Bug 1715414 - Start PBackground earlier in content processes. r=nika

Previously we were staring `PBackground` in content processes in
response to receiving the `SetXPCOMProcessAttributes` IPC message, which
is sent immediately after the process is launched.  Meanwhile, the
idle scheduler tries to use PBackground when the main thread considers
itself idle.  But if thread scheduling is such that the content process
main thread becomes idle before the IPC I/O thread has received and
dispatched that message, then we have a problem (signaled by an assertion
failure).

This patch moves content process `PBackground` startup earlier, to the
end of `ContentProcess::Init`; that point is after enough of IPC and
XPCOM is started for it to work, but before we start spinning the main
thread event loop.

Differential Revision: https://phabricator.services.mozilla.com/D126144

dom/ipc/ContentChild.cpp

@@ -1285,10 +1285,6 @@ void ContentChild::InitSharedUASheets(const Maybe<SharedMemoryHandle>& aHandle,
 void ContentChild::InitXPCOM(
     XPCOMInitData&& aXPCOMInit,
     const mozilla::dom::ipc::StructuredCloneData& aInitialData) {
-  // Do this as early as possible to get the parent process to initialize the
-  // background thread since we'll likely need database information very soon.
-  BackgroundChild::Startup();
-
 #ifdef MOZ_WIDGET_GTK
   // LookAndFeel::NativeInit takes a long time to run on Linux, here we schedule
   // it as soon as possible after BackgroundChild::Startup to give

dom/ipc/ContentProcess.cpp

@@ -27,6 +27,7 @@
 #endif
 
 #include "nsAppRunner.h"
+#include "mozilla/ipc/BackgroundChild.h"
 #include "mozilla/ipc/ProcessUtils.h"
 
 using mozilla::ipc::IOThreadChild;
@@ -218,6 +219,10 @@ bool ContentProcess::Init(int aArgc, char* aArgv[]) {
   SetUpSandboxEnvironment();
 #endif
 
+  // Do this as early as possible to get the parent process to initialize the
+  // background thread since we'll likely need database information very soon.
+  mozilla::ipc::BackgroundChild::Startup();
+
   return true;
 }
 

ipc/glue/BackgroundChild.h

@@ -16,14 +16,13 @@ namespace mozilla {
 namespace dom {
 
 class BlobImpl;
-class ContentChild;
 class ContentParent;
+class ContentProcess;
 
 }  // namespace dom
 
 namespace net {
 
-class SocketProcessImpl;
 class SocketProcessChild;
 
 }  // namespace net
@@ -55,9 +54,8 @@ class PBackgroundChild;
 // The PBackgroundChild actor and all its sub-protocol actors will be
 // automatically destroyed when its designated thread completes.
 class BackgroundChild final {
-  friend class mozilla::dom::ContentChild;
   friend class mozilla::dom::ContentParent;
-  friend class mozilla::net::SocketProcessImpl;
+  friend class mozilla::dom::ContentProcess;
   friend class mozilla::net::SocketProcessChild;
 
   typedef mozilla::ipc::Transport Transport;
@@ -82,7 +80,7 @@ class BackgroundChild final {
       nsIEventTarget* aMainEventTarget = nullptr);
 
  private:
-  // Only called by ContentChild or ContentParent.
+  // Only called by this class's friends.
   static void Startup();
 };