Move PeriodicMonitoringThread to its own file.

And give it unittest coverage. (Using it as an actual thread didn't
produce coverage metrics because we use 'greenlet' concurrency.)

Add a rudimentary scheduler so that functions that don't want to run
as often as the minimum stay (very roughly) on their period.

src/gevent/_monitor.py

+# Copyright (c) 2018 gevent. See LICENSE for details.
+from __future__ import print_function, absolute_import, division
+
+import sys
+import traceback
+
+from weakref import ref as wref
+
+from greenlet import settrace
+from greenlet import getcurrent
+
+from gevent import config as GEVENT_CONFIG
+from gevent.monkey import get_original
+from gevent.util import format_run_info
+
+from gevent._compat import thread_mod_name
+from gevent._util import gmctime
+
+# Clocks
+try:
+    # Python 3.3+ (PEP 418)
+    from time import perf_counter
+except ImportError:
+    import time
+
+    if sys.platform == "win32":
+        perf_counter = time.clock
+    else:
+        perf_counter = time.time
+
+__all__ = [
+    'PeriodicMonitoringThread',
+]
+
+get_thread_ident = get_original(thread_mod_name, 'get_ident')
+start_new_thread = get_original(thread_mod_name, 'start_new_thread')
+thread_sleep = get_original('time', 'sleep')
+
+class _MonitorEntry(object):
+
+    __slots__ = ('function', 'period', 'last_run_time')
+
+    def __init__(self, function, period):
+        self.function = function
+        self.period = period
+        self.last_run_time = 0
+
+    def __eq__(self, other):
+        return self.function == other.function and self.period == other.period
+
+    def __repr__(self):
+        return repr((self.function, self.period, self.last_run_time))
+
+class PeriodicMonitoringThread(object):
+
+    # The amount of seconds we will sleep when we think we have nothing
+    # to do.
+    inactive_sleep_time = 2.0
+
+
+    # A counter, incremented by the greenlet trace function
+    # we install on every greenlet switch. This is reset when the
+    # periodic monitoring thread runs.
+    _greenlet_switch_counter = 0
+    # The greenlet being switched to.
+    _active_greenlet = None
+
+    # The trace function that was previously installed,
+    # if any.
+    previous_trace_function = None
+
+    # The absolute minimum we will sleep, regardless of
+    # what particular monitoring functions want to say.
+    min_sleep_time = 0.005
+
+    # A list of _MonitorEntry objects: [(function(hub), period, last_run_time))]
+    # The first entry is always our entry for self.monitor_blocking
+    _monitoring_functions = None
+
+    # The calculated min sleep time for the monitoring functions list.
+    _calculated_sleep_time = None
+
+    def __init__(self, hub):
+        self._hub_wref = wref(hub, self._on_hub_gc)
+        self.should_run = True
+
+        # Must be installed in the thread that the hub is running in;
+        # the trace function is threadlocal
+        assert get_thread_ident() == hub.thread_ident
+        prev_trace = settrace(self.greenlet_trace)
+        self.previous_trace_function = prev_trace
+
+        self._monitoring_functions = [_MonitorEntry(self.monitor_blocking,
+                                                    GEVENT_CONFIG.max_blocking_time)]
+        self._calculated_sleep_time = GEVENT_CONFIG.max_blocking_time
+        # Create the actual monitoring thread. This is effectively a "daemon"
+        # thread.
+        self.monitor_thread_ident = start_new_thread(self, ())
+
+    @property
+    def hub(self):
+        return self._hub_wref()
+
+    def greenlet_trace(self, event, args):
+        # This function runs in the thread we are monitoring.
+        self._greenlet_switch_counter += 1
+        if event in ('switch', 'throw'):
+            # args is (origin, target). This is the only defined
+            # case
+            self._active_greenlet = args[1]
+        else:
+            self._active_greenlet = None
+        if self.previous_trace_function is not None:
+            self.previous_trace_function(event, args)
+
+    def monitoring_functions(self):
+        # Return a list of _MonitorEntry objects
+
+        # Update max_blocking_time each time.
+        mbt = GEVENT_CONFIG.max_blocking_time # XXX: Events so we know when this changes.
+        if mbt != self._monitoring_functions[0].period:
+            self._monitoring_functions[0].period = mbt
+            self._calculated_sleep_time = min(x.period for x in self._monitoring_functions)
+        return self._monitoring_functions
+
+    def add_monitoring_function(self, function, period):
+        """
+        Schedule the *function* to be called approximately every *period* fractional seconds.
+
+        The *function* receives one argument, the hub being monitored. It is called
+        in the monitoring thread, *not* the hub thread.
+
+        If the *function* is already a monitoring function, then its *period*
+        will be updated for future runs.
+
+        If the *period* is ``None``, then the function will be removed.
+
+        A *period* less than or equal to zero is not allowed.
+
+        """
+        if not callable(function):
+            raise ValueError("function must be callable")
+
+        if period is None:
+            # Remove.
+            self._monitoring_functions = [
+                x for x in self._monitoring_functions
+                if x.function != function
+            ]
+        elif period <= 0:
+            raise ValueError("Period must be positive.")
+        else:
+            # Add or update period
+            entry = _MonitorEntry(function, period)
