Do not use Entropic and Focus Function together (OSS-Fuzz#4242). (#1942)

* Do not use Entropic and Focus Function together (#4242).

* reorder entropic and dataflow as per review suggestion

Author: Dor1s

src/python/bot/fuzzers/libfuzzer.py

@@ -1740,10 +1740,16 @@ def pick_strategies(strategy_pool,
   is_mutations_run = (not environment.is_ephemeral() and
                       candidate_generator != engine_common.Generator.NONE)
 
+  if strategy_pool.do_strategy(strategy.ENTROPIC_STRATEGY):
+    arguments.append(constants.ENTROPIC_ARGUMENT)
+    fuzzing_strategies.append(strategy.ENTROPIC_STRATEGY.name)
+
   # Depends on the presense of DFSan instrumented build.
   dataflow_build_dir = environment.get_value('DATAFLOW_BUILD_DIR')
   use_dataflow_tracing = (
       dataflow_build_dir and
+      # Focus function is not compatible with entropic mode.
+      strategy.ENTROPIC_STRATEGY.name not in fuzzing_strategies and
       strategy_pool.do_strategy(strategy.DATAFLOW_TRACING_STRATEGY))
   if use_dataflow_tracing:
     dataflow_binary_path = os.path.join(
@@ -1789,10 +1795,6 @@ def pick_strategies(strategy_pool,
                                  fuzzer_path)):
     fuzzing_strategies.append(strategy.RECOMMENDED_DICTIONARY_STRATEGY.name)
 
-  if strategy_pool.do_strategy(strategy.ENTROPIC_STRATEGY):
-    arguments.append(constants.ENTROPIC_ARGUMENT)
-    fuzzing_strategies.append(strategy.ENTROPIC_STRATEGY.name)
-
   if strategy_pool.do_strategy(strategy.VALUE_PROFILE_STRATEGY):
     arguments.append(constants.VALUE_PROFILE_ARGUMENT)
     fuzzing_strategies.append(strategy.VALUE_PROFILE_STRATEGY.name)