Make the spec more robust by guaranteeing the fd is not by anything else

Author: eregon

spec/ruby/core/process/spawn_spec.rb

@@ -706,13 +706,15 @@ def child_pids(pid)
       end
 
       it "maps the key to a file descriptor in the child that inherits the file descriptor from the parent specified by the value" do
-        child_fd = find_unused_fd
-        args = ruby_cmd(fixture(__FILE__, "map_fd.rb"), args: [child_fd.to_s])
-        pid = Process.spawn(*args, { child_fd => @io })
-        Process.waitpid pid
-        @io.rewind
-
-        @io.read.should == "writing to fd: #{child_fd}"
+        File.open(__FILE__, "r") do |f|
+          child_fd = f.fileno
+          args = ruby_cmd(fixture(__FILE__, "map_fd.rb"), args: [child_fd.to_s])
+          pid = Process.spawn(*args, { child_fd => @io })
+          Process.waitpid pid
+          @io.rewind
+
+          @io.read.should == "writing to fd: #{child_fd}"
+        end
       end
     end
   end