Add unit tests for RetryAfterHandler

Covers numeric and HTTP-date Retry-After parsing, past-date clamping,
retry count tracking, and HTTPError raise after max_retries exhausted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: namedgraph

tests/unit/test_retry_after_handler.py

@@ -0,0 +1,124 @@
+"""Tests for RetryAfterHandler — HTTP 429 retry logic (src/web_algebra/client.py).
+
+Not in formal-semantics.md (client infrastructure, not an operation).
+Behaviour spec: on 429, sleep Retry-After seconds then retry; raise HTTPError
+after max_retries exhausted.
+"""
+
+from __future__ import annotations
+
+import urllib.error
+from datetime import datetime, timedelta, timezone
+from email.utils import format_datetime
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from web_algebra.client import RetryAfterHandler
+
+
+def _req(url: str = "http://example.org/resource") -> MagicMock:
+    req = MagicMock()
+    req.full_url = url
+    return req
+
+
+class TestRetryAfterHandlerNumericDelay:
+    def test_sleeps_numeric_retry_after(self):
+        handler = RetryAfterHandler()
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+
+        with patch("web_algebra.client.time.sleep") as mock_sleep:
+            result = handler.http_error_429(
+                _req(), None, 429, "Too Many Requests", {"Retry-After": "2"}
+            )
+
+        mock_sleep.assert_called_once_with(2.0)
+        assert result == "ok"
+
+    def test_defaults_to_1s_when_header_absent(self):
+        handler = RetryAfterHandler()
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+
+        with patch("web_algebra.client.time.sleep") as mock_sleep:
+            handler.http_error_429(_req(), None, 429, "Too Many Requests", {})
+
+        mock_sleep.assert_called_once_with(1.0)
+
+
+class TestRetryAfterHandlerDateDelay:
+    def test_sleeps_computed_seconds_for_future_http_date(self):
+        handler = RetryAfterHandler()
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+
+        future = datetime.now(tz=timezone.utc) + timedelta(seconds=5)
+        hdrs = {"Retry-After": format_datetime(future)}
+
+        with patch("web_algebra.client.time.sleep") as mock_sleep:
+            handler.http_error_429(_req(), None, 429, "Too Many Requests", hdrs)
+
+        delay = mock_sleep.call_args[0][0]
+        assert 4.0 <= delay <= 6.0
+
+    def test_clamps_past_http_date_to_zero(self):
+        handler = RetryAfterHandler()
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+
+        past = datetime.now(tz=timezone.utc) - timedelta(seconds=10)
+        hdrs = {"Retry-After": format_datetime(past)}
+
+        with patch("web_algebra.client.time.sleep") as mock_sleep:
+            handler.http_error_429(_req(), None, 429, "Too Many Requests", hdrs)
+
+        mock_sleep.assert_called_once_with(0.0)
+
+
+class TestRetryAfterHandlerRetryLimit:
+    def test_raises_http_error_when_max_retries_exhausted(self):
+        handler = RetryAfterHandler(max_retries=2)
+        handler.parent = MagicMock()
+        req = _req()
+        handler._retry_counts[req.full_url] = 2
+
+        with patch("web_algebra.client.time.sleep"):
+            with pytest.raises(urllib.error.HTTPError):
+                handler.http_error_429(req, None, 429, "Too Many Requests", {})
+
+    def test_clears_retry_count_after_exhaustion(self):
+        handler = RetryAfterHandler(max_retries=1)
+        handler.parent = MagicMock()
+        req = _req()
+        handler._retry_counts[req.full_url] = 1
+
+        with patch("web_algebra.client.time.sleep"):
+            with pytest.raises(urllib.error.HTTPError):
+                handler.http_error_429(req, None, 429, "Too Many Requests", {})
+
+        assert req.full_url not in handler._retry_counts
+
+    def test_increments_retry_count_on_each_attempt(self):
+        handler = RetryAfterHandler(max_retries=3)
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+        req = _req()
+
+        with patch("web_algebra.client.time.sleep"):
+            handler.http_error_429(req, None, 429, "Too Many Requests", {"Retry-After": "0"})
+
+        assert handler._retry_counts[req.full_url] == 1
+
+    def test_retries_up_to_but_not_exceeding_max_retries(self):
+        handler = RetryAfterHandler(max_retries=2)
+        handler.parent = MagicMock()
+        handler.parent.open.return_value = "ok"
+        req = _req()
+
+        with patch("web_algebra.client.time.sleep"):
+            handler.http_error_429(req, None, 429, "Too Many Requests", {"Retry-After": "0"})
+            handler.http_error_429(req, None, 429, "Too Many Requests", {"Retry-After": "0"})
+            with pytest.raises(urllib.error.HTTPError):
+                handler.http_error_429(req, None, 429, "Too Many Requests", {"Retry-After": "0"})