Add specs for the absolute path of the main script after chdir

* See #2709

Author: eregon

spec/ruby/core/thread/backtrace/location/absolute_path_spec.rb

@@ -17,6 +17,15 @@
     end
   end
 
+  it 'returns the correct absolute path when using a relative main script path and changing CWD' do
+    script = fixture(__FILE__, 'subdir/absolute_path_main_chdir.rb')
+    sibling = fixture(__FILE__, 'subdir/sibling.rb')
+    subdir = File.dirname script
+    Dir.chdir(fixture(__FILE__)) do
+      ruby_exe('subdir/absolute_path_main_chdir.rb').should == "subdir/absolute_path_main_chdir.rb\n#{subdir}\n#{subdir}\n#{script}\n#{sibling}\n"
+    end
+  end
+
   context "when used in eval with a given filename" do
     code = "caller_locations(0)[0].absolute_path"
 

spec/ruby/core/thread/backtrace/location/fixtures/subdir/absolute_path_main_chdir.rb

@@ -0,0 +1,11 @@
+puts __FILE__
+puts __dir__
+Dir.chdir __dir__
+
+# Check __dir__ is still correct after chdir
+puts __dir__
+
+puts caller_locations(0)[0].absolute_path
+
+# require_relative also needs to know the absolute path of the current file so we test it here too
+require_relative 'sibling'

spec/ruby/core/thread/backtrace/location/fixtures/subdir/sibling.rb

@@ -0,0 +1 @@
+puts __FILE__

spec/tags/core/thread/backtrace/location/absolute_path_tags.txt

@@ -1,2 +1,3 @@
 slow:Thread::Backtrace::Location#absolute_path returns an absolute path when using a relative main script path
 fails:Thread::Backtrace::Location#absolute_path canonicalization returns a canonical path without symlinks, even when __FILE__ is removed
+slow:Thread::Backtrace::Location#absolute_path returns the correct absolute path when using a relative main script path and changing CWD