Prevented unnecessary superblock rewrites if old version in superblock chain

Because multiple, out-of-date superblocks can exist in our superblock
chain, we need to be careful to make sure newer superblock entries
override older superblock entries.

If we see an older on-disk minor version in the superblock chain, we
were correctly overriding the on-disk minor version, but we were also
leaving the "needs superblock" bit set in our consistency state.

This isn't a hard-error, but would lead to a superblock rewrite every
mount. The rewrite would make no progress, as the out-of-date version is
effectively immutable at this point, and just waste prog cycles.

This should fix that by clearing the "needs superblock" bit if we see a
newer on-disk minor version.

README.md

@@ -258,6 +258,9 @@ License Identifiers that are here available: http://spdx.org/licenses/
   use with the MirageOS library operating system project. It is interoperable
   with the reference implementation, with some caveats.
 
+- [nim-littlefs] - A Nim wrapper and API for littlefs. Includes a fuse 
+  implementation based on [littlefs-fuse]
+
 [BSD-3-Clause]: https://spdx.org/licenses/BSD-3-Clause.html
 [littlefs-disk-img-viewer]: https://github.com/tniessen/littlefs-disk-img-viewer
 [littlefs-fuse]: https://github.com/geky/littlefs-fuse
@@ -274,3 +277,4 @@ License Identifiers that are here available: http://spdx.org/licenses/
 [littlefs-python]: https://pypi.org/project/littlefs-python/
 [littlefs2-rust]: https://crates.io/crates/littlefs2
 [chamelon]: https://github.com/yomimono/chamelon
+[nim-littlefs]: https://github.com/Graveflo/nim-littlefs

SPEC.md

@@ -441,9 +441,10 @@ Superblock fields:
 
 7. **Attr max (32-bits)** - Maximum size of file attributes in bytes.
 
-The superblock must always be the first entry (id 0) in a metadata pair as well
-as be the first entry written to the block. This means that the superblock
-entry can be read from a device using offsets alone.
+The superblock must always be the first entry (id 0) in the metadata pair, and
+the name tag must always be the first tag in the metadata pair. This makes it
+so that the magic string "littlefs" will always reside at offset=8 in a valid
+littlefs superblock.
 
 ---
 #### `0x2xx` LFS_TYPE_STRUCT

lfs.c

@@ -710,11 +710,14 @@ static lfs_stag_t lfs_dir_getslice(lfs_t *lfs, const lfs_mdir_t *dir,
     lfs_tag_t ntag = dir->etag;
     lfs_stag_t gdiff = 0;
 
+    // synthetic moves
     if (lfs_gstate_hasmovehere(&lfs->gdisk, dir->pair) &&
-            lfs_tag_id(gmask) != 0 &&
-            lfs_tag_id(lfs->gdisk.tag) <= lfs_tag_id(gtag)) {
-        // synthetic moves
-        gdiff -= LFS_MKTAG(0, 1, 0);
+            lfs_tag_id(gmask) != 0) {
+        if (lfs_tag_id(lfs->gdisk.tag) == lfs_tag_id(gtag)) {
+            return LFS_ERR_NOENT;
+        } else if (lfs_tag_id(lfs->gdisk.tag) < lfs_tag_id(gtag)) {
+            gdiff -= LFS_MKTAG(0, 1, 0);
+        }
     }
 
     // iterate over dir block backwards (for faster lookups)
@@ -2188,7 +2191,8 @@ static int lfs_dir_splittingcompact(lfs_t *lfs, lfs_mdir_t *dir,
                 // we can do, we'll error later if we've become frozen
                 LFS_WARN("Unable to expand superblock");
             } else {
-                end = begin;
+                // duplicate the superblock entry into the new superblock
+                end = 1;
             }
         }
     }
@@ -2355,7 +2359,9 @@ fixmlist:;
 
