Ignore EPERM which means already being process-leader

process.c

@@ -7115,7 +7115,8 @@ rb_daemon(int nochdir, int noclose)
       default: _exit(EXIT_SUCCESS);
     }
 
-    if (setsid() < 0) return -1;
+    /* ignore EPERM which means already being process-leader */
+    if (setsid() < 0) (void)0;
 
     if (!nochdir)
         err = chdir("/");

test/ruby/test_process.rb

@@ -1881,17 +1881,19 @@ class TestProcess < Test::Unit::TestCase
         if f
           assert_equal(f.pid, Process.wait(f.pid))
 
-          dpid, ppid = Integer(f.gets), Integer(f.gets)
+          dpid, ppid, dsid = 3.times.map {Integer(f.gets)}
 
           message = "daemon #{dpid} should be detached"
           assert_not_equal($$, ppid, message) # would be 1 almost always
           assert_raise(Errno::ECHILD, message) {Process.wait(dpid)}
           assert_kind_of(Integer, Process.kill(0, dpid), message)
+          assert_equal(dpid, dsid)
 
           break # close f, and let the daemon resume and exit
         end
+        Process.setsid rescue nil
         Process.daemon(false, true)
-        puts $$, Process.ppid
+        puts $$, Process.ppid, Process.getsid
         $stdin.gets # wait for the above assertions using signals
       end
     end