fix(structural): preserve parser failure blob provenance

diffgraph/structural.py

@@ -726,6 +726,19 @@ def _resolution_warning(item: ResolutionWarning) -> Dict[str, str]:
     return _warning(code, item.path, "{}: {}".format(item.code, item.message))
 
 
+def _parse_failure_warning(
+    side: str, path: str, oid: Optional[str], error: Exception,
+) -> Dict[str, str]:
+    """Preserve the exact immutable snapshot that Tree-sitter could not parse."""
+    return _warning(
+        "PARSE_FAILURE",
+        path,
+        "{}-change blob {}: {}: {}".format(
+            side, oid or "absent", type(error).__name__, error,
+        ),
+    )
+
+
 def _symbol_id(path: str, qualified_name: str) -> str:
     return "sym::{}::{}".format(path, qualified_name)
 
@@ -974,15 +987,19 @@ def analyze_local_diff(
                 _parse_python(old) if old is not None else ([], [], [], {}, [])
             )
         except parser_errors as error:
-            warnings.append(_warning("PARSE_FAILURE", entry.old_path or path, "pre-change: {}: {}".format(type(error).__name__, error)))
+            warnings.append(_parse_failure_warning(
+                "pre", entry.old_path or path, entry.old_oid, error,
+            ))
             skipped += 1
             continue
         try:
             new_symbols, new_imports, new_calls, new_bindings, new_rebindings = (
                 _parse_python(new) if new is not None else ([], [], [], {}, [])
             )
         except parser_errors as error:
-            warnings.append(_warning("PARSE_FAILURE", path, "post-change: {}: {}".format(type(error).__name__, error)))
+            warnings.append(_parse_failure_warning(
+                "post", path, entry.new_oid, error,
+            ))
             skipped += 1
             continue
         analyzed += 1

tests/fixtures/python_parser_failure.json

@@ -16,7 +16,7 @@
       {
         "code": "PARSE_FAILURE",
         "file": "broken.py",
-        "detail": "post-change: ValueError: Tree-sitter reported a syntax error at line 1, column 12"
+        "detail": "post-change blob c440d6d7f7fca646fb897d0b5d279ec775aeddfd: ValueError: Tree-sitter reported a syntax error at line 1, column 12"
       }
     ]
   }

tests/test_structural.py

@@ -421,6 +421,30 @@ def test_parse_failure_is_scoped_and_matches_the_golden_fixture(tmp_path):
     assert actual == expected
 
 
+def test_pre_change_parse_failure_names_the_immutable_blob(tmp_path):
+    """A broken pre-change snapshot remains reproducible after it is fixed."""
+    root = repo(tmp_path)
+    write(root, "recovered.py", "def broken(:\n")
+    commit(root)
+    old_oid = git(root, "rev-parse", "HEAD:recovered.py")
+    write(root, "recovered.py", "def recovered():\n    return True\n")
+
+    artifact = analyze_local_diff(str(root))
+
+    assert_valid(artifact)
+    assert artifact["metadata"]["files_analyzed"] == 0
+    assert artifact["metadata"]["files_skipped"] == 1
+    assert artifact["symbols"] == []
+    assert artifact["metadata"]["warnings"] == [{
+        "code": "PARSE_FAILURE",
+        "file": "recovered.py",
+        "detail": (
+            "pre-change blob {}: ValueError: Tree-sitter reported a syntax error "
+            "at line 1, column 12"
+        ).format(old_oid),
+    }]
+
+
 def test_no_network_calls(monkeypatch, tmp_path):
     root = repo(tmp_path)
     write(root, "a.py", "def a():\n    pass\n")
@@ -1254,9 +1278,9 @@ def unavailable():
         "code": "PARSE_FAILURE",
         "file": "app.py",
         "detail": (
-            "post-change: StructuralDependencyError: "
+            "post-change blob {}: StructuralDependencyError: "
             "parser dependency is unavailable"
-        ),
+        ).format(git(root, "hash-object", "app.py")),
     }]