Ensure random suffixes are different after forking

3 files changed, 17 insertions, 9 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index d60cc58e..82bb217a 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -18,6 +18,11 @@ Unreleased
   data, leaving you with an empty .coverage data file. Fixes issues
   `issue 412`_, `issue 516`_, and probably `issue 511`_.
 
+- When forking subprocesses, the coverage data files would have the same random
+  number appended to the file name. This didn't cause problems, because the
+  file names had the process id also, making collisions (nearly) impossible.
+  But it was disconcerting.  This is now fixed.
+
 - Data files are now about 15% smaller.
 
 - Corrected the name of the jquery.ba-throttle-debounce.js library. Thanks,
diff --git a/coverage/data.py b/coverage/data.py
index 78590fea..98b9c8a6 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -657,10 +657,8 @@ class CoverageDataFiles(object):
                 with open(_TEST_NAME_FILE) as f:
                     test_name = f.read()
                 extra = "." + test_name
-            suffix = "%s%s.%s.%06d" % (
-                socket.gethostname(), extra, os.getpid(),
-                random.randint(0, 999999)
-            )
+            dice = random.Random().randint(0, 999999)
+            suffix = "%s%s.%s.%06d" % (socket.gethostname(), extra, os.getpid(), dice)
 
         if suffix:
             filename += "." + suffix
diff --git a/tests/test_process.py b/tests/test_process.py
index ec19aed9..aa2045fd 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -23,13 +23,13 @@ TRY_EXECFILE = os.path.join(os.path.dirname(__file__), "modules/process_test/try
 class ProcessTest(CoverageTest):
     """Tests of the per-process behavior of coverage.py."""
 
+    def data_files(self):
+        """Return the names of coverage data files in this directory."""
+        return [f for f in os.listdir('.') if (f.startswith('.coverage.') or f == '.coverage')]
+
     def number_of_data_files(self):
         """Return the number of coverage data files in this directory."""
-        num = 0
-        for f in os.listdir('.'):
-            if f.startswith('.coverage.') or f == '.coverage':
-                num += 1
-        return num
+        return len(self.data_files())
 
     def test_save_on_exit(self):
         self.make_file("mycode.py", """\
@@ -577,6 +577,11 @@ class ProcessTest(CoverageTest):
         # .coverage.machine.123 files.
         self.assertEqual(self.number_of_data_files(), 2)
 
+        # The two data files should have different random numbers at the end of
+        # the file name.
+        nums = set(name.rpartition(".")[-1] for name in self.data_files())
+        self.assertEqual(len(nums), 2, "Same random: %s" % (self.data_files(),))
+
         # Combine the parallel coverage data files into .coverage .
         self.run_command("coverage combine")
         self.assert_exists(".coverage")