tests: preserve MTA-STS failure diagnostics

contrib/testing/smtp-dane-mta-sts-check.py

@@ -1408,6 +1408,62 @@ def validate_tlsrpt_database(database: Path, start_day: int) -> None:
             "TLSRPT did not aggregate both cached-policy successes")
 
 
+def failure_diagnostics(
+    policy_server: PolicyServer | None,
+    cache_directory: Path | None,
+) -> str:
+    """Capture volatile MTA-STS evidence before the fixture is removed."""
+
+    details: list[str] = []
+    if policy_server is None:
+        details.append("policy-server=stopped")
+    else:
+        requests = list(policy_server.requests)  # type: ignore[attr-defined]
+        details.append(
+            "policy-requests=" + (",".join(requests) if requests else "<none>"))
+
+    dig = shutil.which("dig")
+    if dig is None:
+        details.append("dig=unavailable")
+    else:
+        for route in ROUTES:
+            if route.sts_mode is None:
+                continue
+            completed = run(
+                [
+                    dig,
+                    f"@{DNS_RESOLVER}",
+                    "+time=1",
+                    "+tries=1",
+                    "+short",
+                    "TXT",
+                    f"_mta-sts.{route.domain}",
+                ],
+                check=False,
+            )
+            answer = completed.stdout.strip()
+            error = completed.stderr.strip()
+            details.append(
+                f"dns-txt[{route.domain}]="
+                + (answer or error or f"<exit-{completed.returncode}>"))
+
+    if cache_directory is None or not cache_directory.exists():
+        details.append("policy-cache=<missing>")
+    else:
+        paths = sorted(
+            path for path in cache_directory.iterdir() if path.is_file())
+        if not paths:
+            details.append("policy-cache=<empty>")
+        for path in paths:
+            try:
+                content = path.read_text(
+                    encoding="utf-8", errors="replace").strip()
+            except OSError as error:
+                content = f"<unreadable: {error}>"
+            details.append(f"policy-cache[{path.name}]={content}")
+    return "\n--- MTA-STS diagnostics ---\n" + "\n".join(details)
+
+
 def append_trust_anchor(anchor: str) -> None:
     if ANCHOR_BACKUP.exists():
         raise SMTP27Error(
@@ -1479,6 +1535,7 @@ def perform_test(dns_stack: str) -> None:
     resolver_process: subprocess.Popen | None = None
     mail_servers: list[ThreadingTCPServer] = []
     policy_server: PolicyServer | None = None
+    cache_directory: Path | None = None
     primary_error: BaseException | None = None
 
     def terminate(_signum: int, _frame) -> None:
@@ -1643,7 +1700,8 @@ def perform_test(dns_stack: str) -> None:
         if tlsrpt_database is not None:
             validate_tlsrpt_database(tlsrpt_database, start_day)
     except BaseException as error:
-        primary_error = error
+        diagnostics = failure_diagnostics(policy_server, cache_directory)
+        primary_error = SMTP27Error(f"{error}{diagnostics}")
     finally:
         restoration_errors: list[str] = []
         if policy_server is not None: