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.

4 files changed, 106 insertions, 81 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
diff --git a/components/script/textinput.rs b/components/script/textinput.rs
index a4a5d5e8965..a61799dadb0 100644
--- a/components/script/textinput.rs
+++ b/components/script/textinput.rs
@@ -56,6 +56,18 @@ pub struct TextPoint {
     pub index: usize,
 }
 
+impl TextPoint {
+    /// Returns a TextPoint constrained to be a valid location within lines
+    fn constrain_to(&self, lines: &[DOMString]) -> TextPoint {
+        let line = min(self.line, lines.len() - 1);
+
+        TextPoint {
+            line,
+            index: min(self.index, lines[line].len()),
+        }
+    }
+}
+
 #[derive(Clone, Copy, PartialEq)]
 pub struct SelectionState {
     start: TextPoint,
@@ -68,21 +80,26 @@ pub struct SelectionState {
 pub struct TextInput<T: ClipboardProvider> {
     /// Current text input content, split across lines without trailing '\n'
     lines: Vec<DOMString>,
+
     /// Current cursor input point
-    pub edit_point: TextPoint,
+    edit_point: TextPoint,
+
     /// The current selection goes from the selection_origin until the edit_point. Note that the
     /// selection_origin may be after the edit_point, in the case of a backward selection.
-    pub selection_origin: Option<TextPoint>,
+    selection_origin: Option<TextPoint>,
+    selection_direction: SelectionDirection,
+
     /// Is this a multiline input?
     multiline: bool,
+
     #[ignore_malloc_size_of = "Can't easily measure this generic type"]
     clipboard_provider: T,
+
     /// The maximum number of UTF-16 code units this text input is allowed to hold.
     ///
     /// <https://html.spec.whatwg.org/multipage/#attr-fe-maxlength>
-    pub max_length: Option<usize>,
-    pub min_length: Option<usize>,
-    pub selection_direction: SelectionDirection,
+    max_length: Option<usize>,
+    min_length: Option<usize>,
 }
 
 /// Resulting action to be taken by the owner of a text input that is handling an event.
@@ -175,6 +192,32 @@ impl<T: ClipboardProvider> TextInput<T> {
         i
     }
 
+    pub fn edit_point(&self) -> TextPoint {
+        self.edit_point
+    }
+
+    pub fn selection_origin(&self) -> Option<TextPoint> {
+        self.selection_origin
+    }
+
+    /// The selection origin, or the edit point if there is no selection. Note that the selection
+    /// origin may be after the edit point, in the case of a backward selection.
+    pub fn selection_origin_or_edit_point(&self) -> TextPoint {
+        self.selection_origin.unwrap_or(self.edit_point)
+    }
+
+    pub fn selection_direction(&self) -> SelectionDirection {
+        self.selection_direction
+    }
+
+    pub fn set_max_length(&mut self, length: Option<usize>) {
+        self.max_length = length;
+    }
+
+    pub fn set_min_length(&mut self, length: Option<usize>) {
+        self.min_length = length;
+    }
+
     /// Remove a character at the current editing point
     pub fn delete_char(&mut self, dir: Direction) {
         if self.selection_origin.is_none() || self.selection_origin == Some(self.edit_point) {
@@ -196,12 +239,6 @@ impl<T: ClipboardProvider> TextInput<T> {
         self.replace_selection(DOMString::from(s.into()));
     }
 
-    /// The selection origin, or the edit point if there is no selection. Note that the selection
-    /// origin may be after the edit point, in the case of a backward selection.
-    pub fn selection_origin_or_edit_point(&self) -> TextPoint {
-        self.selection_origin.unwrap_or(self.edit_point)
-    }
-
     /// The start of the selection (or the edit point, if there is no selection). Always less than
     /// or equal to selection_end(), regardless of the selection direction.
     pub fn selection_start(&self) -> TextPoint {
@@ -832,12 +869,6 @@ impl<T: ClipboardProvider> TextInput<T> {
         &self.lines[0]
     }
 
-    /// Get a mutable reference to the contents of a single-line text input. Panics if self is a multiline input.
-    pub fn single_line_content_mut(&mut self) -> &mut DOMString {
-        assert!(!self.multiline);
-        &mut self.lines[0]
-    }
-
     /// Set the current contents of the text input. If this is control supports multiple lines,
     /// any \n encountered will be stripped and force a new logical line.
     pub fn set_content(&mut self, content: DOMString, update_text_cursor: bool) {
@@ -850,11 +881,15 @@ impl<T: ClipboardProvider> TextInput<T> {
         } else {
             vec!(content)
         };
+
         if update_text_cursor {
-            self.edit_point.line = min(self.edit_point.line, self.lines.len() - 1);
-            self.edit_point.index = min(self.edit_point.index, self.current_line_length());
+            self.edit_point = self.edit_point.constrain_to(&self.lines);
         }
-        self.selection_origin = None;
+
+        if let Some(origin) = self.selection_origin {
+            self.selection_origin = Some(origin.constrain_to(&self.lines));
+        }
+
         self.assert_ok_selection();
     }