test/ruby/test_process.rb: test for fd=3 usability in child

Ensure we can redirect anything to fd=3 in a child process.  This
test exists because fd=3 is a commonly reserved FD for a timer
thread pipe in the parent Ruby VM, but fd=3 is the first FD used by
the sd_listen_fds function for systemd.

This means there is a possibility for a bug to slip in where the
redirect to fd=3 fails to work.  This test should ensure the
continued viability of systemd replacements in Ruby :>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51146 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 24 insertions, 0 deletions

diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index a43dcf475c..cb9440c836 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -2029,4 +2029,28 @@ EOS
     }
     assert_equal(th, x)
   end if defined?(fork)
+
+  def test_exec_fd_3_redirect
+    # ensure we can redirect anything to fd=3 in a child process.
+    # fd=3 is a commonly reserved FD for the timer thread pipe in the
+    # parent, but fd=3 is the first FD used by the sd_listen_fds function
+    # for systemd
+    assert_separately(['-', RUBY], <<-INPUT, timeout: 60)
+      ruby = ARGV.shift
+      begin
+        a = IO.pipe
+        b = IO.pipe
+        pid = fork do
+          exec ruby, '-e', 'print IO.for_fd(3).read(1)', 3 => a[0], 1 => b[1]
+        end
+        b[1].close
+        a[0].close
+        a[1].write('.')
+        assert_equal ".", b[0].read(1)
+      ensure
+        a.each(&:close) if a
+        b.each(&:close) if b
+      end
+    INPUT
+  end if defined?(fork)
 end