Fix concurrent map read and map write in short page lookups

Regression introduced in Hugo `v0.137.0`.

Fixes #13019

5 files changed, 169 insertions, 49 deletions

diff --git a/common/maps/cache.go b/common/maps/cache.go
index 0175974b5..cdc31a684 100644
--- a/common/maps/cache.go
+++ b/common/maps/cache.go
@@ -13,11 +13,14 @@
 
 package maps
 
-import "sync"
+import (
+	"sync"
+)
 
 // Cache is a simple thread safe cache backed by a map.
 type Cache[K comparable, T any] struct {
-	m map[K]T
+	m                  map[K]T
+	hasBeenInitialized bool
 	sync.RWMutex
 }
 
@@ -34,11 +37,16 @@ func (c *Cache[K, T]) Get(key K) (T, bool) {
 		return zero, false
 	}
 	c.RLock()
-	v, found := c.m[key]
+	v, found := c.get(key)
 	c.RUnlock()
 	return v, found
 }
 
+func (c *Cache[K, T]) get(key K) (T, bool) {
+	v, found := c.m[key]
+	return v, found
+}
+
 // GetOrCreate gets the value for the given key if it exists, or creates it if not.
 func (c *Cache[K, T]) GetOrCreate(key K, create func() (T, error)) (T, error) {
 	c.RLock()
@@ -61,13 +69,49 @@ func (c *Cache[K, T]) GetOrCreate(key K, create func() (T, error)) (T, error) {
 	return v, nil
 }
 
+// InitAndGet initializes the cache if not already done and returns the value for the given key.
+// The init state will be reset on Reset or Drain.
+func (c *Cache[K, T]) InitAndGet(key K, init func(get func(key K) (T, bool), set func(key K, value T)) error) (T, error) {
+	var v T
+	c.RLock()
+	if !c.hasBeenInitialized {
+		c.RUnlock()
+		if err := func() error {
+			c.Lock()
+			defer c.Unlock()
+			// Double check in case another goroutine has initialized it in the meantime.
+			if !c.hasBeenInitialized {
+				err := init(c.get, c.set)
+				if err != nil {
+					return err
+				}
+				c.hasBeenInitialized = true
+			}
+			return nil
+		}(); err != nil {
+			return v, err
+		}
+		// Reacquire the read lock.
+		c.RLock()
+	}
+
+	v = c.m[key]
+	c.RUnlock()
+
+	return v, nil
+}
+
 // Set sets the given key to the given value.
 func (c *Cache[K, T]) Set(key K, value T) {
 	c.Lock()
-	c.m[key] = value
+	c.set(key, value)
 	c.Unlock()
 }
 
+func (c *Cache[K, T]) set(key K, value T) {
+	c.m[key] = value
+}
+
 // ForEeach calls the given function for each key/value pair in the cache.
 func (c *Cache[K, T]) ForEeach(f func(K, T)) {
 	c.RLock()
@@ -81,6 +125,7 @@ func (c *Cache[K, T]) Drain() map[K]T {
 	c.Lock()
 	m := c.m
 	c.m = make(map[K]T)
+	c.hasBeenInitialized = false
 	c.Unlock()
 	return m
 }
@@ -94,6 +139,7 @@ func (c *Cache[K, T]) Len() int {
 func (c *Cache[K, T]) Reset() {
 	c.Lock()
 	c.m = make(map[K]T)
+	c.hasBeenInitialized = false
 	c.Unlock()
 }
 
diff --git a/hugolib/content_map_page.go b/hugolib/content_map_page.go
index 5e8646b21..8c9e4a31a 100644
--- a/hugolib/content_map_page.go
+++ b/hugolib/content_map_page.go
@@ -37,7 +37,6 @@ import (
 	"github.com/gohugoio/hugo/hugolib/doctree"
 	"github.com/gohugoio/hugo/hugolib/pagesfromdata"
 	"github.com/gohugoio/hugo/identity"
-	"github.com/gohugoio/hugo/lazy"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/output"
 	"github.com/gohugoio/hugo/resources"
@@ -925,59 +924,58 @@ func newPageMap(i int, s *Site, mcache *dynacache.Cache, pageTrees *pageTrees) *
 		s: s,
 	}
 
