diff -r 39122c2442cd -r 4e27c0a70574 tests/test-ctxmanager.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-ctxmanager.py	Mon Jan 11 15:25:43 2016 -0800
@@ -0,0 +1,77 @@
+from __future__ import absolute_import
+
+import silenttestrunner
+import unittest
+
+from mercurial.util import ctxmanager
+
+class contextmanager(object):
+    def __init__(self, name, trace):
+        self.name = name
+        self.entered = False
+        self.exited = False
+        self.trace = trace
+
+    def __enter__(self):
+        self.entered = True
+        self.trace(('enter', self.name))
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.exited = exc_type, exc_val, exc_tb
+        self.trace(('exit', self.name))
+
+    def __repr__(self):
+        return '<ctx %r>' % self.name
+
+class ctxerror(Exception):
+    pass
+
+class raise_on_enter(contextmanager):
+    def __enter__(self):
+        self.trace(('raise', self.name))
+        raise ctxerror(self.name)
+
+class raise_on_exit(contextmanager):
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.trace(('raise', self.name))
+        raise ctxerror(self.name)
+
+def ctxmgr(name, trace):
+    return lambda: contextmanager(name, trace)
+
+class test_ctxmanager(unittest.TestCase):
+    def test_basics(self):
+        trace = []
+        addtrace = trace.append
+        with ctxmanager(ctxmgr('a', addtrace), ctxmgr('b', addtrace)) as c:
+            a, b = c()
+            c.atexit(addtrace, ('atexit', 'x'))
+            c.atexit(addtrace, ('atexit', 'y'))
+        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'),
+                                 ('atexit', 'y'), ('atexit', 'x'),
+                                 ('exit', 'b'), ('exit', 'a')])
+
+    def test_raise_on_enter(self):
+        trace = []
+        addtrace = trace.append
+        with self.assertRaises(ctxerror):
+            with ctxmanager(ctxmgr('a', addtrace),
+                           lambda: raise_on_enter('b', addtrace)) as c:
+                c()
+                addtrace('unreachable')
+        self.assertEqual(trace, [('enter', 'a'), ('raise', 'b'), ('exit', 'a')])
+
+    def test_raise_on_exit(self):
+        trace = []
+        addtrace = trace.append
+        with self.assertRaises(ctxerror):
+            with ctxmanager(ctxmgr('a', addtrace),
+                           lambda: raise_on_exit('b', addtrace)) as c:
+                c()
+                addtrace('running')
+        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'), 'running',
+                                 ('raise', 'b'), ('exit', 'a')])
+
+if __name__ == '__main__':
+    silenttestrunner.main(__name__)