+            self._monitoring_functions = [
+                x if x.function != function else entry
+                for x in self._monitoring_functions
+            ]
+            if entry not in self._monitoring_functions:
+                self._monitoring_functions.append(entry)
+        self._calculated_sleep_time = min(x.period for x in self._monitoring_functions)
+
+    def calculate_sleep_time(self):
+        min_sleep = self._calculated_sleep_time
+        if min_sleep <= 0:
+            # Everyone wants to be disabled. Sleep for a longer period of
+            # time than usual so we don't spin unnecessarily. We might be
+            # enabled again in the future.
+            return self.inactive_sleep_time
+        return max((min_sleep, self.min_sleep_time))
+
+    def kill(self):
+        if not self.should_run:
+            # Prevent overwriting trace functions.
+            return
+        # Stop this monitoring thread from running.
+        self.should_run = False
+        # Uninstall our tracing hook
+        settrace(self.previous_trace_function)
+        self.previous_trace_function = None
+
+    def _on_hub_gc(self, _):
+        self.kill()
+
+    def __call__(self):
+        # The function that runs in the monitoring thread.
+        # We cannot use threading.current_thread because it would
+        # create an immortal DummyThread object.
+        getcurrent().gevent_monitoring_thread = wref(self)
+
+        try:
+            while self.should_run:
+                functions = self.monitoring_functions()
+                assert functions
+                sleep_time = self.calculate_sleep_time()
+
+                thread_sleep(sleep_time)
+
+                # Make sure the hub is still around, and still active,
+                # and keep it around while we are here.
+                hub = self.hub
+                if not hub:
+                    self.kill()
+
+                if self.should_run:
+                    this_run = perf_counter()
+                    for entry in functions:
+                        f = entry.function
+                        period = entry.period
+                        last_run = entry.last_run_time
+                        if period and last_run + period <= this_run:
+                            entry.last_run_time = this_run
+                            f(hub)
+                del hub # break our reference to hub while we sleep
+
+        except SystemExit:
+            pass
+        except: # pylint:disable=bare-except
+            # We're a daemon thread, so swallow any exceptions that get here
+            # during interpreter shutdown.
+            if not sys or not sys.stderr: # pragma: no cover
+                # Interpreter is shutting down
+                pass
+            else:
+                hub = self.hub
+                if hub is not None:
+                    # XXX: This tends to do bad things like end the process, because we
+                    # try to switch *threads*, which can't happen. Need something better.
+                    hub.handle_error(self, *sys.exc_info())
+
+    def monitor_blocking(self, hub):
+        # Called periodically to see if the trace function has
+        # fired to switch greenlets. If not, we will print
+        # the greenlet tree.
+
+        # For tests, we return a true value when we think we found something
+        # blocking
+
+        # There is a race condition with this being incremented in the
+        # thread we're monitoring, but probably not often enough to lead
+        # to annoying false positives.
+        active_greenlet = self._active_greenlet
+        did_switch = self._greenlet_switch_counter != 0
+        self._greenlet_switch_counter = 0
+
+        if did_switch or active_greenlet is None or active_greenlet is hub:
+            # Either we switched, or nothing is running (we got a
+            # trace event we don't know about or were requested to
+            # ignore), or we spent the whole time in the hub, blocked
+            # for IO. Nothing to report.
+            return
+
+        report = ['=' * 80,
+                  '\n%s : Greenlet %s appears to be blocked' %
+                  (gmctime(), active_greenlet)]
+        report.append("    Reported by %s" % (self,))
+        try:
+            frame = sys._current_frames()[hub.thread_ident]
+        except KeyError:
+            # The thread holding the hub has died. Perhaps we shouldn't
+            # even report this?
+            stack = ["Unknown: No thread found for hub %r\n" % (hub,)]
+        else:
+            stack = traceback.format_stack(frame)
+        report.append('Blocked Stack (for thread id %s):' % (hex(hub.thread_ident),))
+        report.append(''.join(stack))
+        report.append("Info:")
+        report.extend(format_run_info(greenlet_stacks=False,
+                                      current_thread_ident=self.monitor_thread_ident))
+        report.append(report[0])
+        hub.exception_stream.write('\n'.join(report))
+        return (active_greenlet, report)
+
+    def ignore_current_greenlet_blocking(self):
+        # Don't pay attention to the current greenlet.
+        self._active_greenlet = None
+
+    def monitor_current_greenlet_blocking(self):
+        self._active_greenlet = getcurrent()
+
+    def __repr__(self):
+        return '<%s at %s in thread %s greenlet %r for %r>' % (
+            self.__class__.__name__,
+            hex(id(self)),
+            hex(self.monitor_thread_ident),
+            getcurrent(),
+            self._hub_wref())

