Support for UNIX endpoints on the abstract file system

[herwinw]

https://www.man7.org/linux/man-pages/man7/unix.7.html has a very succinct description on it (search for "abstract"). Short version: these are not actual files, but identifiers that start with a null byte. The changes in 0.17.2 have broken them.

Minimal example:

#!/usr/bin/env -S falcon host
# frozen_string_literal: true
load :rack

rack 'localhost' do
  endpoint Async::HTTP::Endpoint.parse('http://localhost:9292')
end

rack 'unixsocket' do
  uri = URI.for('unix', nil, "\x00Falcontest", nil, nil, '', nil, nil, nil, URI)
  endpoint Async::HTTP::Endpoint.new(uri, IO::Endpoint.unix("\x00FalconTest"))
end

(I'm not sure if this is the best way to create a unix socket for an abstract namespace, but this one worked with io-endpoint 0.17.1)

This results in an error:

 0.02s    error: Falcon::Command [ec=0x328] [pid=95335] [2026-03-31 14:22:47 +0200]
               | path name contains null byte
               |   ArgumentError: path name contains null byte
               |   
               |   			FileUtils.mkdir_p(File.dirname(@path))
               |   			                               ^^^^^
               |   → .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/io-endpoint-0.17.2/lib/io/endpoint/unix_endpoint.rb:121 in 'File.dirname'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/io-endpoint-0.17.2/lib/io/endpoint/unix_endpoint.rb:121 in 'IO::Endpoint::UNIXEndpoint#create_symlink_if_required!'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/io-endpoint-0.17.2/lib/io/endpoint/unix_endpoint.rb:95 in 'IO::Endpoint::UNIXEndpoint#bind'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/async-http-0.88.0/lib/async/http/endpoint.rb:216 in 'Async::HTTP::Endpoint#bind'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/io-endpoint-0.17.2/lib/io/endpoint/bound_endpoint.rb:19 in 'IO::Endpoint::BoundEndpoint.bound'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/io-endpoint-0.17.2/lib/io/endpoint/bound_endpoint.rb:105 in 'IO::Endpoint::Generic#bound'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/falcon-0.51.1/lib/falcon/service/server.rb:42 in 'block in Falcon::Service::Server#start'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/async-2.23.1/lib/async/task.rb:200 in 'block in Async::Task#run'
               |     .local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/async-2.23.1/lib/async/task.rb:438 in 'block in Async::Task#schedule'

I would guess the easiest fix is to never try to do anything on the file system if the first byte of the name is a null byte.