Handle FX API usage limit fallback

Closes #211

agenda/fx.py

@@ -9,6 +9,9 @@ from decimal import Decimal
 import flask
 import httpx
 
+DEFAULT_FX_CACHE_TTL_HOURS = 24
+DEFAULT_FX_FAILURE_RETRY_HOURS = 24
+
 
 async def get_gbpusd(config: flask.config.Config) -> Decimal:
     """Get the current value for GBPUSD, with caching."""
@@ -54,18 +57,51 @@ def read_cached_rates(
     with open(filename) as file:
         data = json.load(file, parse_float=Decimal)
 
+    quotes = data.get("quotes")
+    if not isinstance(quotes, dict):
+        return {}
+
     return {
-        cur: Decimal(data["quotes"][f"GBP{cur}"])
-        for cur in currencies
-        if f"GBP{cur}" in data["quotes"]
+        cur: Decimal(quotes[f"GBP{cur}"]) for cur in currencies if f"GBP{cur}" in quotes
     }
 
 
+def _fx_cache_datetime(filename: str) -> datetime:
+    """Extract the cache timestamp from an FX cache filename."""
+    return datetime.strptime(filename[:16], "%Y-%m-%d_%H:%M")
+
+
+def _has_required_quotes(filename: str, currencies: list[str]) -> bool:
+    """Return true if the cache file contains the requested GBP quotes."""
+    try:
+        return len(read_cached_rates(filename, currencies)) == len(currencies)
+    except (OSError, json.JSONDecodeError):
+        return False
+
+
+def _latest_file(
+    fx_dir: str, filenames: list[str], currencies: list[str], *, valid_only: bool
+) -> str | None:
+    """Return the newest matching FX cache filename."""
+    matching_files = [
+        filename
+        for filename in filenames
+        if not valid_only
+        or _has_required_quotes(os.path.join(fx_dir, filename), currencies)
+    ]
+
+    return max(matching_files) if matching_files else None
+
+
 def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
     """Get current values of exchange rates for a list of currencies against GBP."""
     currencies = config["CURRENCIES"]
     access_key = config["EXCHANGERATE_ACCESS_KEY"]
     data_dir = config["DATA_DIR"]
+    cache_ttl_hours = int(config.get("FX_CACHE_TTL_HOURS", DEFAULT_FX_CACHE_TTL_HOURS))
+    failure_retry_hours = int(
+        config.get("FX_FAILURE_RETRY_HOURS", DEFAULT_FX_FAILURE_RETRY_HOURS)
+    )
 
     now = datetime.now()
     now_str = now.strftime("%Y-%m-%d_%H:%M")
@@ -75,20 +111,26 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
     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(".json")]
+    existing_files = [f for f in existing_data if f.endswith(file_suffix)]
 
-    full_path: str | None = None
+    latest_attempt = _latest_file(fx_dir, existing_files, currencies, valid_only=False)
+    latest_valid = _latest_file(fx_dir, existing_files, currencies, valid_only=True)
+    latest_valid_path = (
+        os.path.join(fx_dir, latest_valid) if latest_valid is not None else None
+    )
 
-    if existing_files:
-        recent_filename = max(existing_files)
-        recent = datetime.strptime(recent_filename[:16], "%Y-%m-%d_%H:%M")
+    if latest_valid is not None:
+        recent = _fx_cache_datetime(latest_valid)
         delta = now - recent
 
-        full_path = os.path.join(fx_dir, recent_filename)
-        if recent_filename.endswith(file_suffix) and (
-            delta < timedelta(hours=12) or config["OFFLINE_MODE"]
-        ):
-            return read_cached_rates(full_path, currencies)
+        if delta < timedelta(hours=cache_ttl_hours) or config["OFFLINE_MODE"]:
+            return read_cached_rates(latest_valid_path, currencies)
+
+    if latest_attempt is not None:
+        recent_attempt = _fx_cache_datetime(latest_attempt)
+        attempt_delta = now - recent_attempt
+        if attempt_delta < timedelta(hours=failure_retry_hours):
+            return read_cached_rates(latest_valid_path, currencies)
 
     url = "http://api.exchangerate.host/live"
     params = {"currencies": currency_string, "source": "GBP", "access_key": access_key}
@@ -98,12 +140,17 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
         with httpx.Client() as client:
             response = client.get(url, params=params, timeout=10)
     except (httpx.ConnectError, httpx.ReadTimeout):
-        return read_cached_rates(full_path, currencies)
+        return read_cached_rates(latest_valid_path, currencies)
 
     try:
         data = json.loads(response.text, parse_float=Decimal)
     except json.decoder.JSONDecodeError:
-        return read_cached_rates(full_path, currencies)
+        return read_cached_rates(latest_valid_path, currencies)
+
+    if not data.get("success", True) or not isinstance(data.get("quotes"), dict):
+        with open(os.path.join(fx_dir, filename), "w") as file:
+            file.write(response.text)
+        return read_cached_rates(latest_valid_path, currencies)
 
     with open(os.path.join(fx_dir, filename), "w") as file:
         file.write(response.text)