Write rendered track to a temporary file

With this commit we write the full track to a temporary file instead of
dumping it directly into Rally's log file. This helps users to interpret
error messages later on that involve line numbers. It also avoids
bloating Rally's log file.

Closes elastic#554
Relates elastic#559

esrally/track/loader.py

@@ -3,6 +3,7 @@
 import os
 import glob
 import urllib.error
+import tempfile
 
 import jinja2
 import jinja2.exceptions
@@ -622,7 +623,12 @@ def read(self, track_name, track_spec_file, mapping_dir):
         self.logger.info("Reading track specification file [%s].", track_spec_file)
         try:
             rendered = render_template_from_file(track_spec_file, self.track_params)
-            self.logger.info("Final rendered track for '%s': %s", track_spec_file, rendered)
+            # render the track to a temporary file instead of dumping it into the logs. It is easier to check for error messages
+            # involving lines numbers and it also does not bloat Rally's log file so much.
+            tmp = tempfile.NamedTemporaryFile(delete=False, suffix=".json")
+            with open(tmp.name, "wt", encoding="utf-8") as f:
+                f.write(rendered)
+            self.logger.info("Final rendered track for '%s' has been written to '%s'.", track_spec_file, tmp.name)
             track_spec = json.loads(rendered)
         except jinja2.exceptions.TemplateNotFound:
             self.logger.exception("Could not load [%s]", track_spec_file)