src/gevent/_util.py

@@ -139,3 +139,10 @@ class readproperty(object):
             return self
 
         return self.func(inst)
+
+def gmctime():
+    """
+    Returns the current time as a string in RFC3339 format.
+    """
+    import time
+    return time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())

src/gevent/hub.py

@@ -14,7 +14,7 @@ from weakref import ref as wref
 from greenlet import greenlet as RawGreenlet
 from greenlet import getcurrent
 from greenlet import GreenletExit
-from greenlet import settrace
+
 
 
 __all__ = [
@@ -37,10 +37,11 @@ from gevent._compat import thread_mod_name
 from gevent._util import _NONE
 from gevent._util import readproperty
 from gevent._util import Lazy
+from gevent._util import gmctime
 from gevent._ident import IdentRegistry
 
 from gevent.monkey import get_original
-from gevent.util import format_run_info
+
 
 
 # These must be the "real" native thread versions,
@@ -453,174 +454,6 @@ def _config(default, envvar):
 
 hub_ident_registry = IdentRegistry()
 
-def _gmctime():
-    import time
-    return time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
-
-class PeriodicMonitoringThread(object):
-
-    # A counter, incremented by the greenlet trace function
-    # we install on every greenlet switch. This is reset when the
-    # periodic monitoring thread runs.
-    _greenlet_switch_counter = 0
-    # The greenlet being switched to.
-    _active_greenlet = None
-
-    # The trace function that was previously installed,
-    # if any.
-    previous_trace_function = None
-
-    # The absolute minimum we will sleep, regardless of
-    # what particular monitoring functions want to say.
-    min_sleep_time = 0.005
-
-    # A list of tuples: [(function(hub), period))]
-    _additional_monitoring_functions = ()
-
-    def __init__(self, hub):
-        self._hub_wref = wref(hub, self.kill)
-        self.should_run = True
-
-        # Must be installed in the thread that the hub is running in;
-        # the trace function is threadlocal
-        assert get_thread_ident() == hub.thread_ident
-        prev_trace = settrace(self.greenlet_trace)
-        self.previous_trace_function = prev_trace
-
-        # Create the actual monitoring thread. This is effectively a "daemon"
-        # thread.
-        self.monitor_thread_ident = get_original(thread_mod_name, 'start_new_thread')(self, ())
-
-    def greenlet_trace(self, event, args):
-        # This function runs in the thread we are monitoring.
-        self._greenlet_switch_counter += 1
-        if event in ('switch', 'throw'):
-            # args is (origin, target). This is the only defined
-            # case
-            self._active_greenlet = args[1]
-        else:
-            self._active_greenlet = None
-        if self.previous_trace_function is not None:
-            self.previous_trace_function(event, args)
-
-    def monitoring_functions(self):
-        # Return a list of tuples: [(function, period)]
-
-        # Calculate our hardcoded list every time so that changes to max_blocking_time can
-        # happen at runtime.
-        monitoring_functions = [(self.monitor_blocking, GEVENT_CONFIG.max_blocking_time)]
-        monitoring_functions.extend(self._additional_monitoring_functions)
-        return monitoring_functions
-
-    def add_monitoring_function(self, function, period):
-        self._additional_monitoring_functions += ((function, period),)
-
-    def calculate_sleep_time(self, monitoring_functions):
-        min_sleep = min(x[1] or 0 for x in monitoring_functions)
-        return  max((min_sleep, self.min_sleep_time))
-
-    def kill(self):
-        # Stop this monitoring thread from running.
-        self.should_run = False
-        # Uninstall our tracing hook
-        settrace(self.previous_trace_function)
-
-    def __call__(self):
-        # The function that runs in the monitoring thread.
-        # We cannot use threading.current_thread because it would
-        # create an immortal DummyThread object.
