unix: Use MP_STREAM_GET_FILENO to allow uselect to poll general objects.

This mechanism will scale to to an arbitrary number of pollable objects, so
long as they implement the MP_STREAM_GET_FILENO ioctl.  Since ussl objects
pass through ioctl requests transparently to the underlying socket object,
it will allow ussl sockets to be polled.  And a user object with uio.IOBase
as a base could support polling.

ports/unix/file.c

@@ -124,6 +124,8 @@ STATIC mp_uint_t fdfile_ioctl(mp_obj_t o_in, mp_uint_t request, uintptr_t arg, i
             o->fd = -1;
             #endif
             return 0;
+        case MP_STREAM_GET_FILENO:
+            return o->fd;
         default:
             *errcode = EINVAL;
             return MP_STREAM_ERROR;

ports/unix/moduselect.c

@@ -34,6 +34,7 @@
 #include <poll.h>
 
 #include "py/runtime.h"
+#include "py/stream.h"
 #include "py/obj.h"
 #include "py/objlist.h"
 #include "py/objtuple.h"
@@ -65,19 +66,15 @@ typedef struct _mp_obj_poll_t {
 } mp_obj_poll_t;
 
 STATIC int get_fd(mp_obj_t fdlike) {
-    int fd;
-    // Shortcut for fdfile compatible types
-    if (MP_OBJ_IS_TYPE(fdlike, &mp_type_fileio)
-        #if MICROPY_PY_SOCKET
-        || MP_OBJ_IS_TYPE(fdlike, &mp_type_socket)
-        #endif
-        ) {
-        mp_obj_fdfile_t *fdfile = MP_OBJ_TO_PTR(fdlike);
-        fd = fdfile->fd;
-    } else {
-        fd = mp_obj_get_int(fdlike);
+    if (MP_OBJ_IS_OBJ(fdlike)) {
+        const mp_stream_p_t *stream_p = mp_get_stream_raise(fdlike, MP_STREAM_OP_IOCTL);
+        int err;
+        mp_uint_t res = stream_p->ioctl(fdlike, MP_STREAM_GET_FILENO, 0, &err);
+        if (res != MP_STREAM_ERROR) {
+            return res;
+        }
     }
-    return fd;
+    return mp_obj_get_int(fdlike);
 }
 
 /// \method register(obj[, eventmask])

ports/unix/modusocket.c

@@ -123,6 +123,9 @@ STATIC mp_uint_t socket_ioctl(mp_obj_t o_in, mp_uint_t request, uintptr_t arg, i
             close(self->fd);
             return 0;
 
+        case MP_STREAM_GET_FILENO:
+            return self->fd;
+
         default:
             *errcode = MP_EINVAL;
             return MP_STREAM_ERROR;