Replace processObservers with a class that doesn't constantly recreate threads

I'm not saying this is why bot_update is failing, but I never trust python threads, especially not at scale. I also think the new implementation is easier to understand and less likely to have subtle mistakes if it is extended later

Change-Id: I29be55594a400fda845ea31893789104373fd0f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2892185Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -114,68 +114,49 @@ OK = object()
 FAIL = object()
 
 
-class ProcessObservers(object):
-  """ProcessObservers allows monitoring of child process."""
-
-  def poke(self):
-    """poke is called when child process sent `BUF_SIZE` data to stdout."""
-    pass
-
-  def cancel(self):
-    """cancel is called once proc exists successfully."""
-    pass
-
-
-class PsPrinter(ProcessObservers):
-  def __init__(self, interval=300):
+class RepeatingTimer(threading.Thread):
+  """Call a function every n seconds, unless reset."""
+  def __init__(self, interval, function, args=None, kwargs=None):
+    threading.Thread.__init__(self)
     self.interval = interval
-    self.active = sys.platform.startswith('linux2')
-    self.thread = None
-
-  def print_pstree(self):
-    """Debugging function used to print "ps auxwwf" for stuck processes."""
+    self.function = function
+    self.args = args if args is not None else []
+    self.kwargs = kwargs if kwargs is not None else {}
+    self.cond = threading.Condition()
+    self.is_shutdown = False
+
+  def reset(self):
+    """Resets timer interval."""
+    with self.cond:
+      self.is_reset = True
+      self.cond.notify_all()
+
+  def shutdown(self):
+    """Stops repeating timer."""
+    with self.cond:
+      self.is_shutdown = True
+      self.cond.notify_all()
+
+  def run(self):
+    with self.cond:
+      while not self.is_shutdown:
+        self.cond.wait(self.interval)
+        if not self.is_reset and not self.is_shutdown:
+          self.function(*self.args, **self.kwargs)
+        self.is_reset = False
+
+
+def _print_pstree():
+  """Debugging function used to print "ps auxwwf" for stuck processes."""
+  if sys.platform.startswith('linux2'):
     # Add new line for cleaner output
     print()
     subprocess.call(['ps', 'auxwwf'])
 
-    # Restart timer, we want to continue printing until the process is
-    # terminated.
-    self.poke()
-
-  def poke(self):
-    if self.active:
-      self.cancel()
-      self.thread = threading.Timer(self.interval, self.print_pstree)
-      self.thread.start()
-
-  def cancel(self):
-    if self.active and self.thread is not None:
-      self.thread.cancel()
-      self.thread = None
 
-
-class StaleProcess(ProcessObservers):
-  '''StaleProcess terminates process if there is no poke call in `interval`. '''
-
-  def __init__(self, interval, proc):
-    self.interval = interval
-    self.proc = proc
-    self.thread = None
-
-  def _terminate_process(self):
-    print('Terminating stale process...')
-    self.proc.terminate()
-
-  def poke(self):
-    self.cancel()
-    if self.interval > 0:
-      self.thread = threading.Timer(self.interval, self._terminate_process)
-      self.thread.start()
-
-  def cancel(self):
-    if self.thread is not None:
-      self.thread.cancel()
-      self.thread = None
+def _terminate_process(proc):
+  print('Terminating stale process...')
+  proc.terminate()
 
 
 def call(*args, **kwargs):  # pragma: no cover
@@ -206,17 +187,17 @@ def call(*args, **kwargs):  # pragma: no cover
     proc.stdin.close()
   stale_process_duration = env.get('STALE_PROCESS_DURATION',
                                    STALE_PROCESS_DURATION)
-  observers = [PsPrinter(), StaleProcess(int(stale_process_duration), proc)]
+  observers = [
+      RepeatingTimer(300, _print_pstree),
+      RepeatingTimer(int(stale_process_duration), _terminate_process, [proc])]
+  for observer in observers:
+    observer.start()
   # This is here because passing 'sys.stdout' into stdout for proc will
   # produce out of order output.
   hanging_cr = False
   while True:
     for observer in observers:
-      try:
-        observer.poke()
-      except:
-        print('failed to poke, active thread count is %d' % threading.active_count())
-        raise
+      observer.reset()
     buf = proc.stdout.read(BUF_SIZE)
     if not buf:
       break
@@ -232,7 +213,7 @@ def call(*args, **kwargs):  # pragma: no cover
     sys.stdout.write('\n')
     out.write('\n')
   for observer in observers:
-    observer.cancel()
+    observer.shutdown()
 
   code = proc.wait()
   elapsed_time = ((time.time() - start_time) / 60.0)