Bug 1616186 - Don't try to write StartupCache if it doesn't exist r=froydnj

The exact circumstances of how this is showing up in the wild aren't
clear - there seem to be a couple of ways we can get here. However it
all revolves around early shutdowns (i.e., from the select profile popup)
- before the StartupCache is ever initialized. In any case, the solution
shouldn't change based on the exact circumstances - if we don't have a
StartupCache, there's no need to write one. Also, don't bother lazy
initializing it if it doesn't exist yet.

Differential Revision: https://phabricator.services.mozilla.com/D63208

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

startupcache/StartupCache.cpp

@@ -102,6 +102,10 @@ static nsresult MapLZ4ErrorToNsresult(size_t aError) {
   return NS_ERROR_FAILURE;
 }
 
+StartupCache* StartupCache::GetSingletonNoInit() {
+  return StartupCache::gStartupCache;
+}
+
 StartupCache* StartupCache::GetSingleton() {
   if (!gStartupCache) {
     if (!XRE_IsParentProcess()) {

startupcache/StartupCache.h

@@ -175,6 +175,7 @@ class StartupCache : public nsIMemoryReporter {
   nsresult GetDebugObjectOutputStream(nsIObjectOutputStream* aStream,
                                       nsIObjectOutputStream** outStream);
 
+  static StartupCache* GetSingletonNoInit();
   static StartupCache* GetSingleton();
   static void DeleteSingleton();
 

xpcom/base/AppShutdown.cpp

@@ -127,7 +127,11 @@ void AppShutdown::Init(AppShutdownMode aMode) {
       StaticPrefs::toolkit_shutdown_lateWriteChecksStage();
   sLateWriteChecksPhase = GetShutdownPhaseFromPrefValue(lateWriteChecksPref);
 
-  scache::StartupCache::GetSingleton()->MaybeInitShutdownWrite();
+  // Very early shutdowns can happen before the startup cache is even
+  // initialized; don't bother initializing it during shutdown.
+  if (auto* cache = scache::StartupCache::GetSingletonNoInit()) {
+    cache->MaybeInitShutdownWrite();
+  }
 }
 
 void AppShutdown::MaybeFastShutdown(ShutdownPhase aPhase) {