Merge pull request #236 from googlefonts/watchfiles-workaround

On exit, suppress a buggy exception from watchfiles

src/fontra/core/fonthandler.py

@@ -66,7 +66,7 @@ def __post_init__(self):
     async def startTasks(self):
         if hasattr(self.backend, "watchExternalChanges"):
             self._watcherTask = asyncio.create_task(self.processExternalChanges())
-            self._watcherTask.add_done_callback(taskDoneHelper)
+            self._watcherTask.add_done_callback(taskDoneHelperWatchFiles)
         self._processWritesError = None
         self._processWritesEvent = asyncio.Event()
         self._processWritesTask = asyncio.create_task(self.processWrites())
@@ -468,6 +468,16 @@ def taskDoneHelper(task):
         )
 
 
+def taskDoneHelperWatchFiles(task):
+    if not task.cancelled() and task.exception() is not None:
+        exception = task.exception()
+        if isinstance(exception, RuntimeError) and str(exception) == "Already borrowed":
+            # Suppress RuntimeError("Already borrowed"), to work around this watchfiles
+            # issue: https://github.com/samuelcolvin/watchfiles/issues/200
+            return
+        logger.exception(f"fatal exception in asyncio task {task}", exc_info=exception)
+
+
 def _writeKeyToPattern(writeKey):
     if not isinstance(writeKey, tuple):
         writeKey = (writeKey,)