Modified lfs_dir_compact to avoid redundant erases during split

The commit machine in littlefs has three stages: commit, compact, and
then split. First we try to append our commit to the metadata log, if
that fails we try to compact the metadata log to remove duplicates and make
room for the commit, if that still fails we split the metadata into two
metadata-pairs and try again. Each stage is less efficient but also less
frequent.

However, in the case that we're filling up a directory with new files,
such as the bootstrap process in setting up a new system, we must pass
through all three stages rather quickly in order to get enough
metadata-pairs to hold all of our files. This means we'll compact,
split, and then need to compact again. This creates more erases than is
needed in the optimal case, which can be a big cost on disks with an
expensive erase operation.

In theory, we can actually avoid this redundant erase by reusing the
data we wrote out in the first attempt to compact. In practice, this
trick is very complicated to pull off.

1. We may need to cache a half-completed program while we write out the
   new metadata-pair. We need to write out the second pair first in
   order to get our new tail before we complete our first metadata-pair.
   This requires two pcaches, which we don't have

   The solution here is to just drop our cache and reconstruct what if
   would have been. This needs to be perfect down to the byte level
   because we don't have knowledge of where our cache lines are.

2. We may have written out entries that are then moved to the new
   metadata-pair.

   The solution here isn't pretty but it works, we just add a delete
   tag for any entry that was moved over.

In the end the solution ends up a bit hacky, with different layers poked
through the commit logic in order to manage writes at the byte level
from where we manage splits. But it works fairly well and saves erases.

lfs.c

@@ -39,6 +39,9 @@ static int lfs_bd_read(lfs_t *lfs,
         void *buffer, lfs_size_t size) {
     uint8_t *data = buffer;
     LFS_ASSERT(block != 0xffffffff);
+    if (off+size > lfs->cfg->block_size) {
+        return LFS_ERR_CORRUPT;
+    }
 
     while (size > 0) {
         if (pcache && block == pcache->block &&
@@ -452,6 +455,7 @@ struct lfs_commit {
 
     lfs_off_t begin;
     lfs_off_t end;
+    lfs_off_t ack;
 };
 
 struct lfs_diskoff {
@@ -503,6 +507,24 @@ static int32_t lfs_commit_get(lfs_t *lfs, lfs_block_t block, lfs_off_t off,
     return LFS_ERR_NOENT;
 }
 
+static int lfs_commit_prog(lfs_t *lfs, struct lfs_commit *commit,
+        const void *buffer, lfs_size_t size) {
+    lfs_off_t skip = lfs_min(lfs_max(commit->ack, commit->off)
+            - commit->off, size);
+    int err = lfs_bd_prog(lfs,
+            &lfs->pcache, &lfs->rcache, false,
+            commit->block, commit->off + skip,
+            (const uint8_t*)buffer + skip, size - skip);
+    if (err) {
+        return err;
+    }
+
+    commit->crc = lfs_crc(commit->crc, buffer, size);
+    commit->off += size;
+    commit->ack = lfs_max(commit->off, commit->ack);
+    return 0;
+}
+
 static int lfs_commit_attrs(lfs_t *lfs, struct lfs_commit *commit,
         uint16_t id, const struct lfs_attr *attrs);
 
@@ -532,21 +554,14 @@ static int lfs_commit_attr(lfs_t *lfs, struct lfs_commit *commit,
 
     // write out tag
     uint32_t ntag = lfs_tole32((tag & 0x7fffffff) ^ commit->ptag);
-    commit->crc = lfs_crc(commit->crc, &ntag, sizeof(ntag));
-    int err = lfs_bd_prog(lfs,
-            &lfs->pcache, &lfs->rcache, false,
-            commit->block, commit->off, &ntag, sizeof(ntag));
+    int err = lfs_commit_prog(lfs, commit, &ntag, sizeof(ntag));
     if (err) {
         return err;
     }
-    commit->off += sizeof(ntag);
 
     if (!(tag & 0x80000000)) {
         // from memory
-        commit->crc = lfs_crc(commit->crc, buffer, size);
-        err = lfs_bd_prog(lfs,
-                &lfs->pcache, &lfs->rcache, false,
-                commit->block, commit->off, buffer, size);
+        err = lfs_commit_prog(lfs, commit, buffer, size);
         if (err) {
             return err;
         }
@@ -563,17 +578,13 @@ static int lfs_commit_attr(lfs_t *lfs, struct lfs_commit *commit,
                 return err;
             }
 
-            commit->crc = lfs_crc(commit->crc, &dat, 1);
-            err = lfs_bd_prog(lfs,
-                    &lfs->pcache, &lfs->rcache, false,
-                    commit->block, commit->off+i, &dat, 1);
+            err = lfs_commit_prog(lfs, commit, &dat, 1);
             if (err) {
                 return err;
             }
         }
     }
 
-    commit->off += size;
     commit->ptag = tag & 0x7fffffff;
     return 0;
 }
@@ -677,13 +688,11 @@ static int lfs_commit_crc(lfs_t *lfs, struct lfs_commit *commit) {
 
