test: harden retriable predicate tests

planning/changes/2026-06-26.02-retriable-test-hardening/change.md

@@ -0,0 +1,37 @@
+---
+summary: Hardened tests/test_retriable.py — made the `.orig` invariant explicit in the helper and added a cycle case whose loop contains a retriable DBAPIError link.
+---
+
+# Change: Harden the retriable predicate tests
+
+**Lane:** lightweight — ≲30 LOC net, ≤2 files, no new file, no public-API
+change, a single straightforward test.
+
+## Goal
+
+Fold the two deferred Minor findings from the [retriable-error-seam
+review](../2026-06-26.01-retriable-error-seam/design.md)'s final whole-branch
+pass into the test suite, so the predicate's invariants are explicit rather than
+implicit.
+
+## Approach
+
+Test-only hardening of the in-memory predicate suite. No production code or
+capability contract changes, so no `architecture/` promotion.
+
+- The `_make_dbapi_error` helper now asserts `err.orig is not None`. The whole
+  matrix hinges on `.orig` being set (the predicate reads `.orig.__cause__`);
+  the assertion makes that load-bearing invariant explicit instead of relying on
+  the True-expected cases to fail if it ever broke.
+- A new case complements `test_is_retriable_cause_cycle_terminates`: a cyclic
+  chain that *contains* a retriable `DBAPIError` link returns `True` — the walk
+  finds the link before the cycle guard would re-visit a node.
+
+## Files
+
+- `tests/test_retriable.py` — assertion in the helper; one new cycle-with-link test.
+
+## Verification
+
+- [x] `just test` — full suite green (25 passed, 100% coverage).
+- [x] `just lint-ci` — clean.

tests/test_retriable.py

@@ -8,7 +8,9 @@
 def _make_dbapi_error(cause: BaseException) -> DBAPIError:
     orig = Exception("db error")
     orig.__cause__ = cause
-    return DBAPIError("SELECT 1", None, orig)
+    err = DBAPIError("SELECT 1", None, orig)
+    assert err.orig is not None  # the predicate hinges on .orig being set; make the invariant explicit
+    return err
 
 
 def test_retriable_asyncpg_errors_contains_expected_classes() -> None:
@@ -55,3 +57,13 @@ def test_is_retriable_cause_cycle_terminates() -> None:
     a.__cause__ = b
     b.__cause__ = a
     assert is_retriable(a) is False
+
+
+def test_is_retriable_cycle_containing_retriable_link() -> None:
+    # A cyclic chain that *contains* a retriable DBAPIError still returns True:
+    # the walk finds the retriable link before the cycle guard would re-visit a node.
+    outer = ValueError("outer")
+    dbapi_err = _make_dbapi_error(asyncpg.SerializationError())
+    outer.__cause__ = dbapi_err
+    dbapi_err.__cause__ = outer
+    assert is_retriable(outer) is True