extmod/vfs_fat: Mount FatFS on creation so VFS methods can be used.

It's possible to use the methods (eg ilistdir) of a VFS FatFS object
without it being mounted in the VFS itself.  This previously worked but
only because FatFS was "mounting" the filesystem automatically when any
function (eg f_opendir) was called.  But it didn't work for ports that used
synchronisation objects (_FS_REENTRANT) because they are only initialised
via a call to f_mount.  So, call f_mount explicitly when creating a new
FatFS object so that everything is set up correctly.  Then also provide a
finaliser to do the f_umount call, but only if synchronisation objects are
enabled (since otherwise the f_umount call does nothing).

extmod/vfs_fat.c

@@ -69,9 +69,28 @@ STATIC mp_obj_t fat_vfs_make_new(const mp_obj_type_t *type, size_t n_args, size_
         mp_load_method(args[0], MP_QSTR_count, vfs->u.old.count);
     }
 
+    // mount the block device so the VFS methods can be used
+    FRESULT res = f_mount(&vfs->fatfs);
+    if (res == FR_NO_FILESYSTEM) {
+        // don't error out if no filesystem, to let mkfs()/mount() create one if wanted
+        vfs->flags |= FSUSER_NO_FILESYSTEM;
+    } else if (res != FR_OK) {
+        mp_raise_OSError(fresult_to_errno_table[res]);
+    }
+
     return MP_OBJ_FROM_PTR(vfs);
 }
 
+#if _FS_REENTRANT
+STATIC mp_obj_t fat_vfs_del(mp_obj_t self_in) {
+    mp_obj_fat_vfs_t *self = MP_OBJ_TO_PTR(self_in);
+    // f_umount only needs to be called to release the sync object
+    f_umount(&self->fatfs);
+    return mp_const_none;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(fat_vfs_del_obj, fat_vfs_del);
+#endif
+
 STATIC mp_obj_t fat_vfs_mkfs(mp_obj_t bdev_in) {
     // create new object
     fs_user_mount_t *vfs = MP_OBJ_TO_PTR(fat_vfs_make_new(&mp_fat_vfs_type, 1, 0, &bdev_in));
@@ -291,10 +310,8 @@ STATIC mp_obj_t vfs_fat_mount(mp_obj_t self_in, mp_obj_t readonly, mp_obj_t mkfs
         self->writeblocks[0] = MP_OBJ_NULL;
     }
 
-    // mount the block device
-    FRESULT res = f_mount(&self->fatfs);
-
     // check if we need to make the filesystem
+    FRESULT res = (self->flags & FSUSER_NO_FILESYSTEM) ? FR_NO_FILESYSTEM : FR_OK;
     if (res == FR_NO_FILESYSTEM && mp_obj_is_true(mkfs)) {
         uint8_t working_buf[_MAX_SS];
         res = f_mkfs(&self->fatfs, FM_FAT | FM_SFD, 0, working_buf, sizeof(working_buf));
@@ -302,22 +319,23 @@ STATIC mp_obj_t vfs_fat_mount(mp_obj_t self_in, mp_obj_t readonly, mp_obj_t mkfs
     if (res != FR_OK) {
         mp_raise_OSError(fresult_to_errno_table[res]);
     }
+    self->flags &= ~FSUSER_NO_FILESYSTEM;
 
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_3(vfs_fat_mount_obj, vfs_fat_mount);
 
 STATIC mp_obj_t vfs_fat_umount(mp_obj_t self_in) {
-    fs_user_mount_t *self = MP_OBJ_TO_PTR(self_in);
-    FRESULT res = f_umount(&self->fatfs);
-    if (res != FR_OK) {
-        mp_raise_OSError(fresult_to_errno_table[res]);
-    }
+    (void)self_in;
+    // keep the FAT filesystem mounted internally so the VFS methods can still be used
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(fat_vfs_umount_obj, vfs_fat_umount);
 
 STATIC const mp_rom_map_elem_t fat_vfs_locals_dict_table[] = {
+    #if _FS_REENTRANT
+    { MP_ROM_QSTR(MP_QSTR___del__), MP_ROM_PTR(&fat_vfs_del_obj) },
+    #endif
     { MP_ROM_QSTR(MP_QSTR_mkfs), MP_ROM_PTR(&fat_vfs_mkfs_obj) },
     { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&fat_vfs_open_obj) },
     { MP_ROM_QSTR(MP_QSTR_ilistdir), MP_ROM_PTR(&fat_vfs_ilistdir_obj) },

extmod/vfs_fat.h

@@ -35,6 +35,7 @@
 #define FSUSER_NATIVE       (0x0001) // readblocks[2]/writeblocks[2] contain native func
 #define FSUSER_FREE_OBJ     (0x0002) // fs_user_mount_t obj should be freed on umount
 #define FSUSER_HAVE_IOCTL   (0x0004) // new protocol with ioctl
+#define FSUSER_NO_FILESYSTEM (0x0008) // the block device has no filesystem on it
 
 typedef struct _fs_user_mount_t {
     mp_obj_base_t base;