src/Pure/System/build.scala

             echo(Library.trim_line(err))
 
             if (rc == 0) {
               val sources = make_stamp(name)
               val heap = heap_stamp(job.output_path)
+              (output_dir + log(name)).file.delete
               File.write_gzip(output_dir + log_gz(name), sources + "\n" + heap + "\n" + out)
             }
             else {
+              (output_dir + log_gz(name)).file.delete
               File.write(output_dir + log(name), out)
               echo(name + " FAILED")
               echo("(see also " + log(name).file.toString + ")")
               val lines = split_lines(out)
               val tail = lines.drop(lines.length - 20 max 0)