repository: assert compact_pack complete coverage reaches pack end

src/borg/repository.py

@@ -864,6 +864,12 @@ class Repository:
             if keep:
                 kept.append((offset, obj_id, size))
 
+        if complete:  # the listed objects must reach the pack's end, no trailing object unaccounted for
+            pack_size = self.store.info(pack_key).size
+            assert (
+                covered == pack_size
+            ), f"pack {bin_to_hex(pack_id)}: {pack_size - covered} trailing bytes unaccounted for"
+
         for drop_id in drop_ids:  # remove dropped objects from the index
             del self.chunks[drop_id]
 

src/borg/testsuite/repository_test.py

@@ -267,6 +267,22 @@ def test_compact_pack_keep_all_is_noop(repo_fixtures, request):
         assert new_pack_id == old_pack_id
         assert pdchunk(repository.get(H(0))) == b"DATA0"
         assert pdchunk(repository.get(H(1))) == b"DATA1"
+
+
+def test_compact_pack_complete_detects_short_coverage(repo_fixtures, request):
+    # complete=True must catch a pack whose listed objects do not reach its end: shrink the last
+    # object's recorded obj_size so the summed coverage falls short of the actual pack file size.
+    chunk0 = fchunk(b"DATA0", chunk_id=H(0))
+    chunk1 = fchunk(b"DATA1", chunk_id=H(1))
+    repository = get_repository_from_fixture(repo_fixtures, request)
+    build_one_pack(repository, [(H(0), chunk0), (H(1), chunk1)])
+    with repository:
+        old_pack_id = repository.chunks[H(0)].pack_id
+        entry = repository.chunks[H(1)]
+        repository.chunks[H(1)] = entry._replace(obj_size=entry.obj_size - 1)  # leave 1 trailing byte unaccounted
+
+        with pytest.raises(AssertionError):
+            repository.compact_pack(old_pack_id, keep_ids={H(0), H(1)}, drop_ids=set())
         assert bin_to_hex(old_pack_id) in [info.name for info in repository.store_list("packs")]