Add a test that .pth measurement doesn't include existing data

2 files changed, 12 insertions, 0 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index bacf3339..14300c8f 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -8,6 +8,11 @@ Change history for Coverage.py
 Unreleased
 ----------
 
+- Subprocesses that are measured with `automatic subprocess measurement`_ used
+  to read in any pre-existing data file.  This meant data would be incorrectly
+  carried forward from run to run.  Now those files are not read, so each
+  subprocess only writes its own data.
+
 - Corrected the name of the jquery.ba-throttle-debounce.js library. Thanks,
   Ben Finney.  Closes `issue 505`_.
 
diff --git a/tests/test_process.py b/tests/test_process.py
index 5848b9ff..53d2362a 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -1088,6 +1088,13 @@ class ProcessStartupTest(ProcessCoverageMixin, CoverageTest):
         if env.METACOV:
             self.skipTest("Can't test sub-process pth file suppport during metacoverage")
 
+        # An existing data file should not be read when a subprocess gets
+        # measured automatically.  Create the data file here with bogus data in
+        # it.
+        data = coverage.CoverageData()
+        data.add_lines({os.path.abspath('sub.py'): dict.fromkeys(range(100))})
+        data.write_file(".mycovdata")
+
         self.make_file("coverage.ini", """\
             [run]
             data_file = .mycovdata