Bug 1539959 - Ensure origin process is subscribed when attaching remote BC, r=farre a=pascalc We were incorrectly not performing an EnsureSubscribed for the origin process which a BrowsingContext came from. Normally this would mean that the BrowsingContext could die in the parent process before the WindowGlobalParent listener arrived, but that didn't generally happen due to the low likelihood of a CC occuring between the two messages. It's likely that this crash was caused by people on lower-end hardware triggering a print. This would be a longer-running operation in the content process between the first and second message than usual. On systems with memory pressure, there would be a higher chance of a CC occuring between the messages, which would then cause this crash. This patch correctly connects the origin ContentParent to the BrowsingContextGroup, which will prevent the CC from destroying our objects. In the future, it may be desirable to ensure that this doesn't happen more reliably by using a ContentParent-local table for looking up BrowsingContext IDs sent over IPC. This would prevent our current dependency on the weak pointer behaviour of the current global ID cache. Unfortunately, this patch adds no tests, and I'm not aware of a good way to test this edge case to confirm it has been fixed. I believe that this patch should fix the issue I mention, however. Differential Revision: https://phabricator.services.mozilla.com/D29563

--- a/docshell/base/BrowsingContext.cpp
+++ b/docshell/base/BrowsingContext.cpp
@@ -142,17 +142,21 @@ already_AddRefed<BrowsingContext> Browsi
 
 /* static */
 already_AddRefed<BrowsingContext> BrowsingContext::CreateFromIPC(
     BrowsingContext::IPCInitializer&& aInit, BrowsingContextGroup* aGroup,
     ContentParent* aOriginProcess) {
   MOZ_DIAGNOSTIC_ASSERT(aOriginProcess || XRE_IsContentProcess());
   MOZ_DIAGNOSTIC_ASSERT(aGroup);
 
-  uint64_t originId = aOriginProcess ? aOriginProcess->ChildID() : 0;
+  uint64_t originId = 0;
+  if (aOriginProcess) {
+    originId = aOriginProcess->ChildID();
+    aGroup->EnsureSubscribed(aOriginProcess);
+  }
 
   MOZ_LOG(GetLog(), LogLevel::Debug,
           ("Creating 0x%08" PRIx64 " from IPC (origin=0x%08" PRIx64 ")",
            aInit.mId, originId));
 
   RefPtr<BrowsingContext> parent = aInit.GetParent();
 
   RefPtr<BrowsingContext> context;