Fix incorrect lookahead population before ack

Rather than tracking all in-flight blocks blocks during a lookahead,
littlefs uses an ack scheme to mark the first allocated block that
hasn't reached the disk yet. littlefs assumes all blocks since the
last ack are bad or in-flight, and uses this to know when it's out
of storage.

However, these unacked allocations were still being populated in the
lookahead buffer. If the whole block device fits in the lookahead
buffer, _and_ littlefs managed to scan around the whole storage while
an unacked block was still in-flight, it would assume the block was
free and misallocate it.

The fix is to only fill the lookahead buffer up to the last ack.
The internal free structure was restructured to simplify the runtime
calculation of lookahead size.

1 files changed, 18 insertions, 20 deletions

diff --git a/lfs.c b/lfs.c
index b21d209..b7cc675 100644
--- a/lfs.c
+++ b/lfs.c
@@ -278,7 +278,7 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
                 % (lfs_soff_t)(lfs->cfg->block_count))
             + lfs->cfg->block_count) % lfs->cfg->block_count;
 
-    if (off < lfs->cfg->lookahead) {
+    if (off < lfs->free.size) {
         lfs->free.buffer[off / 32] |= 1U << (off % 32);
     }
 
@@ -287,18 +287,7 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
 
 static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
     while (true) {
-        while (true) {
-            // check if we have looked at all blocks since last ack
-            if (lfs->free.begin + lfs->free.off == lfs->free.end) {
-                LFS_WARN("No more free space %d", lfs->free.end);
-                return LFS_ERR_NOSPC;
-            }
-
-            if (lfs->free.off >= lfs_min(
-                    lfs->cfg->lookahead, lfs->cfg->block_count)) {
-                break;
-            }
-
+        while (lfs->free.off != lfs->free.size) {
             lfs_block_t off = lfs->free.off;
             lfs->free.off += 1;
 
@@ -309,7 +298,15 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
             }
         }
 
-        lfs->free.begin += lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
+        // check if we have looked at all blocks since last ack
+        if (lfs->free.off == lfs->free.ack - lfs->free.begin) {
+            LFS_WARN("No more free space %d", lfs->free.off + lfs->free.begin);
+            return LFS_ERR_NOSPC;
+        }
+
+        lfs->free.begin += lfs->free.size;
+        lfs->free.size = lfs_min(lfs->cfg->lookahead,
+                lfs->free.ack - lfs->free.begin);
         lfs->free.off = 0;
 
         // find mask of free blocks from tree
@@ -322,7 +319,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
 }
 
 static void lfs_alloc_ack(lfs_t *lfs) {
-    lfs->free.end = lfs->free.begin + lfs->free.off + lfs->cfg->block_count;
+    lfs->free.ack = lfs->free.off-1 + lfs->free.begin + lfs->cfg->block_count;
 }
 
 
@@ -2035,11 +2032,11 @@ int lfs_format(lfs_t *lfs, const struct lfs_config *cfg) {
     // create free lookahead
     memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
     lfs->free.begin = 0;
+    lfs->free.size = lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
     lfs->free.off = 0;
-    lfs->free.end = lfs->free.begin + lfs->free.off + lfs->cfg->block_count;
+    lfs_alloc_ack(lfs);
 
     // create superblock dir
-    lfs_alloc_ack(lfs);
     lfs_dir_t superdir;
     err = lfs_dir_alloc(lfs, &superdir);
     if (err) {
@@ -2112,9 +2109,10 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
     }
 
     // setup free lookahead
-    lfs->free.begin = -lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
-    lfs->free.off = -lfs->free.begin;
-    lfs->free.end = lfs->free.begin + lfs->free.off + lfs->cfg->block_count;
+    lfs->free.begin = 0;
+    lfs->free.size = 0;
+    lfs->free.off = 0;
+    lfs_alloc_ack(lfs);
 
     // load superblock
     lfs_dir_t dir;