-        getcurrent().gevent_monitoring_thread = wref(self)
-        thread_sleep = get_original('time', 'sleep')
-        try:
-            while self.should_run:
-                functions = self.monitoring_functions()
-                assert functions
-                sleep_time = self.calculate_sleep_time(functions)
-
-                thread_sleep(sleep_time)
-
-                # Make sure the hub is still around, and still active,
-                # and keep it around while we are here.
-                hub = self._hub_wref()
-                if not hub:
-                    self.kill()
-
-                if self.should_run:
-                    for f, period in functions:
-                        if period:
-                            f(hub)
-        except SystemExit:
-            pass
-        except: # pylint:disable=bare-except
-            # We're a daemon thread, so swallow any exceptions that get here
-            # during interpreter shutdown.
-            if not sys or not sys.stderr:
-                # Interpreter is shutting down
-                pass
-            else:
-                hub = self._hub_wref()
-                if hub is not None:
-                    hub.handle_error(self, *sys.exc_info())
-
-    def monitor_blocking(self, hub):
-        # Called periodically to see if the trace function has
-        # fired to switch greenlets. If not, we will print
-        # the greenlet tree.
-
-        # There is a race condition with this being incremented in the
-        # thread we're monitoring, but probably not often enough to lead
-        # to annoying false positives.
-
-        did_switch = self._greenlet_switch_counter != 0
-        self._greenlet_switch_counter = 0
-
-        if did_switch or self._active_greenlet is None or isinstance(self._active_greenlet, Hub):
-            # Either we switched, or nothing is running (we got a
-            # trace event we don't know about or were requested to
-            # ignore), or we spent the whole time in the hub, blocked
-            # for IO. Nothing to report.
-            return
-
-        if (self._active_greenlet is getcurrent()
-                or getattr(self._active_greenlet, 'gevent_monitoring_thread', None)):
-            # Ourself or another monitoring thread for the same hub thread.
-            # This happens if multiple hubs are used in quick succession (probably only in tests)
-            # Ignore it. (XXX: Maybe we should kill() ourself?)
-            return
-
-        report = ['\n%s : Greenlet %s appears to be blocked' %
-                  (_gmctime(), self._active_greenlet)]
-        report.append("    Reported by %s" % (self,))
-        try:
-            frame = sys._current_frames()[hub.thread_ident]
-        except KeyError:
-            # The thread holding the hub has died. Perhaps we shouldn't
-            # even report this?
-            stack = ["Unknown: No thread found for hub %r\n" % (hub,)]
-        else:
-            stack = traceback.format_stack(frame)
-        report.append('Blocked Stack (for thread id %s):' % (hex(hub.thread_ident),))
-        report.append(''.join(stack))
-        report.append("Info:")
-        report.extend(format_run_info(self.monitor_thread_ident))
-        hub.exception_stream.write('\n'.join(report))
-
-    def ignore_current_greenlet_blocking(self):
-        # Don't pay attention to the current greenlet.
-        self._active_greenlet = None
-
-    def monitor_current_greenlet_blocking(self):
-        self._active_greenlet = getcurrent()
-
-    def __repr__(self):
-        return '<%s at %s in thread %s greenlet %r for %r>' % (
-            self.__class__.__name__,
-            hex(id(self)),
-            hex(self.monitor_thread_ident),
-            getcurrent(),
-            self._hub_wref())
-
-
 class Hub(TrackedRawGreenlet):
     """
     A greenlet that runs the event loop.
@@ -798,7 +631,7 @@ class Hub(TrackedRawGreenlet):
         del tb
 
