spec/ruby/core/process/setpriority_spec.rb: allow to run under docker

Docker container is not always able to lower the nice value even if the
euid is a root.  It depends upon the configuration of docker which
cannot check from the container itself.

This change does check it by actually trying to lower the value first.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66570 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/process/setpriority_spec.rb

@@ -29,11 +29,26 @@ describe "Process.setpriority" do
     end
 
     as_superuser do
-      it "sets the scheduling priority for a specified user" do
+      p = Process.getpriority(Process::PRIO_USER, 0)
-        p = Process.getpriority(Process::PRIO_USER, 0)
+      # The nice value is a value in the range -20 to 19.
-        Process.setpriority(Process::PRIO_USER, 0, p + 1).should == 0
+      # This test tries to change the nice value to +-1, so it cannot run if p == -20 || p == 19.
-        Process.getpriority(Process::PRIO_USER, 0).should == (p + 1)
+      if -20 < p && p < 19
-        Process.setpriority(Process::PRIO_USER, 0, p).should == 0
+       begin
+         # Check if we can lower the nice value or not.
+         #
+         # We are not always able to do it even as a root.
+         # Docker container is not always able to do it depending upon the configuration,
+         # which cannot know from the container itself.
+          Process.setpriority(Process::PRIO_USER, 0, p - 1)
+          Process.setpriority(Process::PRIO_USER, 0, p)
+
+          it "sets the scheduling priority for a specified user" do
+            Process.setpriority(Process::PRIO_USER, 0, p + 1).should == 0
+            Process.getpriority(Process::PRIO_USER, 0).should == (p + 1)
+            Process.setpriority(Process::PRIO_USER, 0, p).should == 0
+         end
+       rescue Errno::EACCES
+       end
       end
     end
   end