Fixed issue with cold-write after seek to block boundary

This off-by-one error was caused by a slight difference between the
pos argument to lfs_index_find and lfs_index_extend. When pos is on
a block boundary, lfs_index_extend expects block to point before pos,
as it would when writing a file linearly. But when seeking to that
pos, the lfs_index_find to warm things up just supplies the block it
expects pos to be in.

Fixed the off-by-one error and added a test case for several of these
cold seek+writes.

lfs.c

@@ -975,6 +975,7 @@ static int lfs_index_find(lfs_t *lfs,
  return err;
  }
 
+ assert(head >= 2 && head <= lfs->cfg->block_count);
  current -= 1 << skip;
  }
 
@@ -1054,6 +1055,8 @@ static int lfs_index_extend(lfs_t *lfs,
  return err;
  }
  }
+
+ assert(head >= 2 && head <= lfs->cfg->block_count);
  }
 
  *off = 4*skips;
@@ -1428,18 +1431,17 @@ lfs_ssize_t lfs_file_write(lfs_t *lfs, lfs_file_t *file,
  // check if we need a new block
  if (!(file->flags & LFS_F_WRITING) ||
  file->off == lfs->cfg->block_size) {
- if (!(file->flags & LFS_F_WRITING)) {
+ if (!(file->flags & LFS_F_WRITING) && file->pos > 0) {
  // find out which block we're extending from
  int err = lfs_index_find(lfs, &file->cache, NULL,
  file->head, file->size,
- file->pos, &file->block, &file->off);
+ file->pos-1, &file->block, &file->off);
  if (err) {
  return err;
  }
 
  // mark cache as dirty since we may have read data into it
  file->cache.block = 0xffffffff;
- file->flags |= LFS_F_WRITING;
  }
 
  // extend file with new blocks
@@ -1450,6 +1452,8 @@ lfs_ssize_t lfs_file_write(lfs_t *lfs, lfs_file_t *file,
  if (err) {
  return err;
  }
+
+ file->flags |= LFS_F_WRITING;
  }
 
  // program as much as we can in current block

tests/test_seek.sh

@@ -277,5 +277,33 @@ tests/test.py << TEST
  lfs_unmount(&lfs) => 0;
 TEST
 
+echo "--- Boundary seek and write ---"
+tests/test.py << TEST
+ lfs_mount(&lfs, &cfg) => 0;
+ lfs_file_open(&lfs, &file[0], "hello/kitty42", LFS_O_RDWR) => 0;
+
+ size = strlen("hedgehoghog");
+ const lfs_soff_t offsets[] = {512, 1020, 513, 1021, 511, 1019};
+
+ for (int i = 0; i < sizeof(offsets) / sizeof(offsets[0]); i++) {
+ lfs_soff_t off = offsets[i];
+ memcpy(buffer, "hedgehoghog", size);
+ lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) >= 0 => 1;
+ lfs_file_write(&lfs, &file[0], buffer, size) => size;
+ lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) => off+size;
+ lfs_file_read(&lfs, &file[0], buffer, size) => size;
+ memcmp(buffer, "hedgehoghog", size) => 0;
+
+ lfs_file_seek(&lfs, &file[0], 0, LFS_SEEK_SET) => off+size;
+ lfs_file_read(&lfs, &file[0], buffer, size) => size;
+ memcmp(buffer, "kittycatcat", size) => 0;
+
+ lfs_file_sync(&lfs, &file[0]) => 0;
+ }
+
+ lfs_file_close(&lfs, &file[0]) => 0;
+ lfs_unmount(&lfs) => 0;
+TEST
+
 echo "--- Results ---"
 tests/stats.py