Test Retry-After support with multiple local timezones

dev-requirements.txt

@@ -15,3 +15,6 @@ gcp-devrel-py-tools==0.0.15
 
 # https://github.com/GoogleCloudPlatform/python-repo-tools/issues/23
 pylint<2.0;python_version<="2.7"
+
+pytz==2020.1
+tzlocal==2.1

test/conftest.py

@@ -8,6 +8,8 @@
 import trustme
 from tornado import web, ioloop
 
+from .tz_fake import fake_timezone_ctx
+
 from dummyserver.handlers import TestingApp
 from dummyserver.server import run_tornado_app
 from dummyserver.server import HAS_IPV6
@@ -96,3 +98,12 @@ def ipv6_san_server(tmp_path_factory):
 
     with run_server_in_thread("https", "::1", tmpdir, ca, server_cert) as cfg:
         yield cfg
+
+
+@pytest.yield_fixture
+def fake_timezone(request):
+    """
+    A pytest fixture that runs the test with a fake timezone.
+    """
+    with fake_timezone_ctx(request.param):
+        yield

test/test_retry.py

@@ -332,6 +332,16 @@ def test_respect_retry_after_header_propagated(self, respect_retry_after_header)
             ("Mon Jun  3 11:30:12 2019", True, 1812),
         ],
     )
+    @pytest.mark.parametrize(
+        "fake_timezone",
+        [
+            "UTC",
+            "Asia/Jerusalem",
+            None,
+        ],
+        indirect=True,
+    )
+    @pytest.mark.usefixtures("fake_timezone")
     def test_respect_retry_after_header_sleep(
         self, retry_after_header, respect_retry_after_header, sleep_duration
     ):

test/tz_fake.py

@@ -0,0 +1,37 @@
+import pytz
+from contextlib import contextmanager
+import time
+import os
+import pytest
+import tzlocal
+
+
+@contextmanager
+def fake_timezone_ctx(tz):
+    """
+    Switch to a locally-known timezone specified by `tz`.
+    On exit, restore the previous timezone.
+    If `tz` is `None`, do nothing.
+    """
+    if tz is None:
+        yield
+        return
+
+    if not hasattr(time, "tzset"):
+        pytest.skip("Timezone patching is not supported")
+
+    # Make sure the new timezone exists, at least in pytz
+    if tz not in pytz.all_timezones:
+        raise ValueError("Invalid timezone specified: %r" % (tz,))
+
+    # Get the current timezone
+    try:
+        old_tz = tzlocal.get_localzone().zone
+    except ValueError:
+        pytest.skip("Cannot determine current timezone")
+
+    os.environ["TZ"] = tz
+    time.tzset()
+    yield
+    os.environ["TZ"] = old_tz
+    time.tzset()