Bug 1555488 - Part 1: Clear WindowProxy cache when flipping processes, r=peterv,mccr8

This is sorta-part of the work for bug 1510760. Currently if you do a process
switch, the window proxy state is completely borked, and a lot of stuff doesn't
work.

This patch aims to make it so that, while not transplanted correctly, methods
generally don't return the wrong WindowProxy object anymore after a process
swap.

Differential Revision: https://phabricator.services.mozilla.com/D33082

--HG--
extra : moz-landing-system : lando

docshell/base/BrowsingContext.cpp

@@ -311,6 +311,26 @@ void BrowsingContext::Detach(bool aFromIPC) {
   }
 }
 
+void BrowsingContext::PrepareForProcessChange() {
+  MOZ_LOG(GetLog(), LogLevel::Debug,
+          ("%s: Preparing 0x%08" PRIx64 " for a process change",
+           XRE_IsParentProcess() ? "Parent" : "Child", Id()));
+
+  MOZ_ASSERT(mIsInProcess, "Must currently be an in-process frame");
+  MOZ_ASSERT(!mClosed, "We're already closed?");
+
+  mIsInProcess = false;
+
+  // XXX: We should transplant our WindowProxy into a Cross-Process WindowProxy
+  // if mWindowProxy is non-nullptr. (bug 1510760)
+  mWindowProxy = nullptr;
+
+  // NOTE: For now, clear our nsDocShell reference, as we're primarily in a
+  // different process now. This may need to change in the future with
+  // Cross-Process BFCache.
+  mDocShell = nullptr;
+}
+
 void BrowsingContext::CacheChildren(bool aFromIPC) {
   MOZ_LOG(GetLog(), LogLevel::Debug,
           ("%s: Caching children of 0x%08" PRIx64 "",

docshell/base/BrowsingContext.h

@@ -145,6 +145,9 @@ class BrowsingContext : public nsWrapperCache, public BrowsingContextBase {
   // child and the parent process.
   void Detach(bool aFromIPC = false);
 
+  // Prepare this BrowsingContext to leave the current process.
+  void PrepareForProcessChange();
+
   // Remove all children from the current BrowsingContext and cache
   // them to allow them to be attached again.
   void CacheChildren(bool aFromIPC = false);

docshell/base/nsDocShell.cpp

@@ -5079,9 +5079,11 @@ nsDocShell::Destroy() {
     mSessionHistory = nullptr;
   }
 
-  // This will be skipped in cases where we want to preserve the browsing
-  // context between loads.
-  if (!mSkipBrowsingContextDetachOnDestroy) {
+  // Either `Detach` our BrowsingContext if this window is closing, or prepare
+  // the BrowsingContext for the switch to continue.
+  if (mSkipBrowsingContextDetachOnDestroy) {
+    mBrowsingContext->PrepareForProcessChange();
+  } else {
     mBrowsingContext->Detach();
   }