Handle new fuzz targets in split builds properly (#4436)

Previously we tried to handle it by finding the fuzz targets in the
list. But we still failed because it wasn't in the db. Save it to the db
to solve this problem.

src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py

@@ -2025,18 +2025,25 @@ def _pick_fuzz_target():
   return build_manager.pick_random_fuzz_target(target_weights)
 
 
-def _get_fuzz_target_from_db(engine_name, fuzz_target_binary, job_type):
+def _get_or_create_fuzz_target(engine_name, fuzz_target_binary, job_type):
+  """Gets or creates a FuzzTarget db entity."""
   project = data_handler.get_project_name(job_type)
   qualified_name = data_types.fuzz_target_fully_qualified_name(
       engine_name, project, fuzz_target_binary)
   key = ndb.Key(data_types.FuzzTarget, qualified_name)
-  return key.get()
+  fuzz_target = key.get()
+  if fuzz_target:
+    return fuzz_target
+  fuzz_target = data_types.FuzzTarget(
+      engine=engine_name, binary=fuzz_target_binary, project=project)
+  fuzz_target.put()
+  return fuzz_target
 
 
 def _preprocess_get_fuzz_target(fuzzer_name, job_type):
   fuzz_target_name = _pick_fuzz_target()
   if fuzz_target_name:
-    return _get_fuzz_target_from_db(fuzzer_name, fuzz_target_name, job_type)
+    return _get_or_create_fuzz_target(fuzzer_name, fuzz_target_name, job_type)
   return None