         try:
-            errstream.write(_gmctime())
+            errstream.write(gmctime())
             errstream.write(' ' if context is not None else '\n')
         except: # pylint:disable=bare-except
             # Possible not safe to import under certain
@@ -917,6 +750,7 @@ class Hub(TrackedRawGreenlet):
             # if hubs are started and stopped within the thread. This shows up
             # in the threadpool tests. The monitoring threads will eventually notice their
             # hub object is gone.
+            from gevent._monitor import PeriodicMonitoringThread
             self.periodic_monitoring_thread = PeriodicMonitoringThread(self)
         return self.periodic_monitoring_thread
 

src/gevent/util.py

@@ -81,9 +81,11 @@ class wrap_errors(object):
     def __getattr__(self, name):
         return getattr(self.__func, name)
 
-def format_run_info(_current_thread_ident=None):
+def format_run_info(thread_stacks=True,
+                    greenlet_stacks=True,
+                    current_thread_ident=None):
     """
-    format_run_info() -> []
+    format_run_info(thread_stacks=True, greenlet_stacks=True) -> [str]
 
     Request information about the running threads of the current process.
 
@@ -107,11 +109,11 @@ def format_run_info(_current_thread_ident=None):
 
     lines = []
 
-    _format_thread_info(lines, _current_thread_ident)
-    _format_greenlet_info(lines)
+    _format_thread_info(lines, thread_stacks, current_thread_ident)
+    _format_greenlet_info(lines, greenlet_stacks)
     return lines
 
-def _format_thread_info(lines, current_thread_ident):
+def _format_thread_info(lines, thread_stacks, current_thread_ident):
     import threading
     import sys
 
@@ -131,7 +133,10 @@ def _format_thread_info(lines, current_thread_ident):
         if current_thread_ident == thread_ident:
             name = '%s) (CURRENT' % (name,)
         lines.append('Thread 0x%x (%s)\n' % (thread_ident, name))
-        lines.append(''.join(traceback.format_stack(frame, _STACK_LIMIT)))
+        if thread_stacks:
+            lines.append(''.join(traceback.format_stack(frame, _STACK_LIMIT)))
+        else:
+            lines.append('\t...stack elided...')
 
     # We may have captured our own frame, creating a reference
     # cycle, so clear it out.
@@ -140,14 +145,14 @@ def _format_thread_info(lines, current_thread_ident):
     del lines
     del threads
 
-def _format_greenlet_info(lines):
+def _format_greenlet_info(lines, greenlet_stacks):
     # Use the gc module to inspect all objects to find the greenlets
     # since there isn't a global registry
     lines.append('*' * 80)
     lines.append('* Greenlets')
     lines.append('*' * 80)
     for tree in GreenletTree.forest():
-        lines.extend(tree.format_lines(details=True))
+        lines.extend(tree.format_lines(details={'running_stacks': greenlet_stacks}))
 
     del lines
 
@@ -266,6 +271,12 @@ class GreenletTree(object):
     def __getattr__(self, name):
         return getattr(self.greenlet, name)
 
+    DEFAULT_DETAILS = {
+        'running_stacks': True,
+        'spawning_stacks': True,
+        'locals': True,
+    }
+
     def format_lines(self, details=True):
         """
         Return a sequence of lines for the greenlet tree.
@@ -277,7 +288,11 @@ class GreenletTree(object):
             if not details:
                 details = {}
             else:
-                details = {'stacks': True, 'locals': True}
+                details = self.DEFAULT_DETAILS.copy()
+        else:
+            params = details
+            details = self.DEFAULT_DETAILS.copy()
+            details.update(params)
         tree = _TreeFormatter(details, depth=0)
         lines = [l[0] if isinstance(l, tuple) else l
                  for l in self._render(tree)]
@@ -333,12 +348,12 @@ class GreenletTree(object):
         if getattr(self.greenlet, 'gevent_monitoring_thread', None) is not None:
             tree.child_data('Monitoring Thread:' + repr(self.greenlet.gevent_monitoring_thread()))
 
