Bug 1489503 - In docShell, don't set the same title a second time. r=bzbarsky

This avoids setting the title a second time when the title and uri remain the same across setTitle calls. This means we can avoid unnecessary history updates which currently result in extra disk i/o.

Differential Revision: https://phabricator.services.mozilla.com/D5431

docshell/base/nsDocShell.cpp

@@ -384,6 +384,7 @@ nsDocShell::nsDocShell()
   , mInvisible(false)
   , mHasLoadedNonBlankURI(false)
   , mBlankTiming(false)
+  , mTitleValidForCurrentURI(false)
 {
   mHistoryID.m0 = 0;
   mHistoryID.m1 = 0;
@@ -1331,6 +1332,12 @@ nsDocShell::SetCurrentURI(nsIURI* aURI, nsIRequest* aRequest,
     return false;
   }
 
+  bool uriIsEqual = false;
+  if (!mCurrentURI || !aURI ||
+      NS_FAILED(mCurrentURI->Equals(aURI, &uriIsEqual)) || !uriIsEqual) {
+    mTitleValidForCurrentURI = false;
+  }
+
   mCurrentURI = aURI;
 
   if (!NS_IsAboutBlank(mCurrentURI)) {
@@ -5986,8 +5993,15 @@ nsDocShell::GetTitle(nsAString& aTitle)
 NS_IMETHODIMP
 nsDocShell::SetTitle(const nsAString& aTitle)
 {
+  // Avoid unnecessary updates of the title if the URI and the title haven't
+  // changed.
+  if (mTitleValidForCurrentURI && mTitle == aTitle) {
+    return NS_OK;
+  }
+
   // Store local title
   mTitle = aTitle;
+  mTitleValidForCurrentURI = true;
 
   nsCOMPtr<nsIDocShellTreeItem> parent;
   GetSameTypeParent(getter_AddRefs(parent));