Disallow mutating the internals of TextInput

The TextInput::assert_ok_selection() method is meant to ensure that we
are not getting into a state where a selection refers to a location in
the control's contents which doesn't exist.

However, before this change we could have a situation where the
internals of the TextInput are changed by another part of the code,
without using its public API. This could lead to us having an invalid
selection.

I did manage to trigger such a situation (see the test added in this
commit) although it is quite contrived. There may be others that I
didn't think of, and it's also possible that future changes could
introduce new cases. (Including ones which trigger panics, if indexing
is used on the assumption that the selection indices are always valid.)

The current HTML specification doesn't explicitly say that
selectionStart/End must remain within the length of the content, but
that does seems to be the consensus reached in a discussion of this:

https://github.com/whatwg/html/issues/2424

The test case I've added here is currently undefined in the spec which
is why I've added it in tests/wpt/mozilla.

3 files changed, 51 insertions, 61 deletions

diff --git a/components/script/dom/htmlinputelement.rs b/components/script/dom/htmlinputelement.rs
index b0fbfb36f36..05ca796abd2 100755
--- a/components/script/dom/htmlinputelement.rs
+++ b/components/script/dom/htmlinputelement.rs
@@ -48,7 +48,6 @@ use script_traits::ScriptToConstellationChan;
 use servo_atoms::Atom;
 use std::borrow::ToOwned;
 use std::cell::Cell;
-use std::mem;
 use std::ops::Range;
 use style::attr::AttrValue;
 use style::element_state::ElementState;
@@ -990,42 +989,34 @@ impl HTMLInputElement {
     }
 
     // https://html.spec.whatwg.org/multipage/#value-sanitization-algorithm
-    fn sanitize_value(&self) {
+    fn sanitize_value(&self, value: &mut DOMString) {
         match self.input_type() {
             InputType::Text | InputType::Search | InputType::Tel | InputType::Password => {
-                self.textinput.borrow_mut().single_line_content_mut().strip_newlines();
+                value.strip_newlines();
             }
             InputType::Url => {
-                let mut textinput = self.textinput.borrow_mut();
-                let content = textinput.single_line_content_mut();
-                content.strip_newlines();
-                content.strip_leading_and_trailing_ascii_whitespace();
+                value.strip_newlines();
+                value.strip_leading_and_trailing_ascii_whitespace();
             }
             InputType::Date => {
-                let mut textinput = self.textinput.borrow_mut();
-                if !textinput.single_line_content().is_valid_date_string() {
-                    textinput.single_line_content_mut().clear();
+                if !value.is_valid_date_string() {
+                    value.clear();
                 }
             }
             InputType::Month => {
-                let mut textinput = self.textinput.borrow_mut();
-                if !textinput.single_line_content().is_valid_month_string() {
-                    textinput.single_line_content_mut().clear();
+                if !value.is_valid_month_string() {
+                    value.clear();
                 }
             }
             InputType::Week => {
-                let mut textinput = self.textinput.borrow_mut();
-                if !textinput.single_line_content().is_valid_week_string() {
-                    textinput.single_line_content_mut().clear();
+                if !value.is_valid_week_string() {
+                    value.clear();
                 }
             }
             InputType::Color => {
-                let mut textinput = self.textinput.borrow_mut();
-
                 let is_valid = {
-                    let content = textinput.single_line_content();
-                    let mut chars = content.chars();
-                    if content.len() == 7 && chars.next() == Some('#') {
+                    let mut chars = value.chars();
+                    if value.len() == 7 && chars.next() == Some('#') {
                         chars.all(|c| c.is_digit(16))
                     } else {
                         false
@@ -1033,38 +1024,29 @@ impl HTMLInputElement {
                 };
 
                 if is_valid {
-                    let content = textinput.single_line_content_mut();
-                    content.make_ascii_lowercase();
+                    value.make_ascii_lowercase();
                 } else {
-                    textinput.set_content("#000000".into(), true);
+                    *value = "#000000".into();
                 }
             }
             InputType::Time => {
-                let mut textinput = self.textinput.borrow_mut();
-
-                if !textinput.single_line_content().is_valid_time_string() {
-                    textinput.single_line_content_mut().clear();
+                if !value.is_valid_time_string() {
+                    value.clear();
                 }
             }
             InputType::DatetimeLocal => {
-                let mut textinput = self.textinput.borrow_mut();
-                if textinput.single_line_content_mut()
-                    .convert_valid_normalized_local_date_and_time_string().is_err() {
-                        textinput.single_line_content_mut().clear();
+                if value.convert_valid_normalized_local_date_and_time_string().is_err() {
+                    value.clear();
                 }
             }
             InputType::Number => {
-                let mut textinput = self.textinput.borrow_mut();
-                if !textinput.single_line_content().is_valid_floating_point_number_string() {
-                    textinput.single_line_content_mut().clear();
+                if !value.is_valid_floating_point_number_string() {
+                    value.clear();
                 }
             }
             // https://html.spec.whatwg.org/multipage/#range-state-(type=range):value-sanitization-algorithm
             InputType::Range => {
-                self.textinput
-                    .borrow_mut()
-                    .single_line_content_mut()
-                    .set_best_representation_of_the_floating_point_number();
+                value.set_best_representation_of_the_floating_point_number();
             }
             _ => ()
         }
@@ -1075,20 +1057,23 @@ impl HTMLInputElement {
         TextControlSelection::new(&self, &self.textinput)
     }
 
