Ensure build is not destroyed until streaming returns

cmd/vela-worker/exec.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"context"
+	"sync"
 	"time"
 
 	"github.com/go-vela/worker/executor"
@@ -109,7 +110,13 @@ func (w *Worker) exec(index int) error {
 	timeoutCtx, timeout := context.WithTimeout(buildCtx, t)
 	defer timeout()
 
+	// This WaitGroup delays calling DestroyBuild until the StreamBuild goroutine finishes.
+	var wg sync.WaitGroup
+
 	defer func() {
+		// if exec() exits before starting StreamBuild, this returns immediately.
+		wg.Wait()
+
 		logger.Info("destroying build")
 
 		// destroy the build with the executor (pass a background
@@ -139,7 +146,9 @@ func (w *Worker) exec(index int) error {
 	}
 
 	// log/event streaming uses buildCtx so that it is not subject to the timeout.
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		logger.Info("streaming build logs")
 		// execute the build with the executor
 		err = _executor.StreamBuild(buildCtx)