sleep(0) can take a long time on Windows.

Fixes #1314

CHANGES.rst

@@ -55,6 +55,9 @@
 - Make gevent's pywsgi server set the non-standard environment value
   ``wsgi.input_terminated`` to True. See :issue:`1308`.
 
+- Make `gevent.util.assert_switches` produce more informative messages
+  when the assertion fails.
+
 1.3.7 (2018-10-12)
 ==================
 

appveyor.yml

@@ -10,11 +10,8 @@ environment:
 
     # Pre-installed Python versions, which Appveyor may upgrade to
     # a later point release.
-    - PYTHON: "C:\\pypy2-v6.0.0-win32"
-      PYTHON_ID: "pypy"
-      PYTHON_EXE: pypy
-      PYTHON_VERSION: "2.7.x"
-      PYTHON_ARCH: "32"
+
+    # 64-bit
 
     - PYTHON: "C:\\Python37-x64"
       PYTHON_VERSION: "3.7.x"
@@ -31,47 +28,55 @@ environment:
       PYTHON_ARCH: "64"
       PYTHON_EXE: python
 
-    - PYTHON: "C:\\Python34-x64"
-      PYTHON_VERSION: "3.4.x" # currently 3.4.4
+    - PYTHON: "C:\\Python35-x64"
+      PYTHON_VERSION: "3.5.x" # currently 3.5.2
       PYTHON_ARCH: "64"
       PYTHON_EXE: python
 
-    - PYTHON: "C:\\Python35-x64"
-      PYTHON_VERSION: "3.5.x" # currently 3.5.2
+    - PYTHON: "C:\\Python34-x64"
+      PYTHON_VERSION: "3.4.x" # currently 3.4.4
       PYTHON_ARCH: "64"
       PYTHON_EXE: python
 
-    - PYTHON: "C:\\Python35"
-      PYTHON_VERSION: "3.5.x" # currently 3.5.2
+    # 32-bit
+    - PYTHON: "C:\\pypy2-v6.0.0-win32"
+      PYTHON_ID: "pypy"
+      PYTHON_EXE: pypy
+      PYTHON_VERSION: "2.7.x"
+      PYTHON_ARCH: "32"
+
+    # 32-bit, wheel only (no testing)
+
+    - PYTHON: "C:\\Python37"
+      PYTHON_VERSION: "3.7.x"
       PYTHON_ARCH: "32"
       PYTHON_EXE: python
       GWHEEL_ONLY: true
 
-    - PYTHON: "C:\\Python27"
-      PYTHON_VERSION: "2.7.x" # currently 2.7.13
+    - PYTHON: "C:\\Python36"
+      PYTHON_VERSION: "3.6.x" # currently 3.6.3
       PYTHON_ARCH: "32"
       PYTHON_EXE: python
       GWHEEL_ONLY: true
 
-    - PYTHON: "C:\\Python34"
-      PYTHON_VERSION: "3.4.x" # currently 3.4.3
+    - PYTHON: "C:\\Python35"
+      PYTHON_VERSION: "3.5.x" # currently 3.5.2
       PYTHON_ARCH: "32"
       PYTHON_EXE: python
       GWHEEL_ONLY: true
 
-    - PYTHON: "C:\\Python36"
-      PYTHON_VERSION: "3.6.x" # currently 3.6.3
+    - PYTHON: "C:\\Python34"
+      PYTHON_VERSION: "3.4.x" # currently 3.4.3
       PYTHON_ARCH: "32"
       PYTHON_EXE: python
       GWHEEL_ONLY: true
 
-    - PYTHON: "C:\\Python37"
-      PYTHON_VERSION: "3.7.x"
+    - PYTHON: "C:\\Python27"
+      PYTHON_VERSION: "2.7.x" # currently 2.7.13
       PYTHON_ARCH: "32"
       PYTHON_EXE: python
       GWHEEL_ONLY: true
 
-
     # Also test a Python version not pre-installed
     # See: https://github.com/ogrisel/python-appveyor-demo/issues/10
 

src/gevent/tests/test__util.py

@@ -221,23 +221,37 @@ class TestAssertSwitches(unittest.TestCase):
     def test_time_sleep(self):
         # A real blocking function
         from time import sleep
-        with self.assertRaises(util._FailedToSwitch):
+
+        # No time given, we detect the failure to switch immediately
+        with self.assertRaises(util._FailedToSwitch) as exc:
             with util.assert_switches():
                 sleep(0.001)
 
-        # Supply a max allowed and exceed it
+        message = str(exc.exception)
+        self.assertIn('To any greenlet in', message)
+
+        # Supply a max blocking allowed and exceed it
         with self.assertRaises(util._FailedToSwitch):
             with util.assert_switches(0.001):
                 sleep(0.1)
 
 
-        # Stay within it, but don't switch to the hub
-        with self.assertRaises(util._FailedToSwitch):
+        # Supply a max blocking allowed, and exit before that happens,
+        # but don't switch to the hub as requested
+        with self.assertRaises(util._FailedToSwitch) as exc:
             with util.assert_switches(0.001, hub_only=True):
                 sleep(0)
 
-        # Stay within it, and we only watch for any switch
-        with util.assert_switches(0.001, hub_only=False):
+        message = str(exc.exception)
+        self.assertIn('To the hub in', message)
+        self.assertIn('(max allowed 0.0010 seconds)', message)
+
+        # Supply a max blocking allowed, and exit before that happens,
+        # and allow any switch (or no switch).
+        # Note that we need to use a relatively long duration;
+        # sleep(0) on Windows can actually take a substantial amount of time
+        # sometimes (more than 0.001s)
+        with util.assert_switches(1.0, hub_only=False):
             sleep(0)
 
 

src/gevent/util.py

@@ -12,6 +12,7 @@ import traceback
 
 from greenlet import getcurrent
 
+from gevent._compat import perf_counter
 from gevent._compat import PYPY
 from gevent._compat import thread_mod_name
 from gevent._util import _NONE
@@ -541,10 +542,15 @@ class assert_switches(object):
             pass
 
     .. versionadded:: 1.3
+
+    .. versionchanged:: 1.4
+        If an exception is raised, it now includes information about
+        the duration of blocking and the parameters of this object.
     """
 
     hub = None
     tracer = None
+    _entered = None
 
 
     def __init__(self, max_blocking_time=None, hub_only=False):
@@ -567,6 +573,7 @@ class assert_switches(object):
         else:
             self.tracer = _tracer.MaxSwitchTracer(hub, self.max_blocking_time)
 
+        self._entered = perf_counter()
         self.tracer.monitor_current_greenlet_blocking()
         return self
 
@@ -583,6 +590,14 @@ class assert_switches(object):
 
         did_block = tracer.did_block_hub(hub)
         if did_block:
+            execution_time_s = perf_counter() - self._entered
             active_greenlet = did_block[1]
             report_lines = tracer.did_block_hub_report(hub, active_greenlet, {})
-            raise _FailedToSwitch('\n'.join(report_lines))
+
+            message = 'To the hub' if self.hub_only else 'To any greenlet'
+            message += ' in %.4f seconds' % (execution_time_s,)
+            max_block = self.max_blocking_time
+            message += ' (max allowed %.4f seconds)' % (max_block,) if max_block else ''
+            message += '\n'
+            message += '\n'.join(report_lines)
+            raise _FailedToSwitch(message)