     // read erased state from next program unit
     uint32_t tag = 0;
-    if (off < lfs->cfg->block_size) {
-        int err = lfs_bd_read(lfs,
-                &lfs->pcache, &lfs->rcache, lfs->cfg->block_size,
-                commit->block, off, &tag, sizeof(tag));
-        if (err) {
-            return err;
-        }
+    int err = lfs_bd_read(lfs,
+            &lfs->pcache, &lfs->rcache, sizeof(tag),
+            commit->block, off, &tag, sizeof(tag));
+    if (err && err != LFS_ERR_CORRUPT) {
+        return err;
     }
 
     // build crc tag
@@ -697,7 +706,7 @@ static int lfs_commit_crc(lfs_t *lfs, struct lfs_commit *commit) {
     footer[0] = lfs_tole32(tag ^ commit->ptag);
     commit->crc = lfs_crc(commit->crc, &footer[0], sizeof(footer[0]));
     footer[1] = lfs_tole32(commit->crc);
-    int err = lfs_bd_prog(lfs,
+    err = lfs_bd_prog(lfs,
             &lfs->pcache, &lfs->rcache, false,
             commit->block, commit->off, &footer, sizeof(footer));
     if (err) {
@@ -824,12 +833,6 @@ static int32_t lfs_dir_fetchmatch(lfs_t *lfs,
         lfs_global_zero(&templocals);
 
         while (true) {
-            // reached end of block
-            if (off+sizeof(uint32_t) >= lfs->cfg->block_size) {
-                dir->erased = false;
-                break;
-            }
-
             // extract next tag
             uint32_t tag;
             int err = lfs_bd_read(lfs,
@@ -1076,50 +1079,74 @@ static int lfs_dir_compact(lfs_t *lfs,
     lfs_global_zero(&dir->locals);
 
     while (true) {
-        // last complete id
-        dir->count = end - begin;
-        int16_t ack = -1;
+        // setup compaction
+        bool splitted = false;
         bool exhausted = false;
 
-        // increment revision count
-        dir->rev += 1;
-        if (lfs->cfg->block_cycles && dir->rev % lfs->cfg->block_cycles == 0) {
-            if (lfs_pair_cmp(dir->pair, (const lfs_block_t[2]){0, 1}) == 0) {
-                // we're writing too much to the superblock, should we expand?
-                lfs_ssize_t res = lfs_fs_size(lfs);
-                if (res < 0) {
-                    return res;
-                }
+        struct lfs_commit commit;
+        commit.block = dir->pair[1];
+        commit.ack = 0;
+
+commit:
+        // setup erase state
+        exhausted = false;
+        dir->count = end - begin;
+        int16_t ackid = -1;
+
+        // setup commit state
+        commit.off = 0;
+        commit.crc = 0xffffffff;
+        commit.ptag = 0;
+
+        // space is complicated, we need room for tail, crc, globals,
+        // cleanup delete, and we cap at half a block to give room
+        // for metadata updates
+        commit.begin = 0;
+        commit.end = lfs_min(
+             lfs_alignup(lfs->cfg->block_size/2, lfs->cfg->prog_size),
+             lfs->cfg->block_size - 38);
+
+        if (!splitted) {
+            // increment revision count
+            dir->rev += 1;
+            if (lfs->cfg->block_cycles &&
+                    dir->rev % lfs->cfg->block_cycles == 0) {
+                if (lfs_pair_cmp(dir->pair,
+                        (const lfs_block_t[2]){0, 1}) == 0) {
+                    // we're writing too much to the superblock,
+                    // should we expand?
+                    lfs_ssize_t res = lfs_fs_size(lfs);
+                    if (res < 0) {
+                        return res;
+                    }
 
-                // do we have enough space to expand?
-                if (res < lfs->cfg->block_count/2) {
-                    LFS_DEBUG("Expanding superblock at rev %"PRIu32, dir->rev);
+                    // do we have enough space to expand?
+                    if (res < lfs->cfg->block_count/2) {
+                        LFS_DEBUG("Expanding superblock at rev %"PRIu32,
+                                dir->rev);
+                        exhausted = true;
+                        goto split;
+                    }
+                } else {
+                    // we're writing too much, time to relocate
                     exhausted = true;
-                    goto split;
+                    goto relocate;
                 }
-            } else {
-                // we're writing too much, time to relocate
-                exhausted = true;
-                goto relocate;
             }
-        }
 
-        // erase block to write to
-        int err = lfs_bd_erase(lfs, dir->pair[1]);
-        if (err) {
-            if (err == LFS_ERR_CORRUPT) {
-                goto relocate;
+            // erase block to write to
+            int err = lfs_bd_erase(lfs, dir->pair[1]);
+            if (err) {
+                if (err == LFS_ERR_CORRUPT) {
+                    goto relocate;
+                }
+                return err;
             }
-            return err;
         }
 
