Bug 1746524 - Enable browser.tabs.documentchannel.parent-controlled pref. r=smaug

This patch also does some cosmetic changes for when parent controlled
loads should be used, but more importantly introduces the pref
'browser.tabs.documentchannel.speculative-load' with the default value
false.

It turns out that speculatively loading in the parent when a parent
controlled load isn't possible is the cause for performance
regressions.

Differential Revision: https://phabricator.services.mozilla.com/D192489

docshell/base/CanonicalBrowsingContext.cpp

@@ -2188,6 +2188,18 @@ bool CanonicalBrowsingContext::HasCreatedMediaController() const {
 
 bool CanonicalBrowsingContext::SupportsLoadingInParent(
     nsDocShellLoadState* aLoadState, uint64_t* aOuterWindowId) {
+  // We only support parent controlled navigation with Fission.
+  if (!StaticPrefs::browser_tabs_documentchannel_parent_controlled() ||
+      !mozilla::FissionAutostart()) {
+    return false;
+  }
+
+  // We currently only support starting loads directly from the
+  // CanonicalBrowsingContext for top-level BCs.
+  if (!IsTopContent() || !GetContentParent()) {
+    return false;
+  }
+
   // We currently don't support initiating loads in the parent when they are
   // watched by devtools. This is because devtools tracks loads using content
   // process notifications, which happens after the load is initiated in this
@@ -2238,15 +2250,6 @@ bool CanonicalBrowsingContext::SupportsLoadingInParent(
 
 bool CanonicalBrowsingContext::LoadInParent(nsDocShellLoadState* aLoadState,
                                             bool aSetNavigating) {
-  // We currently only support starting loads directly from the
-  // CanonicalBrowsingContext for top-level BCs.
-  // We currently only support starting loads directly from the
-  // CanonicalBrowsingContext for top-level BCs.
-  if (!IsTopContent() || !GetContentParent() ||
-      !StaticPrefs::browser_tabs_documentchannel_parent_controlled()) {
-    return false;
-  }
-
   uint64_t outerWindowId = 0;
   if (!SupportsLoadingInParent(aLoadState, &outerWindowId)) {
     return false;
@@ -2266,17 +2269,9 @@ bool CanonicalBrowsingContext::LoadInParent(nsDocShellLoadState* aLoadState,
 
 bool CanonicalBrowsingContext::AttemptSpeculativeLoadInParent(
     nsDocShellLoadState* aLoadState) {
-  // We currently only support starting loads directly from the
-  // CanonicalBrowsingContext for top-level BCs.
-  // We currently only support starting loads directly from the
-  // CanonicalBrowsingContext for top-level BCs.
-  if (!IsTopContent() || !GetContentParent() ||
-      (StaticPrefs::browser_tabs_documentchannel_parent_controlled())) {
-    return false;
-  }
-
   uint64_t outerWindowId = 0;
-  if (!SupportsLoadingInParent(aLoadState, &outerWindowId)) {
+  if (!StaticPrefs::browser_tabs_documentchannel_speculative_load() ||
+      !SupportsLoadingInParent(aLoadState, &outerWindowId)) {
     return false;
   }
 

modules/libpref/init/StaticPrefList.yaml

@@ -1614,6 +1614,13 @@
 # If set, use DocumentChannel to directly initiate loads entirely
 # from parent-process BrowsingContexts
 - name: browser.tabs.documentchannel.parent-controlled
+  type: bool
+  value: true
+  mirror: always
+
+# If set, use DocumentChannel to attempt a speculative load from the
+# parent whenever it's not possible to directly load from the parent.
+- name: browser.tabs.documentchannel.speculative-load
   type: bool
   value: false
   mirror: always