Fix error in coverage runs under Py3 with a C locale

tornado/test/template_test.py

@@ -6,8 +6,8 @@ import traceback
 
 from tornado.escape import utf8, native_str, to_unicode
 from tornado.template import Template, DictLoader, ParseError, Loader
-from tornado.test.util import unittest
-from tornado.util import ObjectDict, unicode_type
+from tornado.test.util import unittest, is_coverage_running
+from tornado.util import ObjectDict, unicode_type, PY3
 
 
 class TemplateTest(unittest.TestCase):
@@ -175,6 +175,11 @@ try{% set y = 1/x %}
         self.assertEqual(template.generate(), '0')
 
     def test_non_ascii_name(self):
+        if PY3 and is_coverage_running():
+            try:
+                os.fsencode(u"t\u00e9st.html")
+            except UnicodeEncodeError:
+                self.skipTest("coverage tries to access unencodable filename")
         loader = DictLoader({u"t\u00e9st.html": "hello"})
         self.assertEqual(loader.load(u"t\u00e9st.html").generate(), b"hello")
 

tornado/test/util.py

@@ -76,3 +76,21 @@ def exec_test(caller_globals, caller_locals, s):
     local_namespace = {}
     exec(textwrap.dedent(s), global_namespace, local_namespace)
     return local_namespace
+
+
+def is_coverage_running():
+    """Return whether coverage is currently running.
+    """
+    if 'coverage' not in sys.modules:
+        return False
+    tracer = sys.gettrace()
+    if tracer is None:
+        return False
+    try:
+        mod = tracer.__module__
+    except AttributeError:
+        try:
+            mod = tracer.__class__.__module__
+        except AttributeError:
+            return False
+    return mod.startswith('coverage')