1642401 - Change CancelCPOWs() back to a release assert on ParentProcessIsBlocked()

Status: NEW

(Core :: IPC, task, P3)

Description

[Andrew McCreight [:mccr8]]

CancelCPOWs() calls CancelCurrentTransaction() if a sync message is currently in progress. It was added in https://hg.mozilla.org/releases/mozilla-aurora/rev/a237339bf4c8 . Prior to that, we'd hit a release assert in those situations (something about entering a nested event loop and some Windows thing). This also triggers telemetry, IPC_TRANSACTION_CANCEL. It looks like nobody has hit this condition on recent Nightlies, so I think we can revert to the asserting behavior.

The patch I linked above also added a block of code that does a cancel from Send under certain conditions I don't really understand:

  if (msg->nested_level() < DispatchingSyncMessageNestedLevel() ||
      msg->nested_level() < AwaitingSyncReplyNestedLevel()) {
    MOZ_RELEASE_ASSERT(DispatchingSyncMessage() || DispatchingAsyncMessage());
    MOZ_RELEASE_ASSERT(!mIsPostponingSends);
    IPC_LOG("Cancel from Send");
    CancelMessage* cancel =
        new CancelMessage(CurrentNestedInsideSyncTransaction());
    CancelTransaction(CurrentNestedInsideSyncTransaction());
    mLink->SendMessage(cancel);
  }

It doesn't seem super CPOW-specific, so maybe it should be turned into a release assert that we never hit this condition? There's no telemetry on it.

Comment 1

[Andrew McCreight [:mccr8]]

If the "cancel from send" can be removed, then there might be some further clean up around removing cancelling stuff.