fix(lint-gates): fail on an empty base scan instead of blaming the change for every violation (#45314)

Co-authored-by: mateo-berri <277851410+mateo-berri@users.noreply.github.com>

scripts/lint_base_counts.py

@@ -276,8 +276,9 @@ def base_counts_cached(
 ) -> Counts:
     """`compute` memoized on disk. The base tree at a given commit is immutable,
     so its counts are a pure function of the merge-base plus the checker's
-    fingerprints in the cache key; an empty result is never stored because it is
-    the signature of a crashed pass, not a clean tree. On a disk miss the counts
+    fingerprints in the cache key. An empty local result fails the gate instead of
+    being returned, because it is the signature of a crashed pass, not a clean
+    tree, and would make every existing violation look new. On a disk miss the counts
     CI already published for the merge-base are fetched before the expensive
     local base scan; a fetch miss of any kind computes locally."""
     directory: Final = default_cache_dir() if cache_dir is None else cache_dir
@@ -289,8 +290,14 @@ def base_counts_cached(
         store_counts(directory, checker, base_point, fetched)
         return fetched
     counts: Final = compute(base_point)
-    if counts:
-        store_counts(directory, checker, base_point, counts)
+    if not counts:
+        print(
+            f"FAIL: {checker.name} produced no violations for the base tree at {base_point[:12]}, "
+            "so every rule would look freshly added. The base pass almost certainly "
+            "crashed; refusing to blame this change for it."
+        )
+        raise SystemExit(1)
+    store_counts(directory, checker, base_point, counts)
     return counts
 
 

scripts/type_check_gate.py

@@ -279,15 +279,7 @@ def cmd_check(head: Mapping[str, int], base_ref: str) -> None:
         )
         raise SystemExit(1)
     base_point: Final = resolve_base_point(base_ref)
-    base: Final = base_counts_cached(checker_identity(), base_point, base_counts)
-    if not base:
-        print(
-            f"FAIL: basedpyright produced no errors for the base tree at {base_point[:12]}, "
-            "so every rule would look freshly added. The base pass almost certainly "
-            "crashed; refusing to blame this change for it."
-        )
-        raise SystemExit(1)
-    judge(head, base, base_point)
+    judge(head, base_counts_cached(checker_identity(), base_point, base_counts), base_point)
 
 
 def judge(head: Mapping[str, int], base: Mapping[str, int], base_point: str) -> None:

tests/unit/test_lint_base_counts.py

@@ -203,16 +203,15 @@ def test_base_counts_cached_keeps_each_checker_apart(tmp_path: Path) -> None:
     ) == {"reportAny": 4}
 
 
-def test_an_empty_base_pass_is_never_cached(tmp_path: Path) -> None:
-    calls: Final[list[str]] = []
-
-    def crashed(ref: str) -> counts.Counts:
-        calls.append(ref)
-        return {}
-
-    assert counts.base_counts_cached(_CHECKER, "abc123", crashed, cache_dir=tmp_path, fetch=_no_fetch) == {}
-    assert counts.base_counts_cached(_CHECKER, "abc123", crashed, cache_dir=tmp_path, fetch=_no_fetch) == {}
-    assert calls == ["abc123", "abc123"]
+def test_an_empty_base_pass_fails_the_gate_instead_of_blaming_the_change(
+    tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    with pytest.raises(SystemExit) as exit_info:
+        counts.base_counts_cached(
+            _CHECKER, "abc123def4567", lambda ref: {}, cache_dir=tmp_path, fetch=lambda checker, base_point: {}
+        )
+    assert exit_info.value.code == 1
+    assert f"FAIL: {_CHECKER.name} produced no violations for the base tree at abc123def456," in capsys.readouterr().out
     assert list(tmp_path.iterdir()) == []