Fixed issue with aggressively rounding down lookahead configuration

The littlefs allows buffers to be passed statically in the case
that a system does not have a heap. Unfortunately, this means we
can't round up in the case of an unaligned lookahead buffer.

Double unfortunately, rounding down after clamping to the block device
size could result in a lookahead of zero for block devices < 32 blocks
large.

The assert in littlefs does catch this case, but rounding down prevents
support for < 32 block devices.

The solution is to simply require a 32-bit aligned buffer with an
assert. This avoids runtime problems while allowing a user to pass
in the correct buffer for < 32 block devices. Rounding up can be
handled at higher API levels.

.travis.yml

@@ -15,7 +15,7 @@ script:
     - CFLAGS="-DLFS_READ_SIZE=1   -DLFS_PROG_SIZE=1"   make test
     - CFLAGS="-DLFS_READ_SIZE=512 -DLFS_PROG_SIZE=512" make test
     - CFLAGS="-DLFS_BLOCK_COUNT=1023" make test
-    - CFLAGS="-DLFS_LOOKAHEAD=2047"   make test
+    - CFLAGS="-DLFS_LOOKAHEAD=2048"   make test
 
     # self-host with littlefs-fuse for fuzz test
     - make -C littlefs-fuse

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->free.lookahead) {
+    if (off < lfs->cfg->lookahead) {
         lfs->free.buffer[off / 32] |= 1U << (off % 32);
     }
 
@@ -294,7 +294,8 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
                 return LFS_ERR_NOSPC;
             }
 
-            if (lfs->free.off >= lfs->free.lookahead) {
+            if (lfs->free.off >= lfs_min(
+                    lfs->cfg->lookahead, lfs->cfg->block_count)) {
                 break;
             }
 
@@ -308,11 +309,11 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
             }
         }
 
-        lfs->free.begin += lfs->free.lookahead;
+        lfs->free.begin += lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
         lfs->free.off = 0;
 
         // find mask of free blocks from tree
-        memset(lfs->free.buffer, 0, lfs->free.lookahead/8);
+        memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
         int err = lfs_traverse(lfs, lfs_alloc_lookahead, lfs);
         if (err) {
             return err;
@@ -1870,13 +1871,12 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     }
 
     // setup lookahead, round down to nearest 32-bits
-    lfs->free.lookahead = lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
-    lfs->free.lookahead = 32 * (lfs->free.lookahead / 32);
-    assert(lfs->free.lookahead > 0);
+    assert(lfs->cfg->lookahead % 32 == 0);
+    assert(lfs->cfg->lookahead > 0);
     if (lfs->cfg->lookahead_buffer) {
         lfs->free.buffer = lfs->cfg->lookahead_buffer;
     } else {
-        lfs->free.buffer = malloc(lfs->free.lookahead/8);
+        lfs->free.buffer = malloc(lfs->cfg->lookahead/8);
         if (!lfs->free.buffer) {
             return LFS_ERR_NOMEM;
         }
@@ -1919,7 +1919,7 @@ int lfs_format(lfs_t *lfs, const struct lfs_config *cfg) {
     }
 
     // create free lookahead
-    memset(lfs->free.buffer, 0, lfs->free.lookahead/8);
+    memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
     lfs->free.begin = 0;
     lfs->free.off = 0;
     lfs->free.end = lfs->free.begin + lfs->cfg->block_count;
@@ -1998,8 +1998,8 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
     }
 
     // setup free lookahead
-    lfs->free.begin = -lfs->free.lookahead;
-    lfs->free.off = lfs->free.lookahead;
+    lfs->free.begin = -lfs->cfg->lookahead;
+    lfs->free.off = lfs->cfg->lookahead;
     lfs->free.end = lfs->free.begin + lfs->cfg->block_count;
 
     // load superblock

lfs.h

@@ -236,7 +236,6 @@ typedef struct lfs_superblock {
 } lfs_superblock_t;
 
 typedef struct lfs_free {
-    lfs_size_t lookahead;
     lfs_block_t begin;
     lfs_block_t end;
     lfs_block_t off;