Add output to persisted engine run, add code docs

engine/persist.go

@@ -9,6 +9,16 @@ import (
 	"git.defalsify.org/festive/resource"
 )
 
+// RunPersisted performs a single vm execution from client input using a persisted state.
+//
+// State is first loaded from storage. The vm is initialized with the state and executed. The new state is then saved to storage.
+//
+// The resulting output of the execution will be written to the provided writer.
+//
+// The state is identified by the SessionId member of the Config. Before first execution, the caller must ensure that an
+// initialized state actually is available for the identifier, otherwise the method will fail.
+//
+// It will also fail if execution by the underlying Engine fails.
 func RunPersisted(cfg Config, rs resource.Resource, pr persist.Persister, input []byte, w io.Writer, ctx context.Context) error {
 	err := pr.Load(cfg.SessionId)
 	if err != nil {
@@ -24,5 +34,5 @@ func RunPersisted(cfg Config, rs resource.Resource, pr persist.Persister, input
 			return err
 		}
 	}
-	return nil
+	return en.WriteResult(w, ctx)
 }