lazy: Fix concurrent initialization order

Fixes #5901

lazy/init.go

@@ -77,47 +77,45 @@ func (ini *Init) Do() (interface{}, error) {
 	}
 
 	ini.init.Do(func() {
-		var (
-			dependencies []*Init
-			children     []*Init
-		)
-
 		prev := ini.prev
-		for prev != nil {
+		if prev != nil {
+			// A branch. Initialize the ancestors.
 			if prev.shouldInitialize() {
-				dependencies = append(dependencies, prev)
-			}
-			prev = prev.prev
-		}
-
-		for _, child := range ini.children {
-			if child.shouldInitialize() {
-				children = append(children, child)
-			}
-		}
-
-		for _, dep := range dependencies {
-			_, err := dep.Do()
-			if err != nil {
-				ini.err = err
-				return
+				_, err := prev.Do()
+				if err != nil {
+					ini.err = err
+					return
+				}
+			} else if prev.inProgress() {
+				// Concurrent initialization. The following init func
+				// may depend on earlier state, so wait.
+				prev.wait()
 			}
 		}
 
 		if ini.f != nil {
 			ini.out, ini.err = ini.f()
 		}
 
-		for _, dep := range children {
-			_, err := dep.Do()
-			if err != nil {
-				ini.err = err
-				return
+		for _, child := range ini.children {
+			if child.shouldInitialize() {
+				_, err := child.Do()
+				if err != nil {
+					ini.err = err
+					return
+				}
 			}
 		}
-
 	})
 
+	ini.wait()
+
+	return ini.out, ini.err
+
+}
+
+// TODO(bep) investigate if we can use sync.Cond for this.
+func (ini *Init) wait() {
 	var counter time.Duration
 	for !ini.init.Done() {
 		counter += 10
@@ -126,8 +124,10 @@ func (ini *Init) Do() (interface{}, error) {
 		}
 		time.Sleep(counter * time.Microsecond)
 	}
+}
 
-	return ini.out, ini.err
+func (ini *Init) inProgress() bool {
+	return ini != nil && ini.init.InProgress()
 }
 
 func (ini *Init) shouldInitialize() bool {
@@ -147,20 +147,19 @@ func (ini *Init) add(branch bool, initFn func() (interface{}, error)) *Init {
 	ini.mu.Lock()
 	defer ini.mu.Unlock()
 
-	if !branch {
-		ini.checkDone()
-	}
-
-	init := &Init{
-		f:    initFn,
-		prev: ini,
+	if branch {
+		return &Init{
+			f:    initFn,
+			prev: ini,
+		}
 	}
 
-	if !branch {
-		ini.children = append(ini.children, init)
-	}
+	ini.checkDone()
+	ini.children = append(ini.children, &Init{
+		f: initFn,
+	})
 
-	return init
+	return ini
 }
 
 func (ini *Init) checkDone() {

lazy/init_test.go

@@ -25,32 +25,41 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+var (
+	rnd        = rand.New(rand.NewSource(time.Now().UnixNano()))
+	bigOrSmall = func() int {
+		if rnd.Intn(10) < 5 {
+			return 10000 + rnd.Intn(100000)
+		}
+		return 1 + rnd.Intn(50)
+	}
+)
+
+func doWork() {
+	doWorkOfSize(bigOrSmall())
+}
+
+func doWorkOfSize(size int) {
+	_ = strings.Repeat("Hugo Rocks! ", size)
+}
+
 func TestInit(t *testing.T) {
 	assert := require.New(t)
 
 	var result string
 
-	bigOrSmall := func() int {
-		if rand.Intn(10) < 3 {
-			return 10000 + rand.Intn(100000)
-		}
-		return 1 + rand.Intn(50)
-	}
-
 	f1 := func(name string) func() (interface{}, error) {
 		return func() (interface{}, error) {
 			result += name + "|"
-			size := bigOrSmall()
-			_ = strings.Repeat("Hugo Rocks! ", size)
+			doWork()
 			return name, nil
 		}
 	}
 
 	f2 := func() func() (interface{}, error) {
 		return func() (interface{}, error) {
-			size := bigOrSmall()
-			_ = strings.Repeat("Hugo Rocks! ", size)
-			return size, nil
+			doWork()
+			return nil, nil
 		}
 	}
 
@@ -73,16 +82,15 @@ func TestInit(t *testing.T) {
 		go func(i int) {
 			defer wg.Done()
 			var err error
-			if rand.Intn(10) < 5 {
+			if rnd.Intn(10) < 5 {
 				_, err = root.Do()
 				assert.NoError(err)
 			}
 
 			// Add a new branch on the fly.
-			if rand.Intn(10) > 5 {
+			if rnd.Intn(10) > 5 {
 				branch := branch1_2.Branch(f2())
-				init := branch.Add(f2())
-				_, err = init.Do()
+				_, err = branch.Do()
 				assert.NoError(err)
 			} else {
 				_, err = branch1_2_1.Do()
@@ -148,3 +156,71 @@ func TestInitAddWithTimeoutError(t *testing.T) {
 
 	assert.Error(err)
 }
+
+type T struct {
+	sync.Mutex
+	V1 string
+	V2 string
+}
+
+func (t *T) Add1(v string) {
+	t.Lock()
+	t.V1 += v
+	t.Unlock()
+}
+
+func (t *T) Add2(v string) {
+	t.Lock()
+	t.V2 += v
+	t.Unlock()
+}
+
+// https://github.com/gohugoio/hugo/issues/5901
+func TestInitBranchOrder(t *testing.T) {
+	assert := require.New(t)
+
+	base := New()
+
+	work := func(size int, f func()) func() (interface{}, error) {
+		return func() (interface{}, error) {
+			doWorkOfSize(size)
+			if f != nil {
+				f()
+			}
+
+			return nil, nil
+		}
+	}
+
+	state := &T{}
+
+	base = base.Add(work(10000, func() {
+		state.Add1("A")
+	}))
+
+	inits := make([]*Init, 2)
+	for i := range inits {
+		inits[i] = base.Branch(work(i+1*100, func() {
+			// V1 is A
+			ab := state.V1 + "B"
+			state.Add2(ab)
+
+		}))
+	}
+
+	var wg sync.WaitGroup
+
+	for _, v := range inits {
+		v := v
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, err := v.Do()
+			assert.NoError(err)
+		}()
+	}
+
+	wg.Wait()
+
+	assert.Equal("ABAB", state.V2)
+}