-        if self.greenlet and tree.details and tree.details['stacks']:
+        if self.greenlet and tree.details and tree.details['running_stacks']:
             self.__render_tb(tree, 'Running:', self.greenlet.gr_frame)
 
 
         spawning_stack = getattr(self.greenlet, 'spawning_stack', None)
-        if spawning_stack and tree.details and tree.details['stacks']:
+        if spawning_stack and tree.details and tree.details['spawning_stacks']:
             self.__render_tb(tree, 'Spawned at:', spawning_stack)
 
         spawning_parent = self.__spawning_parent(self.greenlet)

src/greentest/test___monitor.py

+# Copyright 2018 gevent contributors. See LICENSE for details.
+
+import gc
+import unittest
+
+
+from greenlet import gettrace
+from greenlet import settrace
+
+from gevent.monkey import get_original
+from gevent._compat import thread_mod_name
+from gevent._compat import NativeStrIO
+
+from gevent import _monitor as monitor
+
+get_ident = get_original(thread_mod_name, 'get_ident')
+
+class MockHub(object):
+    def __init__(self):
+        self.thread_ident = get_ident()
+        self.exception_stream = NativeStrIO()
+        self.dead = False
+
+    def __bool__(self):
+        return not self.dead
+
+    __nonzero__ = __bool__
+
+    def handle_error(self, *args): # pylint:disable=unused-argument
+        raise # pylint:disable=misplaced-bare-raise
+
+class TestPeriodicMonitoringThread(unittest.TestCase):
+
+    def setUp(self):
+        self._orig_start_new_thread = monitor.start_new_thread
+        self._orig_thread_sleep = monitor.thread_sleep
+        monitor.thread_sleep = lambda _s: gc.collect() # For PyPy
+        monitor.start_new_thread = lambda _f, _a: 0xDEADBEEF
+        self.hub = MockHub()
+        self.pmt = monitor.PeriodicMonitoringThread(self.hub)
+
+    def tearDown(self):
+        monitor.start_new_thread = self._orig_start_new_thread
+        monitor.thread_sleep = self._orig_thread_sleep
+        prev = self.pmt.previous_trace_function
+        self.pmt.kill()
+        assert gettrace() is prev, (gettrace(), prev)
+        settrace(None)
+
+    def test_constructor(self):
+        self.assertEqual(0xDEADBEEF, self.pmt.monitor_thread_ident)
+        self.assertEqual(gettrace(), self.pmt.greenlet_trace)
+
+    def test_hub_wref(self):
+        self.assertIs(self.hub, self.pmt.hub)
+        del self.hub
+
+        import gc
+        gc.collect()
+        self.assertIsNone(self.pmt.hub)
+
+        # And it killed itself.
+        self.assertFalse(self.pmt.should_run)
+        self.assertIsNone(gettrace())
+
+    def test_previous_trace(self):
+        self.pmt.kill()
+        self.assertIsNone(gettrace())
+
+        called = []
+        def f(*args):
+            called.append(args)
+
+        settrace(f)
+
+        self.pmt = monitor.PeriodicMonitoringThread(self.hub)
+        self.assertEqual(gettrace(), self.pmt.greenlet_trace)
+        self.assertIs(self.pmt.previous_trace_function, f)
+
+        self.pmt.greenlet_trace('event', 'args')
+
+        self.assertEqual([('event', 'args')], called)
+
+    def test_greenlet_trace(self):
+        self.assertEqual(0, self.pmt._greenlet_switch_counter)
+        # Unknown event still counts as a switch (should it?)
+        self.pmt.greenlet_trace('unknown', None)
+        self.assertEqual(1, self.pmt._greenlet_switch_counter)
+        self.assertIsNone(self.pmt._active_greenlet)
+
+        origin = object()
+        target = object()
+
+        self.pmt.greenlet_trace('switch', (origin, target))
+        self.assertEqual(2, self.pmt._greenlet_switch_counter)
+        self.assertIs(target, self.pmt._active_greenlet)
+
+        # Unknown event removes active greenlet
+        self.pmt.greenlet_trace('unknown', self)
+        self.assertEqual(3, self.pmt._greenlet_switch_counter)
+        self.assertIsNone(self.pmt._active_greenlet)
+
+    def test_add_monitoring_function(self):
+
+        self.assertRaises(ValueError, self.pmt.add_monitoring_function, None, 1)
+        self.assertRaises(ValueError, self.pmt.add_monitoring_function, lambda: None, -1)
