fix: retry DNS URL probe failures

scripts/url_probe.py

@@ -94,10 +94,7 @@ def validate_public_url(url: str) -> tuple[str, int, str]:
         raise UnsafeUrlError("port zero is not allowed")
 
     port = explicit_port or (443 if scheme == "https" else 80)
-    try:
-        results = socket.getaddrinfo(hostname, port, type=socket.SOCK_STREAM)
-    except OSError as error:
-        raise UnsafeUrlError("hostname did not resolve") from error
+    results = socket.getaddrinfo(hostname, port, type=socket.SOCK_STREAM)
 
     addresses: list[str] = []
     for _family, _socktype, _protocol, _canonname, sockaddr in results:
@@ -349,5 +346,15 @@ def probe_url(
                     (),
                     f"transport error: {last_transport_error}",
                 )
+        except UnsafeUrlError as error:
+            prefix = "unsafe URL" if current_url == requested_url else "unsafe redirect target"
+            return _http_error(
+                requested_url,
+                current_url,
+                None,
+                attempt,
+                redirects,
+                f"{prefix}: {error}",
+            )
 
     raise AssertionError("retry loop exhausted unexpectedly")

tests/test_url_probe.py

@@ -106,6 +106,43 @@ class UrlProbeTests(unittest.TestCase):
             ):
                 validate_public_url(PUBLIC_URL)
 
+    def test_dns_resolution_failures_are_retried_as_transient(self):
+        delays = []
+
+        with patch(
+            "scripts.url_probe.socket.getaddrinfo",
+            side_effect=socket.gaierror("no dns"),
+        ) as getaddrinfo:
+            observation = probe_url(PUBLIC_URL, sleeper=delays.append)
+
+        self.assertEqual(observation.outcome, Outcome.TRANSIENT)
+        self.assertEqual(observation.attempts, 3)
+        self.assertEqual(observation.error, "transport error: gaierror")
+        self.assertEqual(delays, [0, 1, 2])
+        self.assertEqual(getaddrinfo.call_count, 3)
+
+    def test_default_requester_revalidation_failure_is_an_http_error(self):
+        public_answer = [
+            (socket.AF_INET, socket.SOCK_STREAM, 6, "", ("8.8.8.8", 443))
+        ]
+        private_answer = [
+            (socket.AF_INET, socket.SOCK_STREAM, 6, "", ("127.0.0.1", 443))
+        ]
+
+        with patch(
+            "scripts.url_probe.socket.getaddrinfo",
+            side_effect=[public_answer, private_answer],
+        ) as getaddrinfo:
+            observation = probe_url(PUBLIC_URL, sleeper=lambda _: None)
+
+        self.assertEqual(observation.outcome, Outcome.HTTP_ERROR)
+        self.assertEqual(observation.attempts, 1)
+        self.assertEqual(
+            observation.error,
+            "unsafe URL: resolved address is not globally routable",
+        )
+        self.assertEqual(getaddrinfo.call_count, 2)
+
     def test_missing_redirect_location_is_an_http_error(self):
         with patch("scripts.url_probe.validate_public_url", side_effect=public_validation):
             observation = probe_url(