Raise an error if combine can't find usable data files. #629

3 files changed, 47 insertions, 2 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 8ccc800..d6fbd09 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -24,15 +24,21 @@ Unreleased
   the other.  This is now corrected, fixing `issue 621`_ and `issue 622`_.
   Thanks to Daniel Hahler for seeing more clearly than I could.
 
+- The ``coverage combine`` command used to always overwrite the data file, even
+  when no data had been read from apparently combinable files.  Now, an error
+  is raised if we thought there were files to combine, but in fact none of them
+  could be used.  Fixes `issue 629`_.
+
 - On Windows, the HTML report could fail when source trees are deeply nested,
   due to attempting to create HTML filenames longer than the 250-character
-  maximum.  Now filenames will never exceed 200 characters, fixing `issue
-  627`_.  Thanks to Alex Sandro for helping with the fix.
+  maximum.  Now filenames will never get much larger than 200 characters,
+  fixing `issue 627`_.  Thanks to Alex Sandro for helping with the fix.
 
 .. _issue 563: https://bitbucket.org/ned/coveragepy/issues/563/platform-specific-configuration
 .. _issue 621: https://bitbucket.org/ned/coveragepy/issues/621/include-ignored-warning-when-using
 .. _issue 622: https://bitbucket.org/ned/coveragepy/issues/622/report-omit-overwrites-run-omit
 .. _issue 627: https://bitbucket.org/ned/coveragepy/issues/627/failure-generating-html-reports-when-the
+.. _issue 629: https://bitbucket.org/ned/coveragepy/issues/629/multiple-use-of-combine-leads-to-empty
 .. _issue 631: https://bitbucket.org/ned/coveragepy/issues/631/precise-coverage-percentage-value
 
 
diff --git a/coverage/data.py b/coverage/data.py
index ecfb86b..6f76a72 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -722,6 +722,7 @@ class CoverageDataFiles(object):
         if strict and not files_to_combine:
             raise CoverageException("No data to combine")
 
+        files_combined = 0
         for f in files_to_combine:
             new_data = CoverageData(debug=self.debug)
             try:
@@ -733,10 +734,14 @@ class CoverageDataFiles(object):
                     self.warn(str(exc))
             else:
                 data.update(new_data, aliases=aliases)
+                files_combined += 1
                 if self.debug and self.debug.should('dataio'):
                     self.debug.write("Deleting combined data file %r" % (f,))
                 file_be_gone(f)
 
+        if strict and not files_combined:
+            raise CoverageException("No usable data files")
+
 
 def canonicalize_json_data(data):
     """Canonicalize our JSON data so it can be compared."""
diff --git a/tests/test_process.py b/tests/test_process.py
index 42a6d98..18564cb 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -144,6 +144,40 @@ class ProcessTest(CoverageTest):
         data.read_file(".coverage")
         self.assertEqual(data.line_counts()['b_or_c.py'], 7)
 
+    def test_combine_no_usable_files(self):
+        # https://bitbucket.org/ned/coveragepy/issues/629/multiple-use-of-combine-leads-to-empty
+        self.make_b_or_c_py()
+        out = self.run_command("coverage run b_or_c.py b")
+        self.assertEqual(out, 'done\n')
+        self.assert_exists(".coverage")
+        self.assertEqual(self.number_of_data_files(), 1)
+
+        # Make bogus data files.
+        self.make_file(".coverage.bad1", "This isn't a coverage data file.")
+        self.make_file(".coverage.bad2", "This isn't a coverage data file.")
+
+        # Combine the parallel coverage data files into .coverage, but nothing is readable.
+        status, out = self.run_command_status("coverage combine")
+        self.assertEqual(status, 1)
+
+        for n in "12":
+            self.assert_exists(".coverage.bad{0}".format(n))
+            warning_regex = (
+                r"Coverage.py warning: Couldn't read data from '.*\.coverage\.bad{0}': "
+                r"CoverageException: Doesn't seem to be a coverage\.py data file".format(n)
+            )
+            self.assertRegex(out, warning_regex)
+        self.assertRegex(out, r"No usable data files")
+
+        # After combining, we should have a main file and two parallel files.
+        self.assertEqual(self.number_of_data_files(), 3)
+
+        # Read the coverage file and see that b_or_c.py has 6 lines
+        # executed (we only did b, not c).
+        data = coverage.CoverageData()
+        data.read_file(".coverage")
+        self.assertEqual(data.line_counts()['b_or_c.py'], 6)
+
     def test_combine_parallel_data_in_two_steps(self):
         self.make_b_or_c_py()