fix: shut down "session flusher" more promptly (#4561)

Currently this thread will wait for `sleep(self.flush_interval)` to end
before
checking `self._running`. We can do better by using an Event.

Now it will wait for `self.flush_interval` or until
`self.__shutdown_requested`
is set, whichever is shorter.

Author: bukzor

sentry_sdk/sessions.py

@@ -1,7 +1,6 @@
 import os
-import time
 import warnings
-from threading import Thread, Lock
+from threading import Thread, Lock, Event
 from contextlib import contextmanager
 
 import sentry_sdk
@@ -162,7 +161,7 @@ def __init__(
         self._thread_lock = Lock()
         self._aggregate_lock = Lock()
         self._thread_for_pid = None  # type: Optional[int]
-        self._running = True
+        self.__shutdown_requested = Event()
 
     def flush(self):
         # type: (...) -> None
@@ -208,10 +207,10 @@ def _ensure_running(self):
 
             def _thread():
                 # type: (...) -> None
-                while self._running:
-                    time.sleep(self.flush_interval)
-                    if self._running:
-                        self.flush()
+                running = True
+                while running:
+                    running = not self.__shutdown_requested.wait(self.flush_interval)
+                    self.flush()
 
             thread = Thread(target=_thread)
             thread.daemon = True
@@ -220,7 +219,7 @@ def _thread():
             except RuntimeError:
                 # Unfortunately at this point the interpreter is in a state that no
                 # longer allows us to spawn a thread and we have to bail.
-                self._running = False
+                self.__shutdown_requested.set()
                 return None
 
             self._thread = thread
@@ -271,7 +270,7 @@ def add_session(
 
     def kill(self):
         # type: (...) -> None
-        self._running = False
+        self.__shutdown_requested.set()
 
     def __del__(self):
         # type: (...) -> None