Fix issue in cext_preprocessor.rb if the replacement is empty

* Like in json_patches.rb
* See #2386
* Rename test file to use _ and not - by convention.

(cherry picked from commit a8b7abc)

Author: eregon

lib/truffle/truffle/cext_preprocessor.rb

@@ -96,7 +96,9 @@ def self.patch(file, contents, directory)
             # preprocessor macro which _must_ end with a newline and
             # so requires that we preserve the trailing whitespace.
             patched_file[:patches].each do |patch|
-              last_line = patch[:replacement].rstrip.lines.last.lstrip
+              replacement = patch[:replacement].rstrip
+              last_line = replacement.lines.last || replacement # .lines returns an empty Array if String#empty?
+              last_line = last_line.lstrip
               contents = contents.gsub(patch[:match],
                                        if last_line && last_line.start_with?('#')
                                          patch[:replacement]

test/truffle/cexts/test-preprocess.rb renamed to test/truffle/cexts/test_preprocess.rb

@@ -56,7 +56,22 @@ def test_patch(file, directory, input, expected)
 }
 EOF
 
+json_original = <<-EOF
+# else
+rb_str_resize(*result, RSTRING_LEN(*result));
+# endif
+EOF
+
+json_patched = <<-EOF
+# else
+
+# endif
+EOF
+
 test_patch 'xml_sax_parser.c', 'ext/nokogiri', original, modified
 # Should not patch other files or other gems
 test_patch 'other_file.c', 'ext/nokogiri', original, original
 test_patch 'xml_sax_parser.c', 'ext/other_gem', original, original
+
+# Tests an empty replacement
+test_patch 'parser.c', 'ext/json/ext/parser', json_original, json_patched

tool/jt.rb

@@ -1278,7 +1278,7 @@ def retag(*args)
       case test_name
       when 'tools'
         # Test tools
-        run_ruby 'test/truffle/cexts/test-preprocess.rb'
+        run_ruby 'test/truffle/cexts/test_preprocess.rb'
 
       when 'minimum', 'method', 'module', 'globals', 'backtraces', 'xopenssl'
         # Test that we can compile and run some very basic C extensions