Don't warn about trace=None on PyPy at shutdown

PyPy clears the trace function before calling atexit functions.  So when we
check if the trace function is changed, don't warn in that specific case.

--HG--
extra : amend_source : a4e946f94b9b84d351a9e112a7eea6a3337bacf1

2 files changed, 16 insertions, 3 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index d502612a..e9c32fbc 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -13,6 +13,10 @@ Unreleased
   process, you could crash Python with a "Fatal Python error: deallocating
   None" error.  This is now fixed.  Thanks to Alex Groce for the bug report.
 
+- On PyPy, measuring coverage in subprocesses could produce a warning: "Trace
+  function changed, measurement is likely wrong: None".  This was spurious, and
+  has been suppressed.
+
 
 Version 4.3.1 --- 2016-12-28
 ----------------------------
diff --git a/coverage/pytracer.py b/coverage/pytracer.py
index 23f4946c..13a3b0c8 100644
--- a/coverage/pytracer.py
+++ b/coverage/pytracer.py
@@ -3,6 +3,7 @@
 
 """Raw data collector for coverage.py."""
 
+import atexit
 import dis
 import sys
 
@@ -52,6 +53,10 @@ class PyTracer(object):
         self.thread = None
         self.stopped = False
 
+        self.in_atexit = False
+        # On exit, self.in_atexit = True
+        atexit.register(setattr, self, 'in_atexit', True)
+
     def __repr__(self):
         return "<PyTracer at 0x{0:0x}: {1} lines in {2} files>".format(
             id(self),
@@ -144,9 +149,13 @@ class PyTracer(object):
             return
 
         if self.warn:
-            if sys.gettrace() != self._trace:
-                msg = "Trace function changed, measurement is likely wrong: %r"
-                self.warn(msg % (sys.gettrace(),))
+            # PyPy clears the trace function before running atexit functions,
+            # so don't warn if we are in atexit on PyPy and the trace function
+            # has changed to None.
+            tf = sys.gettrace()
+            dont_warn = (env.PYPY and self.in_atexit and tf is None)
+            if (not dont_warn) and tf != self._trace:
+                self.warn("Trace function changed, measurement is likely wrong: %r" % (tf,))
 
         sys.settrace(None)