-    fn update_text_contents(&self, value: DOMString, update_text_cursor: bool) -> ErrorResult {
+    fn update_text_contents(&self, mut value: DOMString, update_text_cursor: bool) -> ErrorResult {
         match self.value_mode() {
             ValueMode::Value => {
-                // Steps 1-2.
-                let old_value = mem::replace(self.textinput.borrow_mut().single_line_content_mut(), value);
                 // Step 3.
                 self.value_dirty.set(true);
+
                 // Step 4.
-                if update_text_cursor {
-                    self.sanitize_value();
-                }
-                // Step 5.
-                if *self.textinput.borrow().single_line_content() != old_value {
-                    self.textinput.borrow_mut().clear_selection_to_limit(Direction::Forward, update_text_cursor);
+                self.sanitize_value(&mut value);
+
+                let mut textinput = self.textinput.borrow_mut();
+
+                if *textinput.single_line_content() != value {
+                    // Steps 1-2
+                    textinput.set_content(value, update_text_cursor);
+
+                    // Step 5.
+                    textinput.clear_selection_to_limit(Direction::Forward, update_text_cursor);
                 }
             }
             ValueMode::Default |
@@ -1215,11 +1200,14 @@ impl VirtualMethods for HTMLInputElement {
                         }
 
                         // Step 6
-                        self.sanitize_value();
+                        let mut textinput = self.textinput.borrow_mut();
+                        let mut value = textinput.single_line_content().clone();
+                        self.sanitize_value(&mut value);
+                        textinput.set_content(value, true);
 
                         // Steps 7-9
                         if !previously_selectable && self.selection_api_applies() {
-                            self.textinput.borrow_mut().clear_selection_to_limit(Direction::Backward, true);
+                            textinput.clear_selection_to_limit(Direction::Backward, true);
                         }
                     },
                     AttributeMutation::Removed => {
@@ -1240,9 +1228,10 @@ impl VirtualMethods for HTMLInputElement {
             },
             &local_name!("value") if !self.value_dirty.get() => {
                 let value = mutation.new_value(attr).map(|value| (**value).to_owned());
-                self.textinput.borrow_mut().set_content(
-                    value.map_or(DOMString::new(), DOMString::from), true);
-                self.sanitize_value();
+                let mut value = value.map_or(DOMString::new(), DOMString::from);
+
+                self.sanitize_value(&mut value);
+                self.textinput.borrow_mut().set_content(value, true);
                 self.update_placeholder_shown_state();
             },
             &local_name!("name") if self.input_type() == InputType::Radio => {
@@ -1252,10 +1241,12 @@ impl VirtualMethods for HTMLInputElement {
             &local_name!("maxlength") => {
                 match *attr.value() {
                     AttrValue::Int(_, value) => {
+                        let mut textinput = self.textinput.borrow_mut();
+
                         if value < 0 {
-                            self.textinput.borrow_mut().max_length = None
+                            textinput.set_max_length(None);
                         } else {
-                            self.textinput.borrow_mut().max_length = Some(value as usize)
+                            textinput.set_max_length(Some(value as usize))
                         }
                     },
                     _ => panic!("Expected an AttrValue::Int"),
@@ -1264,10 +1255,12 @@ impl VirtualMethods for HTMLInputElement {
             &local_name!("minlength") => {
                 match *attr.value() {
                     AttrValue::Int(_, value) => {
+                        let mut textinput = self.textinput.borrow_mut();
+
                         if value < 0 {
-                            self.textinput.borrow_mut().min_length = None
+                            textinput.set_min_length(None);
                         } else {
-                            self.textinput.borrow_mut().min_length = Some(value as usize)
+                            textinput.set_min_length(Some(value as usize))
                         }
                     },
                     _ => panic!("Expected an AttrValue::Int"),
diff --git a/components/script/dom/htmltextareaelement.rs b/components/script/dom/htmltextareaelement.rs
index a5038918517..a6899b05a06 100755
--- a/components/script/dom/htmltextareaelement.rs
+++ b/components/script/dom/htmltextareaelement.rs
@@ -323,7 +323,6 @@ impl HTMLTextAreaElement {
 
         // Step 1
         let old_value = textinput.get_content();
-        let old_selection = textinput.selection_origin;
 
         // Step 2
         textinput.set_content(value, update_text_cursor);
@@ -334,8 +333,6 @@ impl HTMLTextAreaElement {
         if old_value != textinput.get_content() {
             // Step 4
             textinput.clear_selection_to_limit(Direction::Forward, update_text_cursor);
-        } else {
-            textinput.selection_origin = old_selection;
         }
 
         self.upcast::<Node>().dirty(NodeDamage::OtherNodeDamage);
diff --git a/components/script/dom/textcontrol.rs b/components/script/dom/textcontrol.rs
index b47167ae937..fa6c8d0b604 100644
--- a/components/script/dom/textcontrol.rs
+++ b/components/script/dom/textcontrol.rs
@@ -257,7 +257,7 @@ impl<'a, E: TextControlElement> TextControlSelection<'a, E> {
     }
 
     fn direction(&self) -> SelectionDirection {
-        self.textinput.borrow().selection_direction
+        self.textinput.borrow().selection_direction()
     }
 
     // https://html.spec.whatwg.org/multipage/#set-the-selection-range