From 87b1e1cdc9b380687564ddf0dc894a10e5df7221 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emilio=20Cobos=20=C3=81lvarez?= <emilio@crisal.io>
Date: Tue, 14 Aug 2018 10:47:25 +0200
Subject: [PATCH] style: no-op visited changes earlier if visited links are
 disabled.

We force a repaint from ContentStateChangedInternal if visited links are
disabled, and that's observable. Let's cut it off as early as we can to avoid
timing attacks even when :visited is disabled.

Differential Revision: https://phabricator.services.mozilla.com/D3304
---
 components/style/gecko/data.rs                | 48 ++++---------------
 .../element/state_and_attributes.rs           |  7 ++-
 2 files changed, 14 insertions(+), 41 deletions(-)

diff --git a/components/style/gecko/data.rs b/components/style/gecko/data.rs
index 6b9005c7560..0bd938c67e8 100644
--- a/components/style/gecko/data.rs
+++ b/components/style/gecko/data.rs
@@ -8,7 +8,7 @@ use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use context::QuirksMode;
 use dom::TElement;
 use gecko_bindings::bindings::{self, RawServoStyleSet};
-use gecko_bindings::structs::{self, RawGeckoPresContextOwned, ServoStyleSetSizes, StyleSheet as DomStyleSheet};
+use gecko_bindings::structs::{RawGeckoPresContextOwned, ServoStyleSetSizes, StyleSheet as DomStyleSheet};
 use gecko_bindings::structs::{StyleSheetInfo, nsIDocument};
 use gecko_bindings::sugar::ownership::{HasArcFFI, HasBoxFFI, HasFFI, HasSimpleFFI};
 use invalidation::media_queries::{MediaListKey, ToMediaListKey};
@@ -185,52 +185,20 @@ impl PerDocumentStyleDataImpl {
             .flush(&StylesheetGuards::same(guard), document_element, snapshots)
     }
 
-    /// Returns whether private browsing is enabled.
-    fn is_private_browsing_enabled(&self) -> bool {
-        let doc = self.stylist
-            .device()
-            .pres_context()
-            .mDocument
-            .raw::<nsIDocument>();
-        unsafe { bindings::Gecko_IsPrivateBrowsingEnabled(doc) }
-    }
-
-    /// Returns whether the document is being used as an image.
-    fn is_being_used_as_an_image(&self) -> bool {
-        let doc = self.stylist
-            .device()
-            .pres_context()
-            .mDocument
-            .raw::<nsIDocument>();
-
-        unsafe { (*doc).mIsBeingUsedAsImage() }
-    }
-
     /// Get the default computed values for this document.
     pub fn default_computed_values(&self) -> &Arc<ComputedValues> {
         self.stylist.device().default_computed_values_arc()
     }
 
-    /// Returns whether visited links are enabled.
-    fn visited_links_enabled(&self) -> bool {
-        unsafe { structs::StaticPrefs_sVarCache_layout_css_visited_links_enabled }
-    }
-
     /// Returns whether visited styles are enabled.
+    #[inline]
     pub fn visited_styles_enabled(&self) -> bool {
-        if !self.visited_links_enabled() {
-            return false;
-        }
-
-        if self.is_private_browsing_enabled() {
-            return false;
-        }
-
-        if self.is_being_used_as_an_image() {
-            return false;
-        }
-
-        true
+        let doc = self.stylist
+            .device()
+            .pres_context()
+            .mDocument
+            .raw::<nsIDocument>();
+        unsafe { bindings::Gecko_VisitedStylesEnabled(doc) }
     }
 
     /// Measure heap usage.
diff --git a/components/style/invalidation/element/state_and_attributes.rs b/components/style/invalidation/element/state_and_attributes.rs
index 88b012a2524..0be44c1fc42 100644
--- a/components/style/invalidation/element/state_and_attributes.rs
+++ b/components/style/invalidation/element/state_and_attributes.rs
@@ -167,8 +167,13 @@ where
         // do for this case.
         if state_changes.intersects(ElementState::IN_VISITED_OR_UNVISITED_STATE) {
             trace!(" > visitedness change, force subtree restyle");
+            // If we get here with visited links disabled, we should probably
+            // just avoid the restyle and remove the state change here, not only
+            // as an optimization, but also because it kind of would kill the
+            // point of disabling visited links.
+            debug_assert!(self.shared_context.visited_styles_enabled);
             // We can't just return here because there may also be attribute
-            // changes as well that imply additional hints.
+            // changes as well that imply additional hints for siblings.
             self.data.hint.insert(RestyleHint::restyle_subtree());
         }