logname may not produce a login name

从 spec/ruby/library/etc/getlogin_spec.rb:22 开始，在 GitHub Actions 环境中的 JRuby 上重现失败的 Etc.getlogin 示例。比较可用的登录名命令和 issue 中描述的环境值，然后让 spec 处理没有控制终端的环境。当 spec 在 JRuby 上通过，同时在命令不可用时保留回退方案，即表示完成。

Running the specs on JRuby on Github Actions, we see the following message and failing spec:

...
logname: no login name
...
1)
Etc.getlogin returns the name associated with the current login activity FAILED
Expected "runner" == ""
to be truthy but was false
/home/runner/work/jruby/jruby/spec/ruby/library/etc/getlogin_spec.rb:22:in `block in <main>'
...

This can occur when logname is run without a controlling terminal. I'm unsure whether this GHA env is not setting up a tty, or if there's an issue launching the command in JRuby that prevents it inheriting the parent terminal, but it seems like id would be a more reliable command to use:

diff --git a/spec/ruby/library/etc/getlogin_spec.rb b/spec/ruby/library/etc/getlogin_spec.rb
index 7a4fd79ae2..f0dde84ccb 100644
--- a/spec/ruby/library/etc/getlogin_spec.rb
+++ b/spec/ruby/library/etc/getlogin_spec.rb
@@ -18,11 +18,13 @@ describe "Etc.getlogin" do
         else
           # Etc.getlogin returns the same result of logname(2)
           # if it returns non NULL
-          if system("which logname", out: File::NULL, err: File::NULL)
+          if system("which id", out: File::NULL, err: File::NULL)
+            Etc.getlogin.should == `id -un`.chomp
+          elsif system("which logname", out: File::NULL, err: File::NULL)
+            # fallback to `logname` command since `id` is not available
             Etc.getlogin.should == `logname`.chomp
           else
-            # fallback to `id` command since `logname` is not available
-            Etc.getlogin.should == `id -un`.chomp
+            Etc.getlogin.should == ENV['LOGNAME']
           end
         end
       else

However I think we are also stacking too many conditions here. I'm unsure of the "best" way to get the current login, but clearly logname has issues that make it undesirable.

FWIW some forums suggest logname -t which will ensure a tty is created, but this flag is not present on BSD-likes.