-	m.pageReverseIndex = &contentTreeReverseIndex{
-		initFn: func(rm map[any]contentNodeI) {
-			add := func(k string, n contentNodeI) {
-				existing, found := rm[k]
-				if found && existing != ambiguousContentNode {
-					rm[k] = ambiguousContentNode
-				} else if !found {
-					rm[k] = n
-				}
+	m.pageReverseIndex = newContentTreeTreverseIndex(func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI)) {
+		add := func(k string, n contentNodeI) {
+			existing, found := get(k)
+			if found && existing != ambiguousContentNode {
+				set(k, ambiguousContentNode)
+			} else if !found {
+				set(k, n)
 			}
+		}
 
-			w := &doctree.NodeShiftTreeWalker[contentNodeI]{
-				Tree:     m.treePages,
-				LockType: doctree.LockTypeRead,
-				Handle: func(s string, n contentNodeI, match doctree.DimensionFlag) (bool, error) {
-					p := n.(*pageState)
-					if p.PathInfo() != nil {
-						add(p.PathInfo().BaseNameNoIdentifier(), p)
-					}
-					return false, nil
-				},
-			}
+		w := &doctree.NodeShiftTreeWalker[contentNodeI]{
+			Tree:     m.treePages,
+			LockType: doctree.LockTypeRead,
+			Handle: func(s string, n contentNodeI, match doctree.DimensionFlag) (bool, error) {
+				p := n.(*pageState)
+				if p.PathInfo() != nil {
+					add(p.PathInfo().BaseNameNoIdentifier(), p)
+				}
+				return false, nil
+			},
+		}
 
-			if err := w.Walk(context.Background()); err != nil {
-				panic(err)
-			}
-		},
-		contentTreeReverseIndexMap: &contentTreeReverseIndexMap{},
-	}
+		if err := w.Walk(context.Background()); err != nil {
+			panic(err)
+		}
+	})
 
 	return m
 }
 
+func newContentTreeTreverseIndex(init func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI))) *contentTreeReverseIndex {
+	return &contentTreeReverseIndex{
+		initFn: init,
+		mm:     maps.NewCache[any, contentNodeI](),
+	}
+}
+
 type contentTreeReverseIndex struct {
-	initFn func(rm map[any]contentNodeI)
-	*contentTreeReverseIndexMap
+	initFn func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI))
+	mm     *maps.Cache[any, contentNodeI]
 }
 
 func (c *contentTreeReverseIndex) Reset() {
-	c.init.ResetWithLock().Unlock()
+	c.mm.Reset()
 }
 
 func (c *contentTreeReverseIndex) Get(key any) contentNodeI {
-	c.init.Do(func() {
-		c.m = make(map[any]contentNodeI)
-		c.initFn(c.contentTreeReverseIndexMap.m)
+	v, _ := c.mm.InitAndGet(key, func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI)) error {
+		c.initFn(get, set)
+		return nil
 	})
-	return c.m[key]
-}
-
-type contentTreeReverseIndexMap struct {
-	init lazy.OnceMore
-	m    map[any]contentNodeI
+	return v
 }
 
 type sitePagesAssembler struct {
diff --git a/hugolib/content_map_test.go b/hugolib/content_map_test.go
index bf9920071..a9f719f4a 100644
--- a/hugolib/content_map_test.go
+++ b/hugolib/content_map_test.go
@@ -17,9 +17,11 @@ import (
 	"fmt"
 	"path/filepath"
 	"strings"
+	"sync"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/identity"
 )
 
