

ports/unix/modsocket.c

@@ -140,9 +140,12 @@ static mp_uint_t socket_ioctl(mp_obj_t o_in, mp_uint_t request, uintptr_t arg, i
             // The rationale MicroPython follows is that close() just releases
             // file descriptor. If you're interested to catch I/O errors before
             // closing fd, fsync() it.
-            MP_THREAD_GIL_EXIT();
-            close(self->fd);
-            MP_THREAD_GIL_ENTER();
+            if (self->fd >= 0) {
+                MP_THREAD_GIL_EXIT();
+                close(self->fd);
+                MP_THREAD_GIL_ENTER();
+            }
+            self->fd = -1;
             return 0;
 
         case MP_STREAM_GET_FILENO:

tests/extmod/socket_fileno.py

@@ -0,0 +1,17 @@
+# Test socket.fileno() functionality
+
+try:
+    import socket
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+if not hasattr(socket.socket, "fileno"):
+    print("SKIP")
+    raise SystemExit
+
+s = socket.socket()
+print(s.fileno() >= 0)
+
+s.close()
+print(s.fileno())  #  should print -1