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

@@ -305,7 +305,6 @@ 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;
@@ -1356,6 +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);

tests/test_alloc.sh

@@ -110,35 +110,10 @@ 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;
@@ -393,6 +368,7 @@ echo "--- Outdated lookahead and split dir test ---"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
@@ -448,8 +424,5 @@ tests/test.py << TEST
     lfs_unmount(&lfs) => 0;
 TEST
 
-
-
-
 echo "--- Results ---"
 tests/stats.py