IMPALA-14514: Handle serializing bytes in bin/run-workload.py

On python 3, when Impyla receives a result with a string that is
not valid UTF-8, it returns that as bytes. TPC-DS Q30 on scale 20
has a result that contains invalid UTF-8, so bin/run-workload.py
can fail while trying to dump this to JSON.

This modifies CustomJSONEncoder to handle serializing bytes by
converting it to a string with invalid unicode handled with
backslashes.

Testing:
 - Ran bin/run-workload.py against TPC-DS scale 20

Change-Id: Ibe31c656de4fc65f8580c7b3b49bf655b8a5ecea
Reviewed-on: http://gerrit.cloudera.org:8080/23602
Reviewed-by: Riza Suminto <riza.suminto@cloudera.com>
Reviewed-by: Jason Fehr <jfehr@cloudera.com>
Tested-by: Joe McDonnell <joemcdonnell@cloudera.com>

bin/run-workload.py

@@ -145,6 +145,11 @@ class CustomJSONEncoder(json.JSONEncoder):
     if isinstance(obj, datetime):
       # Convert datetime into an standard iso string
       return obj.isoformat()
+    if isinstance(obj, bytes):
+      # Impyla can leave a string value as bytes when it is unable to decode it to UTF-8.
+      # TPC-DS has queries that produce non-UTF-8 results (e.g. Q30 on scale 20)
+      # Convert bytes to strings to make JSON encoding work
+      return obj.decode(encoding="utf-8", errors="backslashreplace")
     elif isinstance(obj, (Query, HiveQueryResult, QueryExecConfig, TableFormatInfo)):
       # Serialize these objects manually by returning their __dict__ methods.
       return obj.__dict__