 func TestContentMapSite(t *testing.T) {
@@ -396,3 +398,77 @@ irrelevant
 		"<loc>https://example.org/en/sitemap.xml</loc>",
 	)
 }
+
+func TestContentTreeReverseIndex(t *testing.T) {
+	t.Parallel()
+
+	c := qt.New(t)
+
+	pageReverseIndex := newContentTreeTreverseIndex(
+		func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI)) {
+			for i := 0; i < 10; i++ {
+				key := fmt.Sprint(i)
+				set(key, &testContentNode{key: key})
+			}
+		},
+	)
+
+	for i := 0; i < 10; i++ {
+		key := fmt.Sprint(i)
+		v := pageReverseIndex.Get(key)
+		c.Assert(v, qt.Not(qt.IsNil))
+		c.Assert(v.Path(), qt.Equals, key)
+	}
+}
+
+// Issue 13019.
+func TestContentTreeReverseIndexPara(t *testing.T) {
+	t.Parallel()
+
+	var wg sync.WaitGroup
+
+	for i := 0; i < 10; i++ {
+		pageReverseIndex := newContentTreeTreverseIndex(
+			func(get func(key any) (contentNodeI, bool), set func(key any, val contentNodeI)) {
+				for i := 0; i < 10; i++ {
+					key := fmt.Sprint(i)
+					set(key, &testContentNode{key: key})
+				}
+			},
+		)
+
+		for j := 0; j < 10; j++ {
+			wg.Add(1)
+			go func(i int) {
+				defer wg.Done()
+				pageReverseIndex.Get(fmt.Sprint(i))
+			}(j)
+		}
+	}
+}
+
+type testContentNode struct {
+	key string
+}
+
+func (n *testContentNode) GetIdentity() identity.Identity {
+	return identity.StringIdentity(n.key)
+}
+
+func (n *testContentNode) ForEeachIdentity(cb func(id identity.Identity) bool) bool {
+	panic("not supported")
+}
+
+func (n *testContentNode) Path() string {
+	return n.key
+}
+
+func (n *testContentNode) isContentNodeBranch() bool {
+	return false
+}
+
+func (n *testContentNode) resetBuildState() {
+}
+
+func (n *testContentNode) MarkStale() {
+}
diff --git a/lazy/init.go b/lazy/init.go
index bef3867a9..7b88a5351 100644
--- a/lazy/init.go
+++ b/lazy/init.go
@@ -36,7 +36,7 @@ type Init struct {
 	prev     *Init
 	children []*Init
 
-	init OnceMore
+	init onceMore
 	out  any
 	err  error
 	f    func(context.Context) (any, error)
diff --git a/lazy/once.go b/lazy/once.go
index dac689df3..cea096652 100644
--- a/lazy/once.go
+++ b/lazy/once.go
@@ -24,13 +24,13 @@ import (
 // * it can be reset, so the action can be repeated if needed
 // * it has methods to check if it's done or in progress
 
-type OnceMore struct {
+type onceMore struct {
 	mu   sync.Mutex
 	lock uint32
 	done uint32
 }
 
-func (t *OnceMore) Do(f func()) {
+func (t *onceMore) Do(f func()) {
 	if atomic.LoadUint32(&t.done) == 1 {
 		return
 	}
@@ -53,15 +53,15 @@ func (t *OnceMore) Do(f func()) {
 	f()
 }
 
-func (t *OnceMore) InProgress() bool {
+func (t *onceMore) InProgress() bool {
 	return atomic.LoadUint32(&t.lock) == 1
 }
 
-func (t *OnceMore) Done() bool {
+func (t *onceMore) Done() bool {
 	return atomic.LoadUint32(&t.done) == 1
 }
 
-func (t *OnceMore) ResetWithLock() *sync.Mutex {
+func (t *onceMore) ResetWithLock() *sync.Mutex {
 	t.mu.Lock()
 	defer atomic.StoreUint32(&t.done, 0)
 	return &t.mu