Be more strict when recording plugin names

2 files changed, 36 insertions, 1 deletions

diff --git a/coverage/data.py b/coverage/data.py
index e5c37cb7..b1fcd212 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -216,7 +216,20 @@ class CoverageData(object):
         `plugin_data` is { filename: plugin_name, ... }
 
         """
-        self._plugins.update(plugin_data)
+        existing_files = self._arcs or self._lines
+        for filename, plugin_name in iitems(plugin_data):
+            if filename not in existing_files:
+                raise CoverageException(
+                    "Can't add plugin data for unmeasured file '%s'" % (filename,)
+                )
+            existing_plugin = self._plugins.get(filename)
+            if existing_plugin is not None and plugin_name != existing_plugin:
+                raise CoverageException(
+                    "Conflicting plugin name for '%s': %r vs %r" % (
+                        filename, existing_plugin, plugin_name,
+                    )
+                )
+            self._plugins[filename] = plugin_name
 
     def update(self, other_data, aliases=None):
         """Update this data with data from another `CoverageData`.
diff --git a/tests/test_data.py b/tests/test_data.py
index 7083e69a..da16bd76 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -165,6 +165,25 @@ class CoverageDataTest(DataTestHelpers, CoverageTest):
         self.assertEqual(covdata.plugin_name("main.py"), "")
         self.assertIsNone(covdata.plugin_name("p3.not_here"))
 
+    def test_cant_plugin_unmeasured_files(self):
+        covdata = CoverageData()
+        msg = "Can't add plugin data for unmeasured file 'p1.foo'"
+        with self.assertRaisesRegex(CoverageException, msg):
+            covdata.add_plugins({"p1.foo": "p1.plugin"})
+
+        covdata.add_lines({"p2.html": dict.fromkeys([10, 11, 12])})
+        with self.assertRaisesRegex(CoverageException, msg):
+            covdata.add_plugins({"p1.foo": "p1.plugin"})
+
+    def test_cant_change_plugin_name(self):
+        covdata = CoverageData()
+        covdata.add_lines({"p1.foo": dict.fromkeys([1, 2, 3])})
+        covdata.add_plugins({"p1.foo": "p1.plugin"})
+
+        msg = "Conflicting plugin name for 'p1.foo': 'p1.plugin' vs 'p1.plugin.foo'"
+        with self.assertRaisesRegex(CoverageException, msg):
+            covdata.add_plugins({"p1.foo": "p1.plugin.foo"})
+
     def test_update_lines(self):
         covdata1 = CoverageData()
         covdata1.add_lines(LINES_1)
@@ -251,6 +270,9 @@ class CoverageDataTest(DataTestHelpers, CoverageTest):
         with self.assertRaises(CoverageException):
             covdata1.update(covdata2)
 
+        with self.assertRaises(CoverageException):
+            covdata2.update(covdata1)
+
     def test_update_plugin_vs_no_plugin(self):
         covdata1 = CoverageData()
         covdata1.add_lines({"p1.html": dict.fromkeys([1, 2, 3])})