chunk-format: store chunk offset during write

As a preparatory step to allowing trailing table of contents, store the offsets of each chunk as we write them. This replaces an existing use of a local variable, but the stored value will be used in the next change. Signed-off-by: Derrick Stolee <derrickstolee@github.com>

Author: derrickstolee

chunk-format.c

@@ -13,6 +13,7 @@ struct chunk_info {
 chunk_write_fn write_fn;
 
 const void *start;
+off_t offset;
 };
 
 struct chunkfile {
@@ -78,16 +79,16 @@ int write_chunkfile(struct chunkfile *cf, void *data)
 hashwrite_be64(cf->f, cur_offset);
 
 for (i = 0; i < cf->chunks_nr; i++) {
-off_t start_offset = hashfile_total(cf->f);
+cf->chunks[i].offset = hashfile_total(cf->f);
 result = cf->chunks[i].write_fn(cf->f, data);
 
 if (result)
 goto cleanup;
 
-if (hashfile_total(cf->f) - start_offset != cf->chunks[i].size)
+if (hashfile_total(cf->f) - cf->chunks[i].offset != cf->chunks[i].size)
 BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
  cf->chunks[i].size, cf->chunks[i].id,
- hashfile_total(cf->f) - start_offset);
+ hashfile_total(cf->f) - cf->chunks[i].offset);
 }
 
 cleanup: