[ruby-core:80838] [Ruby trunk Bug#13501] Process.kill behaviour for negative pid is not documented and may be wrong

Issue #13501 has been reported by toy (Ivan Kuchin).

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Bug #13501: Process.kill behaviour for negative pid is not documented and may be wrong
https://bugs.ruby-lang.org/issues/13501

* Author: toy (Ivan Kuchin)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.2.7p470 (2017-03-28 revision 58194) [x86_64-darwin14]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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Following code will not fail and will print "TERM caught by NN" both for spawned and forked processes:

~~~ ruby
pid = spawn 'ruby', '-e', <<-'RUBY', :pgroup => true
  trap('TERM'){ print "TERM caught by #{$$}\n" }
  fork
  sleep 10
RUBY
sleep 1
Process.kill('TERM', -pid)
~~~

Documentation explains such behaviour for using negative signal (or a string starting with '-'), but not negative pid.

[linux](https://linux.die.net/man/2/kill), [osx](https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man2/kill.2.html) and [POSIX](http://pubs.opengroup.org/onlinepubs/009695399/functions/kill.html) documentation for `kill` describes behaviour for negative pid is to send signal to process group.

As `Process.kill` is [using `killpg`](https://github.com/ruby/ruby/blob/e97ac02f8466275732a33f704480c03da722f485/signal.c#L464) if it is defined, the behaviour may be undefined on systems with `killpg` present but `kill` not conforming to `POSIX` specification.
Also sending negative `signal` with negative `pid` will call `killpg` with negative `pid`.




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