replaygain: avoid hasattr in favor of None values

to check for an optional ThreadPool. Seems more idiomatic and readable.

beetsplug/replaygain.py

@@ -21,7 +21,7 @@ import signal
 import subprocess
 import sys
 import warnings
-from multiprocessing.pool import ThreadPool, RUN
+from multiprocessing.pool import ThreadPool
 import queue
 from threading import Thread, Event
 
@@ -1177,6 +1177,9 @@ class ReplayGainPlugin(BeetsPlugin):
             raise ui.UserError(
                 f'replaygain initialization failed: {e}')
 
+        # Start threadpool lazily.
+        self.pool = None
+
     def should_use_r128(self, item):
         """Checks the plugin setting to decide whether the calculation
         should be done using the EBU R128 standard and use R128_ tags instead.
@@ -1314,18 +1317,10 @@ class ReplayGainPlugin(BeetsPlugin):
         except FatalReplayGainError as e:
             raise ui.UserError(f"Fatal replay gain error: {e}")
 
-    def _has_pool(self):
-        """Check whether a `ThreadPool` is running instance in `self.pool`
-        """
-        if hasattr(self, 'pool'):
-            if isinstance(self.pool, ThreadPool) and self.pool._state == RUN:
-                return True
-        return False
-
     def open_pool(self, threads):
         """Open a `ThreadPool` instance in `self.pool`
         """
-        if not self._has_pool() and self.backend_instance.do_parallel:
+        if self.pool is None and self.backend_instance.do_parallel:
             self.pool = ThreadPool(threads)
             self.exc_queue = queue.Queue()
 
@@ -1338,7 +1333,7 @@ class ReplayGainPlugin(BeetsPlugin):
             self.exc_watcher.start()
 
     def _apply(self, func, args, kwds, callback):
-        if self._has_pool():
+        if self.pool is not None:
             def handle_exc(exc):
                 """Handle exceptions in the async work.
                 """
@@ -1353,15 +1348,17 @@ class ReplayGainPlugin(BeetsPlugin):
             callback(func(*args, **kwds))
 
     def terminate_pool(self):
-        """Terminate the `ThreadPool` instance in `self.pool`
-            (e.g. stop execution in case of exception)
+        """Forcibly terminate the `ThreadPool` instance in `self.pool`
+
+        Sends SIGTERM to all processes.
         """
-        # Don't call self._as_pool() here,
-        # self.pool._state may not be == RUN
-        if hasattr(self, 'pool') and isinstance(self.pool, ThreadPool):
+        if self.pool is not None:
             self.pool.terminate()
             self.pool.join()
+            # Terminating the processes leaves the ExceptionWatcher's queues
+            # in an unknown state, so don't wait for it.
             # self.exc_watcher.join()
+            self.pool = None
 
     def _interrupt(self, signal, frame):
         try:
@@ -1373,12 +1370,13 @@ class ReplayGainPlugin(BeetsPlugin):
             pass
 
     def close_pool(self):
-        """Close the `ThreadPool` instance in `self.pool` (if there is one)
+        """Regularly lose the `ThreadPool` instance in `self.pool`.
         """
-        if self._has_pool():
+        if self.pool is not None:
             self.pool.close()
             self.pool.join()
             self.exc_watcher.join()
+            self.pool = None
 
     def import_begin(self, session):
         """Handle `import_begin` event -> open pool