1260774 - Create RAII class for IPC Actors that should get __delete__'d unless some conditions are met

Status: NEW

(Core :: IPC, defect, P3)

Description

[Mike Conley (:mconley) (:⚙️)]

In bug 1251032, kanru rightly noted that we have this pattern:

``` Pseudocode

ThingChild* thing = someActor->SendThingConstructor();

if (!foo) {
  // ... things happen in here.
  // Oh no, things have gone wrong - delete the child
  PThingChild::Send__delete__(thing);
} else if (bar) {
  // ... do some things
  nsresult rv = something->DoTheJob();

  if (NS_FAILED(rv)) {
    PThingChild::Send__delete__(thing);
  }
} else {
  // All is well, don't delete the Thing
}
```

This can get more complicated as more checks are put into place for the various conditions where ThingChild should be deleted.

An RAII class might allow us to do something like this:

``` Pseudocode

ThingChild* thing = someActor->SendThingConstructor();

{
  AutoIPCDeleter autoThingDeleter(thing);

  if (!foo) {
    // ... things happen in here.
    // No need to delete the child - that'll occur once AutoIPCDeleter goes out of scope.
  } else if (bar) {
    // ... do some things
    nsresult rv = something->DoTheJob();

    if (NS_FAILED(rv)) {
      // No need to delete the child - that'll occur once AutoIPCDeleter goes out of scope.
    }
  } else {
    // All is well, don't delete the Thing
    autoThingDeleter.Preserve();
  }
}
```

I wonder if something more like this second pattern will allow us to avoid cases where we forget to delete some Actors when things go wrong.