Added error when opening multiple files with a statically allocated buffer

Opening multiple files simultaneously is not supported without dynamic memory,
but the previous behaviour would just let the files overwrite each other, which
could lead to bad errors down the line

found by husigeza

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) {
@@ -283,20 +282,22 @@ 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;
@@ -304,15 +305,15 @@ 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.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;
 }
 
 
@@ -1354,6 +1355,10 @@ 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);
@@ -2103,9 +2108,9 @@ 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 +2187,9 @@ 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;