Add regression tests for redis/requests-cache version compatibility

.github/workflows/test.yml

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+name: Test
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install pipenv
+        run: pip install pipenv
+
+      - name: Export locked dependencies (excluding gdal, which needs a matching system libgdal)
+        run: pipenv requirements | grep -v -e '^-i' -e '^gdal' > requirements-ci.txt
+
+      - name: Install dependencies
+        run: pip install -r requirements-ci.txt pytest
+
+      - name: Run tests
+        run: pytest census_extractomatic/test_aggregate_acs.py census_extractomatic/test_moe.py census_extractomatic/test_redis_compat.py -v

census_extractomatic/test_redis_compat.py

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+"""Regression test for the redis-py/Redis-server version incompatibility that took
+down production on 2026-07-28 (census-api#98).
+
+The deployed Redis server is v5.0.7, which predates Redis 6.0 and doesn't support
+the HELLO command or ACL-style `AUTH user pass`. redis-py >= 6 tries to negotiate
+protocol version via HELLO whenever the connection URL includes a username (ours
+does), and fails outright against this server rather than falling back. This test
+runs a real redis:5.0.7 container (not a mock) so it actually exercises that
+protocol behavior, and exercises the app's real flask_caching -> cachelib -> redis-py
+integration path rather than a bare redis-py client.
+
+Requires Docker to be available; fails (not skips) if it isn't, since this is the
+only way to genuinely catch this class of bug.
+"""
+import shutil
+import socket
+import subprocess
+import time
+import tomllib
+from pathlib import Path
+
+import pytest
+from flask import Flask
+from flask_caching import Cache
+
+REDIS_IMAGE = 'redis:5.0.7'
+REDIS_PASSWORD = 'testpassword'
+
+
+def _free_port():
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(('127.0.0.1', 0))
+        return s.getsockname()[1]
+
+
+@pytest.fixture(scope='module')
+def redis_url():
+    if shutil.which('docker') is None:
+        pytest.fail(
+            'docker is required to run this test (it validates real redis-py '
+            'wire-protocol behavior against a real Redis 5.0.7 server, which a '
+            'mock cannot reproduce)'
+        )
+
+    port = _free_port()
+    container_id = subprocess.run(
+        [
+            'docker', 'run', '-d', '--rm',
+            '-p', f'{port}:6379',
+            REDIS_IMAGE,
+            'redis-server', '--requirepass', REDIS_PASSWORD,
+        ],
+        capture_output=True, text=True, check=True,
+    ).stdout.strip()
+
+    try:
+        for _ in range(30):
+            ping = subprocess.run(
+                ['docker', 'exec', container_id, 'redis-cli', '-a', REDIS_PASSWORD,
+                 '--no-auth-warning', 'ping'],
+                capture_output=True, text=True,
+            )
+            if ping.stdout.strip() == 'PONG':
+                break
+            time.sleep(0.5)
+        else:
+            pytest.fail('redis:5.0.7 container did not become ready in time')
+
+        # Matches the shape of the real production REDIS_URL (dokku's redis
+        # plugin sets a username ahead of the password even though this Redis
+        # version has no concept of named users).
+        yield f'redis://censusreporter:{REDIS_PASSWORD}@127.0.0.1:{port}/0'
+    finally:
+        subprocess.run(['docker', 'stop', container_id], capture_output=True)
+
+
+def test_cache_get_set_against_real_redis_5_0_7(redis_url):
+    app = Flask(__name__)
+    app.config['CACHE_TYPE'] = 'redis'
+    app.config['CACHE_REDIS_URL'] = redis_url
+    cache = Cache(app)
+
+    with app.app_context():
+        assert cache.get('missing-key') is None
+        cache.set('some-key', {'geoid': '16000US1714000'})
+        assert cache.get('some-key') == {'geoid': '16000US1714000'}
+
+
+def test_redis_stays_explicitly_pinned():
+    pipfile = tomllib.loads(
+        (Path(__file__).parent.parent / 'Pipfile').read_text()
+    )
+    version = pipfile['packages']['redis']
+    assert version != '*', (
+        'redis is unpinned again - this silently broke production once already '
+        '(see test_cache_get_set_against_real_redis_5_0_7); pin it explicitly'
+    )