+
+        def f():
+            pass
+
+        # Add
+        self.pmt.add_monitoring_function(f, 1)
+        self.assertEqual(2, len(self.pmt.monitoring_functions()))
+        self.assertEqual(1, self.pmt.monitoring_functions()[1].period)
+
+        # Update
+        self.pmt.add_monitoring_function(f, 2)
+        self.assertEqual(2, len(self.pmt.monitoring_functions()))
+        self.assertEqual(2, self.pmt.monitoring_functions()[1].period)
+
+        # Remove
+        self.pmt.add_monitoring_function(f, None)
+        self.assertEqual(1, len(self.pmt.monitoring_functions()))
+
+    def test_calculate_sleep_time(self):
+        self.assertEqual(
+            self.pmt.monitoring_functions()[0].period,
+            self.pmt.calculate_sleep_time())
+
+        # Pretend that GEVENT_CONFIG.max_blocking_time was set to 0,
+        # to disable this monitor.
+        self.pmt._calculated_sleep_time = 0
+        self.assertEqual(
+            self.pmt.inactive_sleep_time,
+            self.pmt.calculate_sleep_time()
+        )
+
+        # Getting the list of monitoring functions will also
+        # do this, if it looks like it has changed
+        self.pmt.monitoring_functions()[0].period = -1
+        self.pmt._calculated_sleep_time = 0
+        self.pmt.monitoring_functions()
+        self.assertEqual(
+            self.pmt.monitoring_functions()[0].period,
+            self.pmt.calculate_sleep_time())
+        self.assertEqual(
+            self.pmt.monitoring_functions()[0].period,
+            self.pmt._calculated_sleep_time)
+
+
+    def test_monitor_blocking(self):
+        # Initially there's no active greenlet and no switches,
+        # so nothing is considered blocked
+        self.assertFalse(self.pmt.monitor_blocking(self.hub))
+
+        # Give it an active greenlet
+        origin = object()
+        target = object()
+        self.pmt.greenlet_trace('switch', (origin, target))
+
+        # We've switched, so we're not blocked
+        self.assertFalse(self.pmt.monitor_blocking(self.hub))
+
+        # Again without switching is a problem.
+        self.assertTrue(self.pmt.monitor_blocking(self.hub))
+
+        # But we can order it not to be a problem
+        self.pmt.ignore_current_greenlet_blocking()
+        self.assertFalse(self.pmt.monitor_blocking(self.hub))
+
+        # And back again
+        self.pmt.monitor_current_greenlet_blocking()
+        self.assertTrue(self.pmt.monitor_blocking(self.hub))
+
+        # A bad thread_ident in the hub doesn't mess things up
+        self.hub.thread_ident = -1
+        self.assertTrue(self.pmt.monitor_blocking(self.hub))
+
+    def test_call_destroyed_hub(self):
+        # Add a function that destroys the hub so we break out (eventually)
+        # This clears the wref, which eventually calls kill()
+        def f(_hub):
+            _hub = None
+            self.hub = None
+            gc.collect()
+
+        self.pmt.add_monitoring_function(f, 0.1)
+        self.pmt()
+        self.assertFalse(self.pmt.should_run)
+
+    def test_call_dead_hub(self):
+        # Add a function that makes the hub go false (e.g., it quit)
+        # This causes the function to kill itself.
+        def f(hub):
+            hub.dead = True
+        self.pmt.add_monitoring_function(f, 0.1)
+        self.pmt()
+        self.assertFalse(self.pmt.should_run)
+
+    def test_call_SystemExit(self):
+        # breaks the loop
+        def f(_hub):
+            raise SystemExit()
+
+        self.pmt.add_monitoring_function(f, 0.1)
+        self.pmt()
+
+    def test_call_other_error(self):
+        class MyException(Exception):
+            pass
+
+        def f(_hub):
+            raise MyException()
+
+        self.pmt.add_monitoring_function(f, 0.1)
+        with self.assertRaises(MyException):
+            self.pmt()
+
+if __name__ == '__main__':
+    unittest.main()

src/greentest/test__hub.py

@@ -159,7 +159,7 @@ class TestPeriodicMonitoringThread(greentest.TestCase):
     def test_config(self):
         self.assertEqual(0.1, gevent.config.max_blocking_time)
 
