extmod/vfs: Support opening a file descriptor (int) with VfsPosix.

Fixes issue #3865.

extmod/vfs.c

@@ -38,6 +38,10 @@
 #include "extmod/vfs_fat.h"
 #endif
 
+#if MICROPY_VFS_POSIX
+#include "extmod/vfs_posix.h"
+#endif
+
 // For mp_vfs_proxy_call, the maximum number of additional args that can be passed.
 // A fixed maximum size is used to avoid the need for a costly variable array.
 #define PROXY_MAX_ARGS (2)
@@ -264,6 +268,13 @@ mp_obj_t mp_vfs_open(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args)
     mp_arg_val_t args[MP_ARRAY_SIZE(allowed_args)];
     mp_arg_parse_all(n_args, pos_args, kw_args, MP_ARRAY_SIZE(allowed_args), allowed_args, args);
 
+    #if MICROPY_VFS_POSIX
+    // If the file is an integer then delegate straight to the POSIX handler
+    if (MP_OBJ_IS_SMALL_INT(args[ARG_file].u_obj)) {
+        return mp_vfs_posix_file_open(&mp_type_textio, args[ARG_file].u_obj, args[ARG_mode].u_obj);
+    }
+    #endif
+
     mp_vfs_mount_t *vfs = lookup_path(args[ARG_file].u_obj, &args[ARG_file].u_obj);
     return mp_vfs_proxy_call(vfs, MP_QSTR_open, 2, (mp_obj_t*)&args);
 }