Refresh FX rates when configured currencies change

Compare the latest cached currency list before applying cache TTL and match cache filenames exactly. Preserve offline mode and failure retry backoff.

Closes #170

tests/test_fx_currency_changes.py

@@ -0,0 +1,93 @@
+"""Currency configuration changes must bypass the FX cache interval."""
+
+import json
+from datetime import datetime
+from pathlib import Path
+from unittest.mock import patch
+
+import flask
+import pytest
+
+from agenda.fx import get_rates
+
+
+@pytest.mark.parametrize(
+    ("previous", "current", "fetch"),
+    [
+        (["USD"], ["USD", "EUR"], True),
+        (["EUR", "USD"], ["USD"], True),
+        (["EUR", "USD"], ["USD", "EUR"], False),
+    ],
+)
+def test_currency_changes(
+    tmp_path: Path, previous: list[str], current: list[str], fetch: bool
+) -> None:
+    """Fetch additions/removals even if an older matching cache is still fresh."""
+    fx_dir = tmp_path / "fx"
+    fx_dir.mkdir()
+
+    def cache(timestamp: str, currencies: list[str]) -> None:
+        suffix = ",".join(sorted(currencies))
+        filename = fx_dir / f"{timestamp}_frankfurter_{suffix}_to_GBP.json"
+        filename.write_text(
+            json.dumps([{"quote": c, "rate": 1.25} for c in currencies])
+        )
+
+    cache("2026-10-01_10:00", current)
+    cache("2026-10-01_11:00", previous)
+    config = flask.config.Config(
+        "/",
+        {
+            "CURRENCIES": current,
+            "DATA_DIR": str(tmp_path),
+            "OFFLINE_MODE": False,
+            "FX_CACHE_TTL_HOURS": 12,
+        },
+    )
+    with (
+        patch("agenda.fx.datetime") as clock,
+        patch("agenda.fx.httpx.Client") as client,
+    ):
+        clock.now.return_value = datetime(2026, 10, 1, 12)
+        clock.strptime.side_effect = datetime.strptime
+        get = client.return_value.__enter__.return_value.get
+        get.return_value.text = json.dumps([{"quote": c, "rate": 1.3} for c in current])
+        result = get_rates(config)
+        assert set(result) == set(current)
+        assert get.call_count == int(fetch)
+        # The same configuration must use the newly fetched cache.
+        get_rates(config)
+        assert get.call_count == int(fetch)
+
+
+def test_changed_currencies_preserve_failure_backoff(tmp_path: Path) -> None:
+    """A failed fetch for the new list should not be retried on every cron run."""
+    fx_dir = tmp_path / "fx"
+    fx_dir.mkdir()
+    (fx_dir / "2026-10-01_11:00_frankfurter_EUR,USD_to_GBP.json").write_text(
+        json.dumps([{"quote": "EUR", "rate": 1.15}, {"quote": "USD", "rate": 1.25}])
+    )
+    config = flask.config.Config(
+        "/", {"CURRENCIES": ["USD"], "DATA_DIR": str(tmp_path), "OFFLINE_MODE": False}
+    )
+    with (
+        patch("agenda.fx.datetime") as clock,
+        patch("agenda.fx.httpx.Client") as client,
+    ):
+        clock.now.return_value = datetime(2026, 10, 1, 12)
+        clock.strptime.side_effect = datetime.strptime
+        get = client.return_value.__enter__.return_value.get
+        get.return_value.text = "[]"
+        assert get_rates(config) == {}
+        assert get_rates(config) == {}
+        assert get.call_count == 1
+
+
+def test_currency_changes_respect_offline_mode(tmp_path: Path) -> None:
+    """Changing configuration must not trigger network requests when offline."""
+    config = flask.config.Config(
+        "/", {"CURRENCIES": ["USD"], "DATA_DIR": str(tmp_path), "OFFLINE_MODE": True}
+    )
+    with patch("agenda.fx.httpx.Client") as client:
+        assert get_rates(config) == {}
+        client.assert_not_called()