From 0d96627ffba8350ebac14df3ac126c66eb1a9ed8 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <me@cschwarz.com>
Date: Sun, 22 Nov 2020 15:14:26 +0100
Subject: [PATCH] [#388] trace: make WithTaskGroup actually concurrent

---
 daemon/logging/trace/trace_convenience.go     | 12 +++--
 .../logging/trace/trace_convenience_test.go   | 46 +++++++++++++++++++
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/daemon/logging/trace/trace_convenience.go b/daemon/logging/trace/trace_convenience.go
index f14ce53..c6e1888 100644
--- a/daemon/logging/trace/trace_convenience.go
+++ b/daemon/logging/trace/trace_convenience.go
@@ -43,15 +43,19 @@ func WithTaskAndSpan(ctx context.Context, task string, span string) (context.Con
 }
 
 // create a span during which several child tasks are spawned using the `add` function
+//
+// IMPORTANT FOR USERS: Caller must ensure that the capturing behavior is correct, the Go linter doesn't catch this.
 func WithTaskGroup(ctx context.Context, taskGroup string) (_ context.Context, add func(f func(context.Context)), waitEnd DoneFunc) {
 	var wg sync.WaitGroup
 	ctx, endSpan := WithSpan(ctx, taskGroup)
 	add = func(f func(context.Context)) {
 		wg.Add(1)
-		defer wg.Done()
-		ctx, endTask := WithTask(ctx, taskGroup)
-		defer endTask()
-		f(ctx)
+		go func() {
+			defer wg.Done()
+			ctx, endTask := WithTask(ctx, taskGroup)
+			defer endTask()
+			f(ctx)
+		}()
 	}
 	waitEnd = func() {
 		wg.Wait()
diff --git a/daemon/logging/trace/trace_convenience_test.go b/daemon/logging/trace/trace_convenience_test.go
index 3f27bfd..ce2a07b 100644
--- a/daemon/logging/trace/trace_convenience_test.go
+++ b/daemon/logging/trace/trace_convenience_test.go
@@ -1,7 +1,10 @@
 package trace
 
 import (
+	"context"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -14,3 +17,46 @@ func TestGetCallerOrPanic(t *testing.T) {
 	// zrepl prefix is stripped
 	assert.Equal(t, "daemon/logging/trace.TestGetCallerOrPanic", ret)
 }
+
+func TestWithTaskGroupRunTasksConcurrently(t *testing.T) {
+
+	// spawn a task group where each task waits for the other to start
+	// => without concurrency, they would hang
+
+	rootCtx, endRoot := WithTaskFromStack(context.Background())
+	defer endRoot()
+
+	_, add, waitEnd := WithTaskGroup(rootCtx, "test-task-group")
+
+	schedulerTimeout := 2 * time.Second
+	timeout := time.After(schedulerTimeout)
+	var hadTimeout uint32
+	started0, started1 := make(chan struct{}), make(chan struct{})
+	for i := 0; i < 2; i++ {
+		i := i // capture by copy
+		add(func(ctx context.Context) {
+			switch i {
+			case 0:
+				close(started0)
+				select {
+				case <-started1:
+				case <-timeout:
+					atomic.AddUint32(&hadTimeout, 1)
+				}
+			case 1:
+				close(started1)
+				select {
+				case <-started0:
+				case <-timeout:
+					atomic.AddUint32(&hadTimeout, 1)
+				}
+			default:
+				panic("unreachable")
+			}
+		})
+	}
+
+	waitEnd()
+	assert.Zero(t, hadTimeout, "either bad impl or scheduler timeout (which is %v)", schedulerTimeout)
+
+}