Use UNIXSocket to pass file descriptor

This is a more standard way to pass fd in UNIX world.
At the same time, it helps to remove a few hacks and
simplifies the code in the sandbox.

Closes Homebrew/homebrew#38434.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/build.rb

@@ -10,6 +10,7 @@ require "keg"
 require "extend/ENV"
 require "debrew"
 require "fcntl"
+require "socket"
 
 class Build
   attr_reader :formula, :deps, :reqs
@@ -172,7 +173,7 @@ class Build
 end
 
 begin
-  error_pipe = IO.new(ENV["HOMEBREW_ERROR_PIPE"].to_i, "w")
+  error_pipe = UNIXSocket.open(ENV["HOMEBREW_ERROR_PIPE"], &:recv_io)
   error_pipe.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
 
   # Invalidate the current sudo timestamp in case a build script calls sudo

Library/Homebrew/formula_installer.rb

@@ -11,6 +11,8 @@ require 'install_renamed'
 require 'cmd/tap'
 require 'hooks/bottles'
 require 'debrew'
+require 'fcntl'
+require 'socket'
 
 class FormulaInstaller
   include FormulaCellarChecks
@@ -455,6 +457,9 @@ class FormulaInstaller
   end
 
   def build
+    socket_path = "#{Dir.mktmpdir("homebrew", HOMEBREW_TEMP)}/socket"
+    server = UNIXServer.new(socket_path)
+
     FileUtils.rm Dir["#{HOMEBREW_LOGS}/#{formula.name}/*"]
 
     @start_time = Time.now
@@ -463,8 +468,7 @@ class FormulaInstaller
     #    installation has a pristine ENV when it starts, forking now is
     #    the easiest way to do this
     read, write = IO.pipe
-    # I'm guessing this is not a good way to do this, but I'm no UNIX guru
-    ENV['HOMEBREW_ERROR_PIPE'] = write.to_i.to_s
+    ENV["HOMEBREW_ERROR_PIPE"] = socket_path
 
     args = %W[
       nice #{RUBY_PATH}
@@ -475,14 +479,11 @@ class FormulaInstaller
       #{formula.path}
     ].concat(build_argv)
 
-    # Ruby 2.0+ sets close-on-exec on all file descriptors except for
-    # 0, 1, and 2 by default, so we have to specify that we want the pipe
-    # to remain open in the child process.
-    args << { write => write } if RUBY_VERSION >= "2.0"
-
     pid = fork do
       begin
+        server.close
         read.close
+        write.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
         exec(*args)
       rescue Exception => e
         Marshal.dump(e, write)
@@ -492,10 +493,17 @@ class FormulaInstaller
     end
 
     ignore_interrupts(:quietly) do # the child will receive the interrupt and marshal it back
+      begin
+        socket = server.accept_nonblock
+      rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::ECONNABORTED, Errno::EPROTO, Errno::EINTR
+        retry unless Process.waitpid(pid, Process::WNOHANG)
+      else
+        socket.send_io(write)
+      end
       write.close
       data = read.read
       read.close
-      Process.wait(pid)
+      Process.wait(pid) unless socket.nil?
       raise Marshal.load(data) unless data.nil? or data.empty?
       raise Interrupt if $?.exitstatus == 130
       raise "Suspicious installation failure" unless $?.success?
@@ -510,6 +518,9 @@ class FormulaInstaller
       formula.rack.rmdir_if_possible
     end
     raise
+  ensure
+    server.close
+    FileUtils.rm_r File.dirname(socket_path)
   end
 
   def link(keg)