Bug 1652000 - A little lifetime documentation;r=nika

Differential Revision: https://phabricator.services.mozilla.com/D83337

dom/base/ChromeUtils.cpp

@@ -800,6 +800,8 @@ already_AddRefed<Promise> ChromeUtils::RequestProcInfo(GlobalObject& aGlobal,
   // Get all the content parents.
   // Note that this array includes even the long dead content parents, so we
   // might have some garbage, especially with Fission.
+  // SAFETY NOTE: `contentParents` is only valid if used synchronously.
+  // Anything else and you may end up dealing with dangling pointers.
   nsTArray<ContentParent*> contentParents;
   ContentParent::GetAll(contentParents);
 

dom/ipc/ContentParent.h

@@ -238,6 +238,16 @@ class ContentParent final
       const nsACString& aRemoteType, BrowsingContext* aBrowsingContext,
       ContentParent* aOpenerContentParent);
 
+  /**
+   * Get all content parents.
+   *
+   * # Lifetime
+   *
+   * These pointers are ONLY valid for synchronous use from the main thread.
+   *
+   * Do NOT attempt to use them after the main thread has had a chance to handle
+   * messages or you could end up with dangling pointers.
+   */
   static void GetAll(nsTArray<ContentParent*>& aArray);
 
   static void GetAllEvenIfDead(nsTArray<ContentParent*>& aArray);

ipc/glue/GeckoChildProcessHost.h

@@ -180,6 +180,8 @@ class GeckoChildProcessHost : public ChildProcessHost,
   // Iterates over all instances and calls aCallback with each one of them.
   // This method will lock any addition/removal of new processes
   // so you need to make sure the callback is as fast as possible.
+  //
+  // To reiterate: the callbacks are executed synchronously.
   static void GetAll(const GeckoProcessCallback& aCallback);
 
   friend class BaseProcessLauncher;