diff --git a/brozzler/__init__.py b/brozzler/__init__.py
index 6a4303e..0cbaf0d 100644
--- a/brozzler/__init__.py
+++ b/brozzler/__init__.py
@@ -98,31 +98,88 @@ def behavior_script(url, template_parameters=None):
             return script
     return None
 
+def thread_accept_exceptions():
+    import threading
+    thread = threading.current_thread()
+    if hasattr(thread, 'thread_raise_lock'):
+        lock = thread.thread_raise_lock
+    else:
+        lock = threading.Lock()
+    with lock:
+        thread.thread_raise_lock = lock
+        thread.thread_raise_ok = True
+
+def thread_block_exceptions():
+    import threading
+    thread = threading.current_thread()
+    if hasattr(thread, 'thread_raise_lock'):
+        with thread.thread_raise_lock:
+            thread.thread_raise_ok = False
+
 def thread_raise(thread, exctype):
     '''
-    Raises the exception exctype in the thread.
+    If `thread` has declared itself willing to accept exceptions by calling
+    `thread_accept_exceptions`, raises the exception `exctype` in the thread
+    `thread`.
 
     Adapted from http://tomerfiliba.com/recipes/Thread2/ which explains:
     "The exception will be raised only when executing python bytecode. If your
     thread calls a native/built-in blocking function, the exception will be
     raised only when execution returns to the python code."
+
+    Returns:
+        True if exception was raised, False if the thread is not accepting
+        exceptions or another thread is holding `thread.thread_raise_lock`
+
+    Raises:
+        threading.ThreadError if `thread` is not running
+        TypeError if `exctype` is not a class
+        ValueError, SystemError in case of unexpected problems
     '''
-    import ctypes, inspect, threading
-    if not thread.is_alive():
-        raise threading.ThreadError('thread %s is not running' % thread)
+    import ctypes, inspect, threading, logging
+
     if not inspect.isclass(exctype):
         raise TypeError(
                 'cannot raise %s, only exception types can be raised (not '
-                'instances)' % exc_type)
-    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
-            ctypes.c_long(thread.ident), ctypes.py_object(exctype))
-    if res == 0:
-        raise ValueError('invalid thread id? thread.ident=%s' % thread.ident)
-    elif res != 1:
-        # if it returns a number greater than one, you're in trouble,
-        # and you should call it again with exc=NULL to revert the effect
-        ctypes.pythonapi.PyThreadState_SetAsyncExc(thread.ident, 0)
-        raise SystemError('PyThreadState_SetAsyncExc failed')
+                'instances)' % exctype)
+
+    if not hasattr(thread, 'thread_raise_lock'):
+        logging.warn(
+                'thread is not accepting exceptions (no member variable '
+                '"thread_raise_lock"), not raising %s in thread %s',
+                exctype, thread)
+        return False
+
+    got_lock = thread.thread_raise_lock.acquire(timeout=0.5)
+    if not got_lock:
+        logging.warn(
+                'could not get acquire "thread_raise_lock", not raising %s in '
+                'thread %s', exctype, thread)
+        return False
+    try:
+        if not thread.thread_raise_ok:
+            logging.warn(
+                    'thread is not accepting exceptions (thread_raise_ok is '
+                    '%s), not raising %s in thread %s',
+                    thread.thread_raise_ok, exctype, thread)
+            return False
+
+        if not thread.is_alive():
+            raise threading.ThreadError('thread %s is not running' % thread)
+        logging.info('raising %s in thread %s', exctype, thread)
+        res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
+                ctypes.c_long(thread.ident), ctypes.py_object(exctype))
+        if res == 0:
+            raise ValueError(
+                    'invalid thread id? thread.ident=%s' % thread.ident)
+        elif res != 1:
+            # if it returns a number greater than one, you're in trouble,
+            # and you should call it again with exc=NULL to revert the effect
+            ctypes.pythonapi.PyThreadState_SetAsyncExc(thread.ident, 0)
+            raise SystemError('PyThreadState_SetAsyncExc failed')
+        return True
+    finally:
+        thread.thread_raise_lock.release()
 
 def sleep(duration):
     '''
diff --git a/brozzler/browser.py b/brozzler/browser.py
index bd705f0..f4f0e99 100644
--- a/brozzler/browser.py
+++ b/brozzler/browser.py
@@ -427,6 +427,7 @@ class Browser:
             self.websock_thread.on_request = on_request
         if on_response:
             self.websock_thread.on_response = on_response
+        brozzler.thread_accept_exceptions()
         try:
             self.navigate_to_page(
                     page_url, extra_headers=extra_headers,
@@ -444,13 +445,16 @@ class Browser:
             final_page_url = self.url()
             return final_page_url, outlinks
         except brozzler.ReachedLimit:
+            brozzler.thread_block_exceptions()
             # websock_thread has stashed the ReachedLimit exception with
             # more information, raise that one
             raise self.websock_thread.reached_limit
         except websocket.WebSocketConnectionClosedException as e:
+            brozzler.thread_block_exceptions()
             self.logger.error('websocket closed, did chrome die?')
             raise BrowsingException(e)
         finally:
+            brozzler.thread_block_exceptions()
             self.is_browsing = False
             self.websock_thread.on_request = None
             self.websock_thread.on_response = None
diff --git a/brozzler/worker.py b/brozzler/worker.py
index bc5bb17..6625b56 100644
--- a/brozzler/worker.py
+++ b/brozzler/worker.py
@@ -118,6 +118,7 @@ class BrozzlerWorker:
 
