Add priliminary support for "engine-less" builds.

Needed for #925.

infra/base-images/base-builder/compile

@@ -22,8 +22,10 @@ if [ -z "${SANITIZER_FLAGS-}" ]; then
   export SANITIZER_FLAGS=${!FLAGS_VAR-}
 fi
 
-# compile script might override environment, use . to call it.
-. compile_${FUZZING_ENGINE}
+if [[ $FUZZING_ENGINE != "none" ]]; then
+  # compile script might override environment, use . to call it.
+  . compile_${FUZZING_ENGINE}
+fi
 
 if [[ $SANITIZER_FLAGS = *sanitize=memory* ]]
 then

infra/gcb/build.py

@@ -28,6 +28,7 @@ CONFIGURATIONS = {
   'engine-libfuzzer' : [ 'FUZZING_ENGINE=libfuzzer' ],
   'engine-afl' : [ 'FUZZING_ENGINE=afl' ],
   'engine-honggfuzz' : [ 'FUZZING_ENGINE=honggfuzz' ],
+  'engine-none' : [ 'FUZZING_ENGINE=none' ],
 }
 
 EngineInfo = collections.namedtuple(

infra/helper.py

@@ -174,7 +174,7 @@ def _get_command_string(command):
 def _add_engine_args(parser):
   """Add common engine args."""
   parser.add_argument('--engine', default='libfuzzer',
-                      choices=['libfuzzer', 'afl', 'honggfuzz'])
+                      choices=['libfuzzer', 'afl', 'honggfuzz', 'none'])
 
 
 def _add_sanitizer_args(parser):