exchange_rate: historical rates: merge old+new data, don't overwrite

- CoinGecko restricted its historical API to last 365 days
  - we used to ask for, and get, the whole history, but now we can only ask for the last year
  - so change HTTP request to only ask for 365 days
- we cache historical rates to disk
  - previously we used to overwrite what is already stored, with the newly obtained data
  - now this is changed so that we merge the new data into the already stored data

electrum/exchange_rate.py

@@ -8,7 +8,7 @@ import time
 import csv
 import decimal
 from decimal import Decimal
-from typing import Sequence, Optional, Mapping, Dict, Union, Any
+from typing import Sequence, Optional, Mapping, Dict, Union, Any, Tuple
 
 from aiorpcx.curio import timeout_after, TaskTimeout, ignore_after
 import aiohttp
@@ -97,30 +97,49 @@ class ExchangeBase(Logger):
             self._quotes_timestamp = time.time()
             self.on_quotes(received_new_data=True)
 
-    def read_historical_rates(self, ccy: str, cache_dir: str) -> Optional[dict]:
-        filename = os.path.join(cache_dir, self.name() + '_'+ ccy)
+    @staticmethod
+    def _read_historical_rates_from_file(
+        *, exchange_name: str, ccy: str, cache_dir: str,
+    ) -> Tuple[Optional[dict], Optional[float]]:
+        filename = os.path.join(cache_dir, f"{exchange_name}_{ccy}")
         if not os.path.exists(filename):
-            return None
+            return None, None
         timestamp = os.stat(filename).st_mtime
         try:
             with open(filename, 'r', encoding='utf-8') as f:
                 h = json.loads(f.read())
         except Exception:
-            return None
+            return None, None
         if not h:  # e.g. empty dict
-            return None
+            return None, None
         # cast rates to str
         h = {date_str: str(rate) for (date_str, rate) in h.items()}
+        return h, timestamp
+
+    def read_historical_rates(self, ccy: str, cache_dir: str) -> Optional[dict]:
+        h, timestamp = self._read_historical_rates_from_file(
+            exchange_name=self.name(),
+            ccy=ccy,
+            cache_dir=cache_dir,
+        )
         h['timestamp'] = timestamp
         self._history[ccy] = h
         self.on_history()
         return h
 
+    @staticmethod
+    def _write_historical_rates_to_file(
+        *, exchange_name: str, ccy: str, cache_dir: str, history: Dict[str, str],
+    ) -> None:
+        filename = os.path.join(cache_dir, f"{exchange_name}_{ccy}")
+        with open(filename, 'w', encoding='utf-8') as f:
+            f.write(json.dumps(history))
+
     @log_exceptions
     async def get_historical_rates_safe(self, ccy: str, cache_dir: str) -> None:
         try:
             self.logger.info(f"requesting fx history for {ccy}")
-            h = await self.request_history(ccy)
+            h_new = await self.request_history(ccy)
             self.logger.info(f"received fx history for {ccy}")
         except (aiohttp.ClientError, asyncio.TimeoutError, OSError) as e:
             self.logger.info(f"failed fx history: {repr(e)}")
@@ -129,10 +148,16 @@ class ExchangeBase(Logger):
             self.logger.exception(f"failed fx history: {repr(e)}")
             return
         # cast rates to str
-        h = {date_str: str(rate) for (date_str, rate) in h.items()}
-        filename = os.path.join(cache_dir, self.name() + '_' + ccy)
-        with open(filename, 'w', encoding='utf-8') as f:
-            f.write(json.dumps(h))
+        h_new = {date_str: str(rate) for (date_str, rate) in h_new.items()}
+        # merge old history and new history. resolve duplicate dates using new data.
+        h_old, _timestamp = self._read_historical_rates_from_file(
+            exchange_name=self.name(), ccy=ccy, cache_dir=cache_dir,
+        )
+        h = {**h_old, **h_new}
+        # write merged data to disk cache
+        self._write_historical_rates_to_file(
+            exchange_name=self.name(), ccy=ccy, cache_dir=cache_dir, history=h,
+        )
         h['timestamp'] = time.time()
         self._history[ccy] = h
         self.on_history()
@@ -352,8 +377,12 @@ class CoinGecko(ExchangeBase):
         return CURRENCIES[self.name()]
 
     async def request_history(self, ccy):
+        num_days = 365
+        # Setting `num_days = "max"` started erroring (around 2024-04) with:
+        # > Your request exceeds the allowed time range. Public API users are limited to querying
+        # > historical data within the past 365 days. Upgrade to a paid plan to enjoy full historical data access
         history = await self.get_json('api.coingecko.com',
-                                      '/api/v3/coins/bitcoin/market_chart?vs_currency=%s&days=max' % ccy)
+                                      f"/api/v3/coins/bitcoin/market_chart?vs_currency={ccy}&days={num_days}")
 
         return dict([(timestamp_to_datetime(h[0]/1000, utc=True).strftime('%Y-%m-%d'), str(h[1]))
                      for h in history['prices']])