-    def _run_monitoring_threads(self, monitor):
+    def _run_monitoring_threads(self, monitor, kill=True):
         self.assertTrue(monitor.should_run)
         from threading import Condition
         cond = Condition()
@@ -167,20 +167,18 @@ class TestPeriodicMonitoringThread(greentest.TestCase):
 
         def monitor_cond(_hub):
             cond.acquire()
-            # Only run once. Especially helpful on PyPy, where
-            # formatting stacks is expensive.
-            monitor.kill()
             cond.notifyAll()
             cond.release()
+            if kill:
+                # Only run once. Especially helpful on PyPy, where
+                # formatting stacks is expensive.
+                monitor.kill()
 
-        before_funs = monitor._additional_monitoring_functions
-        monitor.add_monitoring_function(
-            monitor_cond,
-            0.01)
+        monitor.add_monitoring_function(monitor_cond, 0.01)
 
         cond.wait()
         cond.release()
-        monitor._additional_monitoring_functions = before_funs
+        monitor.add_monitoring_function(monitor_cond, None)
 
     @greentest.ignores_leakcheck
     def test_kill_removes_trace(self):
@@ -197,10 +195,12 @@ class TestPeriodicMonitoringThread(greentest.TestCase):
         stream = hub.exception_stream = NativeStrIO()
         monitor = hub.start_periodic_monitoring_thread()
         self.assertIsNotNone(monitor)
-        before_funs = monitor._additional_monitoring_functions
 
+        self.assertEqual(1, len(monitor.monitoring_functions()))
         monitor.add_monitoring_function(self._monitor, 0.1)
-        self.assertIn((self._monitor, 0.1), monitor.monitoring_functions())
+        self.assertEqual(2, len(monitor.monitoring_functions()))
+        self.assertEqual(self._monitor, monitor.monitoring_functions()[1].function)
+        self.assertEqual(0.1, monitor.monitoring_functions()[1].period)
 
         # We must make sure we have switched greenlets at least once,
         # otherwise we can't detect a failure.
@@ -210,7 +210,8 @@ class TestPeriodicMonitoringThread(greentest.TestCase):
             time.sleep(0.3) # Thrice the default
             self._run_monitoring_threads(monitor)
         finally:
-            monitor._additional_monitoring_functions = before_funs
+            monitor.add_monitoring_function(self._monitor, None)
+            self.assertEqual(1, len(monitor._monitoring_functions))
             assert hub.exception_stream is stream
             monitor.kill()
             del hub.exception_stream
@@ -295,7 +296,7 @@ class TestPeriodicMonitoringThread(greentest.TestCase):
 
         task = threadpool.spawn(task)
         # Now wait until the monitoring threads have run.
-        self._run_monitoring_threads(worker_monitor)
+        self._run_monitoring_threads(worker_monitor, kill=False)
         # and be sure the task ran
         task.get()
         worker_monitor.kill()

src/greentest/test__util.py

@@ -113,7 +113,8 @@ class TestTree(greentest.TestCase):
         s4.join()
 
         tree = s4.value
-        return tree, str(tree), tree.format(details={'stacks': False})
+        return tree, str(tree), tree.format(details={'running_stacks': False,
+                                                     'spawning_stacks': False})
 
     @greentest.ignores_leakcheck
     def test_tree(self):

src/greentest/tests_that_dont_do_leakchecks.txt

 test___monkey_patching.py
 test__monkey_ssl_warning.py
+test___monitor.py

src/greentest/tests_that_dont_monkeypatch.txt

@@ -15,3 +15,5 @@ test__socket_timeout.py
 test__examples.py
 test__issue330.py
 test___ident.py
+test___config.py
+test___monitor.py

src/greentest/tests_that_dont_use_resolver.txt

@@ -132,3 +132,4 @@ test__issue639.py
 test__issue_728.py
 test__refcount_core.py
 test__api.py
+test__monitor.py