report when blocked package exception is not needed

python/tidy/servo_tidy_tests/blocked_package.lock

@@ -0,0 +1,26 @@
+[root]
+name = "servo"
+version = "0.0.1"
+
+[[package]]
+name = "test_blocked"
+version = "0.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+    "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "test_exception"
+version = "0.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+    "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "test_unneeded_exception"
+version = "0.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+]

python/tidy/servo_tidy_tests/test_tidy.py

@@ -229,6 +229,25 @@ class CheckTidiness(unittest.TestCase):
 
         self.assertNoMoreErrors(errors)
 
+    def test_lock_exceptions(self):
+        tidy.config["blocked-packages"]["rand"] = ["test_exception", "test_unneeded_exception"]
+        errors = tidy.collect_errors_for_files(iterFile('blocked_package.lock'), [tidy.check_lock], [], print_text=False)
+
+        msg = (
+            "Package test_blocked 0.0.2 depends on blocked package rand."
+        )
+
+        msg2 = (
+            "Package test_unneeded_exception is not required to be an exception of blocked package rand."
+        )
+
+        self.assertEqual(msg, errors.next()[2])
+        self.assertEqual(msg2, errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
+        # needed to not raise errors in other test cases
+        tidy.config["blocked-packages"]["rand"] = []
+
     def test_lint_runner(self):
         test_path = base_path + 'lints/'
         runner = tidy.LintRunner(only_changed_files=False, progress=False)