Bug 1589102 - Part 3: Enable about:blank loads to take place via DocumentChannel, r=mattwoodrow

In process selection logic, ensure that we don't use the original URI for
about:blank and instead use the result principal. If the about:blank load has a
null principal, then revert to using the original URI.

Also, remove an extra about:blank load when an nsFrameLoaderOwner is changing
remoteness to prevent races.

Differential Revision: https://phabricator.services.mozilla.com/D85081

dom/base/nsFrameLoaderOwner.cpp

@@ -248,14 +248,6 @@ void nsFrameLoaderOwner::ChangeRemotenessToProcess(
                                         aContentParent);
     }
 
-    // FIXME(bug 1644779): We'd like to stop triggering a load here, as this
-    // reads the attributes, such as `src`, on the <browser> element, and could
-    // start another load which will be clobbered shortly.
-    //
-    // This is OK for now, as we're mimicing the existing process switching
-    // behaviour, and <browser> elements created by tabbrowser don't have the
-    // `src` attribute specified.
-    mFrameLoader->LoadFrame(false);
   };
 
   auto shouldPreserve =

netwerk/ipc/DocumentChannel.cpp

@@ -151,7 +151,7 @@ nsDocShell* DocumentChannel::GetDocShell() {
 // Changes here should also be made in
 // E10SUtils.documentChannelPermittedForURI().
 static bool URIUsesDocChannel(nsIURI* aURI) {
-  if (SchemeIsJavascript(aURI) || NS_IsAboutBlank(aURI)) {
+  if (SchemeIsJavascript(aURI)) {
     return false;
   }
 

toolkit/modules/E10SUtils.jsm

@@ -126,11 +126,7 @@ const kSafeSchemes = [
 ];
 
 const kDocumentChannelDeniedSchemes = ["javascript"];
-const kDocumentChannelDeniedURIs = [
-  "about:blank",
-  "about:crashcontent",
-  "about:printpreview",
-];
+const kDocumentChannelDeniedURIs = ["about:crashcontent", "about:printpreview"];
 
 // Changes here should also be made in URIUsesDocChannel in DocumentChannel.cpp.
 function documentChannelPermittedForURI(aURI) {
@@ -581,12 +577,14 @@ var E10SUtils = {
       return NOT_REMOTE;
     }
 
-    // We want to use the original URI for "about:" (except for "about:srcdoc")
-    // and "chrome://" scheme, so that we can properly determine
-    // the remote type.
+    // We want to use the original URI for "about:" (except for "about:srcdoc"
+    // and "about:blank") and "chrome://" scheme, so that we can properly
+    // determine the remote type.
     let useOriginalURI;
     if (aOriginalURI.scheme == "about") {
-      useOriginalURI = !["srcdoc"].includes(aOriginalURI.spec);
+      useOriginalURI = !["about:srcdoc", "about:blank"].includes(
+        aOriginalURI.spec
+      );
     } else {
       useOriginalURI = aOriginalURI.scheme == "chrome";
     }
@@ -602,7 +600,9 @@ var E10SUtils = {
       // using fission we add the option to force them into the default
       // web process for better test coverage.
       if (aPrincipal.isNullPrincipal) {
-        if (
+        if (aOriginalURI.spec == "about:blank") {
+          useOriginalURI = true;
+        } else if (
           (aRemoteSubframes && useSeparateDataUriProcess) ||
           aPreferredRemoteType == NOT_REMOTE
         ) {