FJP.CommonPool and VM shutdown

[Doug Lea]

While writing some CompletableFuture test code, I
realized that there's a potential unhappy surprise
lurking in code of the form:

class X {
   public static void main(String[] args) {
      new ComputationUsingCommonPool(...).fork();
   }
}


Because the common pool uses daemon threads, it is
OK for the VM to shutdown immediately after the fork().

This is not at all a common usage: normally you'd
use invoke() or some other construction that somehow
uses the computation. But still, it is sure to be the
subject of a bug report someday.

There is an easy although currently non-obvious
workaround: End such programs with a call to
ForkJoinThread.helpQuiescePool().  This could be
made a bit less obscure by adding a (basically
identical) method:
   static void FJP.quiesceCommonPool();
and then document for use in such cases.

Otherwise, I don't know of a good alternative.
We cannot change to use non-daemon threads, because
that would then hold up VM termination until
an explicit common pool shutdown, which we
don't and can't allow. We could ask that the
VM implicitly add a call to quiesceCommonPool
to every main(), but that would probably be seen
as too disruptive and controversial considering
the rare cases it applies. We could also add a
shutdown hook calling quiesceCommonPool but because
of the lack of ordering guarantees for hooks, this may
get called after some other needed resources are blown away.

Other ideas?

-Doug