         // write out header
-        uint32_t crc = 0xffffffff;
         uint32_t rev = lfs_tole32(dir->rev);
-        crc = lfs_crc(crc, &rev, sizeof(rev));
-        err = lfs_bd_prog(lfs,
-                &lfs->pcache, &lfs->rcache, false,
-                dir->pair[1], 0, &rev, sizeof(rev));
+        int err = lfs_commit_prog(lfs, &commit, &rev, sizeof(rev));
         if (err) {
             if (err == LFS_ERR_CORRUPT) {
                 goto relocate;
@@ -1127,28 +1154,13 @@ static int lfs_dir_compact(lfs_t *lfs,
             return err;
         }
 
-        // setup compaction
-        struct lfs_commit commit = {
-            .block = dir->pair[1],
-            .off = sizeof(dir->rev),
-            .crc = crc,
-            .ptag = 0,
-
-            // space is complicated, we need room for tail, crc, globals,
-            // and we cap at half a block to give room for metadata updates
-            .begin = 0,
-            .end = lfs_min(
-                lfs_alignup(lfs->cfg->block_size/2, lfs->cfg->prog_size),
-                lfs->cfg->block_size - 34),
-        };
-
         // commit with a move
-        for (uint16_t id = begin; id < end; id++) {
+        for (uint16_t id = begin; id < end || commit.off < commit.ack; id++) {
             err = lfs_commit_move(lfs, &commit,
                     0x003ff000, LFS_MKTAG(0, id, 0),
                     0x003ff000, LFS_MKTAG(0, id - begin, 0),
                     source, attrs);
-            if (err) {
+            if (err && !(splitted && err == LFS_ERR_NOSPC)) {
                 if (err == LFS_ERR_NOSPC) {
                     goto split;
                 } else if (err == LFS_ERR_CORRUPT) {
@@ -1157,12 +1169,25 @@ static int lfs_dir_compact(lfs_t *lfs,
                 return err;
             }
 
-            ack = id;
+            ackid = id;
         }
 
         // reopen reserved space at the end
         commit.end = lfs->cfg->block_size - 8;
 
+        if (ackid >= end) {
+            // extra garbage attributes were written out during split,
+            // need to clean up
+            err = lfs_commit_attr(lfs, &commit,
+                    LFS_MKTAG(LFS_TYPE_DELETE, ackid, 0), NULL);
+            if (err) {
+                if (err == LFS_ERR_CORRUPT) {
+                    goto relocate;
+                }
+                return err;
+            }
+        }
+
         if (lfs_pair_cmp(dir->pair, (const lfs_block_t[2]){0, 1}) == 0) {
             // move over (duplicate) superblock if we are root
             err = lfs_commit_move(lfs, &commit,
@@ -1178,8 +1203,8 @@ static int lfs_dir_compact(lfs_t *lfs,
         }
 
         if (!relocated) {
-            // commit any globals, unless we're relocating, in which case our
-            // parent will steal our globals
+            // commit any globals, unless we're relocating,
+            // in which case our parent will steal our globals
             err = lfs_commit_globals(lfs, &commit, &dir->locals);
             if (err) {
                 if (err == LFS_ERR_CORRUPT) {
@@ -1222,8 +1247,13 @@ static int lfs_dir_compact(lfs_t *lfs,
 split:
         // commit no longer fits, need to split dir,
         // drop caches and create tail
-        lfs_cache_drop(lfs, &lfs->pcache);
-        if (!exhausted && ack < 0) {
+        splitted = !exhausted;
+        if (lfs->pcache.block != 0xffffffff) {
+            commit.ack -= lfs->pcache.size;
+            lfs_cache_drop(lfs, &lfs->pcache);
+        }
+
+        if (!exhausted && ackid < 0) {
             // If we can't fit in this block, we won't fit in next block
             return LFS_ERR_NOSPC;
         }
@@ -1234,25 +1264,26 @@ split:
             return err;
         }
 
-        if (exhausted) {
-            lfs->root[0] = tail.pair[0];
-            lfs->root[1] = tail.pair[1];
-        }
-
         tail.split = dir->split;
         tail.tail[0] = dir->tail[0];
         tail.tail[1] = dir->tail[1];
 
-        err = lfs_dir_compact(lfs, &tail, attrs, source, ack+1, end);
+        err = lfs_dir_compact(lfs, &tail, attrs, source, ackid+1, end);
         if (err) {
             return err;
         }
 
-        end = ack+1;
+        end = ackid+1;
         dir->tail[0] = tail.pair[0];
         dir->tail[1] = tail.pair[1];
         dir->split = true;
-        continue;
+
+        if (exhausted) {
+            lfs->root[0] = tail.pair[0];
+            lfs->root[1] = tail.pair[1];
+        }
+
+        goto commit;
 
 relocate:
         // commit was corrupted, drop caches and prepare to relocate block
@@ -1363,6 +1394,7 @@ static int lfs_dir_commit(lfs_t *lfs, lfs_mdir_t *dir,
 
             .begin = dir->off,
             .end = lfs->cfg->block_size - 8,
+            .ack = 0,
         };
 
         for (const lfs_mattr_t *a = attrs; a; a = a->next) {