         self._thread = None
         self._start_stop_lock = threading.Lock()
+        self._shutdown = threading.Event()
 
     def _proxy_for(self, site):
         if self._proxy:
@@ -233,7 +234,9 @@ class BrozzlerWorker:
     def _try_youtube_dl(self, ydl, site, page):
         try:
             self.logger.info("trying youtube-dl on {}".format(page))
+            brozzler.thread_accept_exceptions()
             info = ydl.extract_info(page.url)
+            brozzler.thread_block_exceptions()
             self._remember_videos(page, ydl.brozzler_spy)
             # logging.info('XXX %s', json.dumps(info))
             if self._using_warcprox(site):
@@ -517,7 +520,7 @@ class BrozzlerWorker:
         self.logger.info("brozzler worker starting")
         try:
             latest_state = None
-            while True:
+            while not self._shutdown.is_set():
                 self._service_heartbeat_if_due()
                 try:
                     browser = self._browser_pool.acquire()
@@ -543,6 +546,7 @@ class BrozzlerWorker:
                 except brozzler.NothingToClaim:
                     pass
                 time.sleep(0.5)
+            self.logger.info("shutdown requested")
         except brozzler.ShutdownRequested:
             self.logger.info("shutdown requested")
         except:
@@ -586,12 +590,7 @@ class BrozzlerWorker:
         self.stop()
 
     def stop(self):
-        with self._start_stop_lock:
-            if self._thread and self._thread.is_alive():
-                self.logger.info("brozzler worker shutting down")
-                brozzler.thread_raise(self._thread, brozzler.ShutdownRequested)
-                self._thread.join()
-                self._thread = None
+        self._shutdown.set()
 
     def is_alive(self):
         return self._thread and self._thread.is_alive()
diff --git a/setup.py b/setup.py
index 227bf00..8fad7fa 100644
--- a/setup.py
+++ b/setup.py
@@ -32,7 +32,7 @@ def find_package_data(package):
 
 setuptools.setup(
         name='brozzler',
-        version='1.1b11.dev237',
+        version='1.1b11.dev238',
         description='Distributed web crawling with browsers',
         url='https://github.com/internetarchive/brozzler',
         author='Noah Levitt',
diff --git a/tests/test_units.py b/tests/test_units.py
index 911fceb..0af5202 100644
--- a/tests/test_units.py
+++ b/tests/test_units.py
@@ -30,6 +30,13 @@ import requests
 import tempfile
 import uuid
 import socket
+import time
+import sys
+
+logging.basicConfig(
+        stream=sys.stderr, level=logging.INFO, format=(
+            '%(asctime)s %(process)d %(levelname)s %(threadName)s '
+            '%(name)s.%(funcName)s(%(filename)s:%(lineno)d) %(message)s'))
 
 @pytest.fixture(scope='module')
 def httpd(request):
@@ -185,3 +192,67 @@ def test_start_stop_backwards_compat():
     assert not 'started' in job
     assert not 'finished' in job
 
+def test_thread_raise():
+    let_thread_finish = threading.Event()
+    thread_preamble_done = threading.Event()
+    thread_caught_exception = None
+
+    def thread_target(accept_exceptions=False, block_exceptions=False):
+        if accept_exceptions:
+            brozzler.thread_accept_exceptions()
+        if block_exceptions:
+            brozzler.thread_block_exceptions()
+        thread_preamble_done.set()
+
+        try:
+            logging.info('waiting')
+            let_thread_finish.wait()
+        except Exception as e:
+            logging.info('caught exception %s', e)
+            nonlocal thread_caught_exception
+            thread_caught_exception = e
+        finally:
+            logging.info('finishing')
+            let_thread_finish.clear()
+            thread_preamble_done.clear()
+
+    # test that thread_raise does not raise exception in a thread that has not
+    # called thread_accept_exceptions
+    thread_caught_exception = None
+    th = threading.Thread(target=lambda: thread_target())
+    th.start()
+    thread_preamble_done.wait()
+    with pytest.raises(TypeError):
+        brozzler.thread_raise(
+                th, Exception("i'm an instance, which is not allowed"))
+    assert brozzler.thread_raise(th, Exception) is False
+    assert thread_caught_exception is None
+    let_thread_finish.set()
+    th.join()
+    assert thread_caught_exception is None
+
+    # test that thread_raise raises exception in a thread that has called
+    # thread_accept_exceptions
+    thread_caught_exception = None
+    th = threading.Thread(target=lambda: thread_target(accept_exceptions=True))
+    th.start()
+    thread_preamble_done.wait()
+    assert brozzler.thread_raise(th, Exception) is True
+    let_thread_finish.set()
+    th.join()
+    assert thread_caught_exception
+    with pytest.raises(threading.ThreadError): # thread is not running
+        brozzler.thread_raise(th, Exception)
+
+    # test that thread_raise does not raise exception in a thread that has
+    # called thread_block_exceptions
+    thread_caught_exception = None
+    th = threading.Thread(target=lambda: thread_target(block_exceptions=True))
+    th.start()
+    thread_preamble_done.wait()
+    assert brozzler.thread_raise(th, Exception) is False
+    let_thread_finish.set()
+    th.join()
+    assert thread_caught_exception is None
+
+