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

@@ -305,6 +305,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
         }
 
         // check if we have looked at all blocks since last ack
+        //if (lfs->free.i == lfs->free.ack - lfs->free.off) {
         if (lfs->free.ack == 0) {
             LFS_WARN("No more free space %d", lfs->free.i + lfs->free.off);
             return LFS_ERR_NOSPC;
@@ -1355,10 +1356,6 @@ int lfs_file_open(lfs_t *lfs, lfs_file_t *file,
     // allocate buffer if needed
     file->cache.block = 0xffffffff;
     if (lfs->cfg->file_buffer) {
-        if (lfs->files) {
-            // already in use
-            return LFS_ERR_NOMEM;
-        }
         file->cache.buffer = lfs->cfg->file_buffer;
     } else if ((file->flags & 3) == LFS_O_RDONLY) {
         file->cache.buffer = lfs_malloc(lfs->cfg->read_size);

tests/test_alloc.sh

@@ -110,10 +110,35 @@ lfs_alloc_singleproc multiprocreuse
 lfs_verify multiprocreuse
 lfs_verify singleprocreuse
 
-echo "--- Cleanup ---"
 lfs_remove multiprocreuse
 lfs_remove singleprocreuse
 
+echo "--- Lookahead overflow test ---"
+lfs_mkdir overflow
+for name in bacon eggs pancakes
+do
+tests/test.py << TEST
+    lfs_mount(&lfs, &cfg) => 0;
+
+//    // setup lookahead to almost overflow
+//    lfs.free.begin = ((lfs_size_t)-1) - 2*$SIZE;
+//    lfs.free.size = 0;
+//    lfs.free.off = 0;
+
+    lfs_file_open(&lfs, &file[0], "overflow/$name",
+            LFS_O_WRONLY | LFS_O_CREAT | LFS_O_APPEND) => 0;
+    size = strlen("$name");
+    memcpy(buffer, "$name", size);
+    for (int i = 0; i < $SIZE; i++) {
+        lfs_file_write(&lfs, &file[0], buffer, size) => size;
+    }
+    lfs_file_close(&lfs, &file[0]) => 0;
+    lfs_unmount(&lfs) => 0;
+TEST
+done
+lfs_verify overflow
+lfs_remove overflow
+
 echo "--- Exhaustion test ---"
 tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
@@ -368,7 +393,6 @@ echo "--- Outdated lookahead and split dir test ---"
 rm -rf blocks
 tests/test.py << TEST
     lfs_format(&lfs, &cfg) => 0;
-
     lfs_mount(&lfs, &cfg) => 0;
 
     // fill completely with two files
@@ -424,5 +448,8 @@ tests/test.py << TEST
     lfs_unmount(&lfs) => 0;
 TEST
 
+
+
+
 echo "--- Results ---"
 tests/stats.py