Fixed lookahead overflow and removed unbounded lookahead pointers

As pointed out by davidefer, the lookahead pointer modular arithmetic
does not work around integer overflow when the pointer size is not a
multiple of the block count.

To avoid overflow problems, the easy solution is to stop trying to
work around integer overflows and keep the lookahead offset inside the
block device. To make this work, the ack was modified into a resetable
counter that is decremented every block allocation.

As a plus, quite a bit of the allocation logic ended up simplified.

lfs.c

@@ -270,8 +270,7 @@ int lfs_deorphan(lfs_t *lfs);
 static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
  lfs_t *lfs = p;
 
- lfs_block_t off = (((lfs_soff_t)(block - lfs->free.begin)
- % (lfs_soff_t)(lfs->cfg->block_count))
+ lfs_block_t off = ((block - lfs->free.off)
  + lfs->cfg->block_count) % lfs->cfg->block_count;
 
  if (off < lfs->free.size) {
@@ -283,36 +282,38 @@ 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 (lfs->free.off != lfs->free.size) {
- lfs_block_t off = lfs->free.off;
- lfs->free.off += 1;
+ while (lfs->free.i != lfs->free.size) {
+ lfs_block_t off = lfs->free.i;
+ lfs->free.i += 1;
+ lfs->free.ack -= 1;
 
  if (!(lfs->free.buffer[off / 32] & (1U << (off % 32)))) {
  // found a free block
- *block = (lfs->free.begin + off) % lfs->cfg->block_count;
+ *block = (lfs->free.off + off) % lfs->cfg->block_count;
 
  // eagerly find next off so an alloc ack can
  // discredit old lookahead blocks
- while (lfs->free.off != lfs->free.size &&
- (lfs->free.buffer[lfs->free.off / 32] &
- (1U << (lfs->free.off % 32)))) {
- lfs->free.off += 1;
+ while (lfs->free.i != lfs->free.size &&
+ (lfs->free.buffer[lfs->free.i / 32]
+ & (1U << (lfs->free.i % 32)))) {
+ lfs->free.i += 1;
+ lfs->free.ack -= 1;
  }
 
  return 0;
  }
  }
 
  // 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);
+ if (lfs->free.ack == 0) {
+ LFS_WARN("No more free space %d", lfs->free.i + lfs->free.off);
  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;
+ lfs->free.off = (lfs->free.off + lfs->free.size)
+  % lfs->cfg->block_count;
+ lfs->free.size = lfs_min(lfs->cfg->lookahead, lfs->free.ack);
+ lfs->free.i = 0;
 
  // find mask of free blocks from tree
  memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
@@ -324,7 +325,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
 }
 
 static void lfs_alloc_ack(lfs_t *lfs) {
- lfs->free.ack = lfs->free.begin+lfs->free.off + lfs->cfg->block_count;
+ lfs->free.ack = lfs->cfg->block_count;
 }
 
 
@@ -2103,9 +2104,9 @@ 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.size = lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
+ lfs->free.i = 0;
  lfs_alloc_ack(lfs);
 
  // create superblock dir
@@ -2182,9 +2183,9 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
  }
 
  // setup free lookahead
- lfs->free.begin = 0;
- lfs->free.size = 0;
  lfs->free.off = 0;
+ lfs->free.size = 0;
+ lfs->free.i = 0;
  lfs_alloc_ack(lfs);
 
  // load superblock

lfs.h

@@ -259,9 +259,9 @@ typedef struct lfs_superblock {
 } lfs_superblock_t;
 
 typedef struct lfs_free {
- lfs_block_t begin;
- lfs_block_t size;
  lfs_block_t off;
+ lfs_block_t size;
+ lfs_block_t i;
  lfs_block_t ack;
  uint32_t *buffer;
 } lfs_free_t;