Allow shutdown_pipe to be passed in via @config

[wishdev]

I would like to be able to fork a webrick server and then shut it down from the parent process.

The @shutdown_pipe variable within webrick/server is the perfect tool for the job but it is not settable/accessible outside of the class. This patch simply allows for a pipe to be passed in via the server config.

[jeremyevans]

I think it's fine to expose this, but we should add a test for it to prevent regressions.

[wishdev]

Test added - thanks for your time Jeremy!

[jeremyevans]

Thanks for working on this!

[jeremyevans]

@wishdev Looks like there is a nondeterministic failure in the test: https://github.com/ruby/webrick/runs/894956154

Can you please add a pull request to fix that? Otherwise, I'll probably have to revert.

[mame]

I'm unsure if it is good to expose the internal. @akr introduced the mechanism of shutdown_pipe. @akr, what do you think?

[ko1]

  1) Error:
TestWEBrickServer#test_shutdown_pipe:
IOError: stream closed in another thread
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `write'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `puts'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `block in test_shutdown_pipe'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in `loop'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in `test_shutdown_pipe'

on high load-average environments.

http://ci.rvm.jp/logfiles/brlog.trunk-no-theap.20200924-222101

[wishdev]

  1) Error:
TestWEBrickServer#test_shutdown_pipe:
IOError: stream closed in another thread
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `write'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `puts'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in `block in test_shutdown_pipe'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in `loop'
    /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in `test_shutdown_pipe'

on high load-average environments.

http://ci.rvm.jp/logfiles/brlog.trunk-no-theap.20200924-222101

I'm going to discuss this on #54 because that's where the test code in question lives.