correctly spawn parallel experiment tasks

tasks/upload.py

@@ -604,6 +604,7 @@ def _schedule_coverage_processing_task(
                 commit_report,
                 upload_context,
                 checkpoints,
+                parallel_processing.run_fully_parallel,
             )
 
             if parallel_processing.run_fully_parallel:
@@ -654,6 +655,7 @@ def create_parallel_tasks(
         commit_report: CommitReport,
         upload_context: UploadContext,
         checkpoints: CheckpointLogger,
+        run_fully_parallel: bool,
     ):
         report_service = ReportService(commit_yaml)
         sessions = report_service.build_sessions(commit=commit)
@@ -706,7 +708,6 @@ def create_parallel_tasks(
 
         parallel_processing_tasks = [
             upload_processor_task.s(
-                {},
                 repoid=commit.repoid,
                 commitid=commit.commitid,
                 commit_yaml=commit_yaml,
@@ -720,6 +721,11 @@ def create_parallel_tasks(
                 argument_list, parallel_session_ids
             )
         ]
+        if run_fully_parallel:
+            for task in parallel_processing_tasks:
+                # this is the `previous_results`, which celery provides when running
+                # in a chain as part of the experiment, otherwise we have to provide this.
+                task.args = ({},)
 
         finish_parallel_sig = upload_finisher_task.signature(
             kwargs={