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

agenda/fx.py

@@ -150,7 +150,7 @@ def get_rates_exchangerate_host(config: flask.config.Config) -> dict[str, Decima
     currency_string = ",".join(sorted(currencies))
     file_suffix = f"{currency_string}_to_GBP.json"
     existing_data = os.listdir(fx_dir)
-    existing_files = [f for f in existing_data if f.endswith(file_suffix)]
+    existing_files = [f for f in existing_data if f[17:] == file_suffix]
 
     latest_attempt = _latest_file(fx_dir, existing_files, currencies, valid_only=False)
     latest_valid = _latest_file(fx_dir, existing_files, currencies, valid_only=True)
@@ -222,9 +222,17 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
     valid_cache_files = [
         f
         for f in existing_data
-        if f.endswith(legacy_file_suffix) or f.endswith(frankfurter_file_suffix)
+        if f[17:] in (legacy_file_suffix, frankfurter_file_suffix)
     ]
-    attempt_files = [f for f in existing_data if f.endswith(frankfurter_file_suffix)]
+    attempt_files = [f for f in existing_data if f[17:] == frankfurter_file_suffix]
+    # Cache filenames already record the requested currencies. Compare against
+    # the newest request, including failed attempts, so failures retain backoff.
+    all_rate_files = [f for f in existing_data if f.endswith("_to_GBP.json")]
+    latest_request = max(all_rate_files) if all_rate_files else None
+    currencies_changed = latest_request is not None and latest_request[17:] not in (
+        legacy_file_suffix,
+        frankfurter_file_suffix,
+    )
 
     latest_attempt = _latest_file(fx_dir, attempt_files, currencies, valid_only=False)
     latest_source_valid = _latest_file(
@@ -238,7 +246,7 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
     if config["OFFLINE_MODE"]:
         return read_cached_rates(latest_valid_path, currencies)
 
-    if latest_source_valid is not None:
+    if latest_source_valid is not None and not currencies_changed:
         recent = _fx_cache_datetime(latest_source_valid)
         delta = now - recent
 
@@ -247,7 +255,7 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
                 os.path.join(fx_dir, latest_source_valid), currencies
             )
 
-    if latest_attempt is not None:
+    if latest_attempt is not None and not currencies_changed:
         recent_attempt = _fx_cache_datetime(latest_attempt)
         attempt_delta = now - recent_attempt
         if attempt_delta < timedelta(hours=failure_retry_hours):

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()