Auto merge of #14589 - servo:harden-script, r=jdm+Ms2ger

Reduce calls into arbitrary code with the ScriptThread::documents borrow held.

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python/tidy/servo_tidy/tidy.py

@@ -542,6 +542,9 @@ def check_rust(file_name, lines):
                 lambda match, line: line.startswith('use ')),
             (r"^\s*else {", "else braces should be on the same line", no_filter),
             (r"[^$ ]\([ \t]", "extra space after (", no_filter),
+            # This particular pattern is not reentrant-safe in script_thread.rs
+            (r"match self.documents.borrow", "use a separate variable for the match expression",
+             lambda match, line: file_name.endswith('script_thread.rs')),
         ]
 
         for pattern, message, filter_func in regex_rules:

python/tidy/servo_tidy_tests/script_thread.rs

@@ -0,0 +1,18 @@
+fn main() {
+    // This should trigger an error.
+    match self.documents.borrow_mut() {
+        _ => {}
+    }
+    // This should trigger an error.
+    match self.documents.borrow() {
+        _ => {}
+    }
+    // This should not trigger an error.
+    match { self.documents.borrow().find_window(id) } {
+        => {}
+    }
+    // This should not trigger an error.
+    match self.documents_status.borrow() {
+        => {}
+    }
+}

python/tidy/servo_tidy_tests/test_tidy.py

@@ -141,6 +141,12 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('method declared in webidl is missing a comment with a specification link', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
+    def test_script_thread(self):
+        errors = tidy.collect_errors_for_files(iterFile('script_thread.rs'), [], [tidy.check_rust], print_text=False)
+        self.assertEqual('use a separate variable for the match expression', errors.next()[2])
+        self.assertEqual('use a separate variable for the match expression', errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
     def test_webidl(self):
         errors = tidy.collect_errors_for_files(iterFile('spec.webidl'), [tidy.check_webidl_spec], [], print_text=False)
         self.assertEqual('No specification link found.', errors.next()[2])