Bug 1255986 - [moztest] Safe-guard structured logging for exceptions from bad handlers. r=jgraham

Any exception which gets thrown by a log handler while test results are getting generated, should
not cause test harnesses to stop immediately. To achive that the exception details are written to
stderr and not propagated up the stack.

MozReview-Commit-ID: ChyYxApYSGx

--HG--
extra : rebase_source : 9fc3fe597061bedb1df2f5b8de1daa4bd127ea1e

testing/mozbase/mozlog/mozlog/structuredlog.py

@@ -207,7 +207,13 @@ class StructuredLogger(object):
                     return
 
             for handler in self.handlers:
-                handler(data)
+                try:
+                    handler(data)
+                except Exception:
+                    # Write the exception details directly to stderr because
+                    # log() would call this method again which is currently locked.
+                    print >> sys.__stderr__, '%s: Failure calling log handler:' % __name__
+                    print >> sys.__stderr__, traceback.format_exc()
 
     def _make_log_data(self, action, data):
         all_data = {"action": action,