Fix ChildProcess::Unix::Process#exited? method with leader processes

[jdelStrother]

Sorry, I found another edge case following on from my leader-process problems in #151

require "childprocess"

ChildProcess.logger.level = 0

# Start a background thread that launches a long-lived process.
# The child process will be killed once this script finishes (via at_exit)
Thread.new do
  child1 = ChildProcess.build("yes")
  child1.start
  at_exit do
    child1.stop
  end
  status = child1.wait
end

# give the bg thread some time to ensure child1.wait has happened
sleep 0.1

# Start a second child process
child2 = ChildProcess.build("sleep", "100")
child2.leader = true
child2.start
puts "child2.stop"
child2.stop
puts "Child2 is done!"

When child2.stop is called, we send it TERM, then poll for it to exit with ::Process.waitpid2(_pid, ::Process::WNOHANG | ::Process::WUNTRACED). The process dies straight away, but under ruby 2.6, that waitpid2 call always returns nil, until we hit the stop-timeout, at which point we sent the child a KILL.

In OS X, that KILL raises Errno::EPERM. In Linux, it seems like it just swallows the signal being sent to a dead process.

WDYT to this fix? I'm not really familiar with unix leader process, so would welcome additional scrutiny.

[coveralls]

Coverage remained the same at 93.715% when pulling 5d1096d on jdelStrother:patch-1 into 3bbc30f on enkessler:dev.

[sds]

Thanks for the PR.

Supportive of this solution given the documentation for waitpid(2):

wait for any child process whose process group ID is equal to the absolute value of pid.

In this context, we don't want to wait for any process in the process group—we are only interested in the process itself, so we should simply wait on the lone process, not any member of the group.