process.c: fix rubyspec of Process.groups

getgroups(2) may return a GID list that includes duplicated GIDs.
The behavior is totaly depends on what OS is used.

This commit fixes the example of Process.groups so that the example
is independent of this OS-dependent features.

Additonaly, this commit adds the description of such system-dependent
characteristics of Process.groups.

[ruby-dev:50603] [Bug #14969]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64265 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -6136,6 +6136,19 @@ maxgroups(void)
  *
  *     Process.groups   #=> [27, 6, 10, 11]
  *
+ *  Note that this method is just a wrapper of getgroups(2).
+ *  This means that the following characteristics of
+ *  the results are completely depends on your system:
+ *
+ *  - the result is sorted
+ *  - the result includes effective GIDs
+ *  - the result does not include duplicated GIDs
+ *
+ *  You can certainly get a sorted unique GID list of
+ *  the current process by this expression:
+ *
+ *     Process.groups.unique.sort
+ *
  */
 
 static VALUE

spec/ruby/core/process/groups_spec.rb

@@ -6,8 +6,8 @@ describe "Process.groups" do
       groups = `id -G`.scan(/\d+/).map { |i| i.to_i }
       gid = Process.gid
 
-      expected = (groups.sort - [gid]).sort
-      actual = (Process.groups - [gid]).sort
+      expected = (groups.sort - [gid]).uniq.sort
+      actual = (Process.groups - [gid]).uniq.sort
       actual.should == expected
     end
   end