join

        # Thread.join

(from ruby core)
---
    thr.join          -> thr
    thr.join(limit)   -> thr

---

The calling thread will suspend execution and run this `thr`.

Does not return until `thr` exits or until the given `limit` seconds
have passed.

If the time limit expires, `nil` will be returned, otherwise `thr` is
returned.

Any threads not joined will be killed when the main program exits.

If `thr` had previously raised an exception and the ::abort_on_exception
or $DEBUG flags are not set, (so the exception has not yet been
processed), it will be processed at this time.

    a = Thread.new { print "a"; sleep(10); print "b"; print "c" }
    x = Thread.new { print "x"; Thread.pass; print "y"; print "z" }
    x.join # Let thread x finish, thread a will be killed on exit.
    #=> "axyz"

The following example illustrates the `limit` parameter.

    y = Thread.new { 4.times { sleep 0.1; puts 'tick... ' }}
    puts "Waiting" until y.join(0.15)

This will produce:

    tick...
    Waiting
    tick...
    Waiting
    tick...
    tick...