             while (d->id >= d->m.count && d->m.split) {
                 // we split and id is on tail now
-                d->id -= d->m.count;
+                if (lfs_pair_cmp(d->m.tail, lfs->root) != 0) {
+                    d->id -= d->m.count;
+                }
                 int err = lfs_dir_fetch(lfs, &d->m, d->m.tail);
                 if (err) {
                     return err;
@@ -4463,6 +4469,7 @@ static int lfs_mount_(lfs_t *lfs, const struct lfs_config *cfg) {
             // found older minor version? set an in-device only bit in the
             // gstate so we know we need to rewrite the superblock before
             // the first write
+            bool needssuperblock = false;
             if (minor_version < lfs_fs_disk_version_minor(lfs)) {
                 LFS_DEBUG("Found older minor version "
                         "v%"PRIu16".%"PRIu16" < v%"PRIu16".%"PRIu16,
@@ -4470,10 +4477,11 @@ static int lfs_mount_(lfs_t *lfs, const struct lfs_config *cfg) {
                         minor_version,
                         lfs_fs_disk_version_major(lfs),
                         lfs_fs_disk_version_minor(lfs));
-                // note this bit is reserved on disk, so fetching more gstate
-                // will not interfere here
-                lfs_fs_prepsuperblock(lfs, true);
+                needssuperblock = true;
             }
+            // note this bit is reserved on disk, so fetching more gstate
+            // will not interfere here
+            lfs_fs_prepsuperblock(lfs, needssuperblock);
 
             // check superblock configuration
             if (superblock.name_max) {

tests/test_superblocks.toml

@@ -14,6 +14,24 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
+# make sure the magic string "littlefs" is always at offset=8
+[cases.test_superblocks_magic]
+code = '''
+    lfs_t lfs;
+    lfs_format(&lfs, cfg) => 0;
+
+    // check our magic string
+    //
+    // note if we lose power we may not have the magic string in both blocks!
+    // but we don't lose power in this test so we can assert the magic string
+    // is present in both
+    uint8_t magic[lfs_max(16, READ_SIZE)];
+    cfg->read(cfg, 0, 0, magic, lfs_max(16, READ_SIZE)) => 0;
+    assert(memcmp(&magic[8], "littlefs", 8) == 0);
+    cfg->read(cfg, 1, 0, magic, lfs_max(16, READ_SIZE)) => 0;
+    assert(memcmp(&magic[8], "littlefs", 8) == 0);
+'''
+
 # mount/unmount from interpretting a previous superblock block_count
 [cases.test_superblocks_mount_unknown_block_count]
 code = '''
@@ -28,7 +46,6 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
-
 # reentrant format
 [cases.test_superblocks_reentrant_format]
 reentrant = true
@@ -135,6 +152,39 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
+# make sure the magic string "littlefs" is always at offset=8
+[cases.test_superblocks_magic_expand]
+defines.BLOCK_CYCLES = [32, 33, 1]
+defines.N = [10, 100, 1000]
+code = '''
+    lfs_t lfs;
+    lfs_format(&lfs, cfg) => 0;
+    lfs_mount(&lfs, cfg) => 0;
+    for (int i = 0; i < N; i++) {
+        lfs_file_t file;
+        lfs_file_open(&lfs, &file, "dummy",
+                LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
+        lfs_file_close(&lfs, &file) => 0;
+        struct lfs_info info;
+        lfs_stat(&lfs, "dummy", &info) => 0;
+        assert(strcmp(info.name, "dummy") == 0);
+        assert(info.type == LFS_TYPE_REG);
+        lfs_remove(&lfs, "dummy") => 0;
+    }
+    lfs_unmount(&lfs) => 0;
+
+    // check our magic string
+    //
+    // note if we lose power we may not have the magic string in both blocks!
+    // but we don't lose power in this test so we can assert the magic string
+    // is present in both
+    uint8_t magic[lfs_max(16, READ_SIZE)];
+    cfg->read(cfg, 0, 0, magic, lfs_max(16, READ_SIZE)) => 0;
+    assert(memcmp(&magic[8], "littlefs", 8) == 0);
+    cfg->read(cfg, 1, 0, magic, lfs_max(16, READ_SIZE)) => 0;
+    assert(memcmp(&magic[8], "littlefs", 8) == 0);
+'''
+
 # expanding superblock with power cycle
 [cases.test_superblocks_expand_power_cycle]
 defines.BLOCK_CYCLES = [32, 33, 1]
@@ -221,6 +271,7 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
+
 # mount with unknown block_count
 [cases.test_superblocks_unknown_blocks]
 code = '''