avoid race condition described in #141 (#142)

execute.go

@@ -253,6 +253,22 @@ func executeStep(
 		queryResult = resultObj
 	}
 
+	// we need to collect all the dependent steps and execute them at last in this function
+	// to avoid a race condition, where the result of a dependent request is published to the
+	// result channel even before the result created in this iteration
+	type stepArgs struct {
+		step           *QueryPlanStep
+		insertionPoint []string
+	}
+	var dependentSteps []stepArgs
+	// defer the execution of the dependent steps after the main step has been published
+	defer func() {
+		for _, sr := range dependentSteps {
+			log.Info("Spawn ", sr.insertionPoint)
+			go executeStep(ctx, plan, sr.step, sr.insertionPoint, resultLock, queryVariables, resultCh, errCh, stepWg)
+		}
+	}()
+
 	// if there are next steps
 	if len(step.Then) > 0 {
 		log.Debug("Kicking off child queries")
@@ -263,19 +279,24 @@ func executeStep(
 			copy(copiedInsertionPoint, insertionPoint)
 			insertPoints, err := executorFindInsertionPoints(resultLock, dependent.InsertionPoint, step.SelectionSet, queryResult, [][]string{copiedInsertionPoint}, step.FragmentDefinitions)
 			if err != nil {
+				// reset dependent steps - result would be discarded anyways
+				dependentSteps = nil
 				errCh <- err
 				return
 			}
 
 			// this dependent needs to fire for every object that the insertion point references
 			for _, insertionPoint := range insertPoints {
-				log.Info("Spawn ", insertionPoint)
-				stepWg.Add(1)
-				go executeStep(ctx, plan, dependent, insertionPoint, resultLock, queryVariables, resultCh, errCh, stepWg)
+				dependentSteps = append(dependentSteps, stepArgs{
+					step:           dependent,
+					insertionPoint: insertionPoint,
+				})
 			}
 		}
 	}
 
+	// before publishing the current result, tell the wait-group about the dependent steps to wait for
+	stepWg.Add(len(dependentSteps))
 	log.Debug("Pushing Result. Insertion point: ", insertionPoint, ". Value: ", queryResult)
 	// send the result to be stitched in with our accumulator
 	resultCh <- &queryExecutionResult{