Bug 1372988 part A - Warn when user prefs are set after the profile is dead, because the prefs won't get saved anyway. r=milan

MozReview-Commit-ID: IfwL5yYtLcF

--HG--
extra : rebase_source : f03a79b5f7c3b8eb802bfd2db9991e5ed6c74a8f

modules/libpref/Preferences.cpp

@@ -102,6 +102,9 @@ Preferences::DirtyCallback()
   }
   if (gHashTable && sPreferences && !sPreferences->mDirty) {
     sPreferences->mDirty = true;
+
+    NS_WARNING_ASSERTION(!sPreferences->mProfileShutdown,
+                         "Setting user pref after profile shutdown.");
   }
 }
 
@@ -662,7 +665,6 @@ Preferences::Shutdown()
  */
 
 Preferences::Preferences()
-  : mDirty(false)
 {
 }
 
@@ -796,11 +798,19 @@ Preferences::Observe(nsISupports *aSubject, const char *aTopic,
   nsresult rv = NS_OK;
 
   if (!nsCRT::strcmp(aTopic, "profile-before-change")) {
-    rv = SavePrefFile(nullptr);
-  } else if (!nsCRT::strcmp(aTopic, "profile-before-change-telemetry")) {
+    // Normally prefs aren't written after this point, and so we kick off
+    // an asynchronous pref save so that I/O can be done in parallel with
+    // other shutdown.
     if (AllowOffMainThreadSave()) {
-      PreferencesWriter::Flush();
+      SavePrefFile(nullptr);
     }
+  } else if (!nsCRT::strcmp(aTopic, "profile-before-change-telemetry")) {
+    // It's possible that a profile-before-change observer after ours
+    // set a pref. A blocking save here re-saves if necessary and also waits
+    // for any pending saves to complete.
+    SavePrefFileBlocking();
+    MOZ_ASSERT(!mDirty, "Preferences should not be dirty");
+    mProfileShutdown = true;
   } else if (!strcmp(aTopic, "load-extension-defaults")) {
     pref_LoadPrefsInDirList(NS_EXT_PREFS_DEFAULTS_DIR_LIST);
   } else if (!nsCRT::strcmp(aTopic, "reload-default-prefs")) {
@@ -1154,6 +1164,13 @@ Preferences::SavePrefFileInternal(nsIFile *aFile, SaveMethod aSaveMethod)
       return NS_OK;
     }
 
+    // check for profile shutdown after mDirty because the runnables from
+    // DirtyCallback can still be pending
+    if (mProfileShutdown) {
+      NS_WARNING("Cannot save pref file after profile shutdown.");
+      return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+    }
+
     // It's possible that we never got a prefs file.
     nsresult rv = NS_OK;
     if (mCurrentFile) {

modules/libpref/Preferences.h

@@ -485,7 +485,8 @@ protected:
 
 private:
   nsCOMPtr<nsIFile>        mCurrentFile;
-  bool                     mDirty;
+  bool                     mDirty = false;
+  bool                     mProfileShutdown = false;
 
   static Preferences*      sPreferences;
   static nsIPrefBranch*    sRootBranch;