all: Rework page store, add a dynacache, improve partial rebuilds, and some general spring cleaningdevelop2024

There are some breaking changes in this commit, see #11455.

Closes #11455
Closes #11549

This fixes a set of bugs (see issue list) and it is also paying some technical debt accumulated over the years. We now build with Staticcheck enabled in the CI build.

The performance should be about the same as before for regular sized Hugo sites, but it should perform and scale much better to larger data sets, as objects that uses lots of memory (e.g. rendered Markdown, big JSON files read into maps with transform.Unmarshal etc.) will now get automatically garbage collected if needed. Performance on partial rebuilds when running the server in fast render mode should be the same, but the change detection should be much more accurate.

A list of the notable new features:

* A new dependency tracker that covers (almost) all of Hugo's API and is used to do fine grained partial rebuilds when running the server.
* A new and simpler tree document store which allows fast lookups and prefix-walking in all dimensions (e.g. language) concurrently.
* You can now configure an upper memory limit allowing for much larger data sets and/or running on lower specced PCs.
We have lifted the "no resources in sub folders" restriction for branch bundles (e.g. sections).
Memory Limit
* Hugos will, by default, set aside a quarter of the total system memory, but you can set this via the OS environment variable HUGO_MEMORYLIMIT (in gigabytes). This is backed by a partitioned LRU cache used throughout Hugo. A cache that gets dynamically resized in low memory situations, allowing Go's Garbage Collector to free the memory.

New Dependency Tracker: Hugo has had a rule based coarse grained approach to server rebuilds that has worked mostly pretty well, but there have been some surprises (e.g. stale content). This is now revamped with a new dependency tracker that can quickly calculate the delta given a changed resource (e.g. a content file, template, JS file etc.). This handles transitive relations, e.g. $page -> js.Build -> JS import, or $page1.Content -> render hook -> site.GetPage -> $page2.Title, or $page1.Content -> shortcode -> partial -> site.RegularPages -> $page2.Content -> shortcode ..., and should also handle changes to aggregated values (e.g. site.Lastmod) effectively.

This covers all of Hugo's API with 2 known exceptions (a list that may not be fully exhaustive):

Changes to files loaded with template func os.ReadFile may not be handled correctly. We recommend loading resources with resources.Get
Changes to Hugo objects (e.g. Page) passed in the template context to lang.Translate may not be detected correctly. We recommend having simple i18n templates without too much data context passed in other than simple types such as strings and numbers.
Note that the cachebuster configuration (when A changes then rebuild B) works well with the above, but we recommend that you revise that configuration, as it in most situations should not be needed. One example where it is still needed is with TailwindCSS and using changes to hugo_stats.json to trigger new CSS rebuilds.

Document Store: Previously, a little simplified, we split the document store (where we store pages and resources) in a tree per language. This worked pretty well, but the structure made some operations harder than they needed to be. We have now restructured it into one Radix tree for all languages. Internally the language is considered to be a dimension of that tree, and the tree can be viewed in all dimensions concurrently. This makes some operations re. language simpler (e.g. finding translations is just a slice range), but the idea is that it should also be relatively inexpensive to add more dimensions if needed (e.g. role).

Fixes #10169
Fixes #10364
Fixes #10482
Fixes #10630
Fixes #10656
Fixes #10694
Fixes #10918
Fixes #11262
Fixes #11439
Fixes #11453
Fixes #11457
Fixes #11466
Fixes #11540
Fixes #11551
Fixes #11556
Fixes #11654
Fixes #11661
Fixes #11663
Fixes #11664
Fixes #11669
Fixes #11671
Fixes #11807
Fixes #11808
Fixes #11809
Fixes #11815
Fixes #11840
Fixes #11853
Fixes #11860
Fixes #11883
Fixes #11904
Fixes #7388
Fixes #7425
Fixes #7436
Fixes #7544
Fixes #7882
Fixes #7960
Fixes #8255
Fixes #8307
Fixes #8863
Fixes #8927
Fixes #9192
Fixes #9324

8 files changed, 1358 insertions, 0 deletions

diff --git a/hugolib/doctree/dimensions.go b/hugolib/doctree/dimensions.go
new file mode 100644
index 000000000..bcc3cae00
--- /dev/null
+++ b/hugolib/doctree/dimensions.go
@@ -0,0 +1,43 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+const (
+	// Language is currently the only dimension in the Hugo build matrix.
+	DimensionLanguage DimensionFlag = 1 << iota
+)
+
+// Dimension is a row in the Hugo build matrix which currently has one value: language.
+type Dimension [1]int
+
+// DimensionFlag is a flag in the Hugo build matrix.
+type DimensionFlag byte
+
+// Has returns whether the given flag is set.
+func (d DimensionFlag) Has(o DimensionFlag) bool {
+	return d&o == o
+}
+
+// Set sets the given flag.
+func (d DimensionFlag) Set(o DimensionFlag) DimensionFlag {
+	return d | o
+}
+
+// Index returns this flag's index in the Dimensions array.
+func (d DimensionFlag) Index() int {
+	if d == 0 {
+		panic("dimension flag not set")
+	}
+	return int(d - 1)
+}
diff --git a/hugolib/doctree/dimensions_test.go b/hugolib/doctree/dimensions_test.go
new file mode 100644
index 000000000..598f22a2d
--- /dev/null
+++ b/hugolib/doctree/dimensions_test.go
@@ -0,0 +1,37 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestDimensionFlag(t *testing.T) {
+	c := qt.New(t)
+
+	var zero DimensionFlag
+	var d DimensionFlag
+	var o DimensionFlag = 1
+	var p DimensionFlag = 12
+
+	c.Assert(d.Has(o), qt.Equals, false)
+	d = d.Set(o)
+	c.Assert(d.Has(o), qt.Equals, true)
+	c.Assert(d.Has(d), qt.Equals, true)
+	c.Assert(func() { zero.Index() }, qt.PanicMatches, "dimension flag not set")
+	c.Assert(DimensionLanguage.Index(), qt.Equals, 0)
+	c.Assert(p.Index(), qt.Equals, 11)
+}
diff --git a/hugolib/doctree/nodeshiftree_test.go b/hugolib/doctree/nodeshiftree_test.go
new file mode 100644
index 000000000..313be0bc4
--- /dev/null
+++ b/hugolib/doctree/nodeshiftree_test.go
@@ -0,0 +1,374 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree_test
+
+import (
+	"context"
+	"fmt"
+	"math/rand"
+	"path"
+	"strings"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/common/para"
+	"github.com/gohugoio/hugo/hugolib/doctree"
+	"github.com/google/go-cmp/cmp"
+)
+
+var eq = qt.CmpEquals(
+	cmp.Comparer(func(n1, n2 *testValue) bool {
+		if n1 == n2 {
+			return true
+		}
+
+		return n1.ID == n2.ID && n1.Lang == n2.Lang
+	}),
+)
+
+func TestTree(t *testing.T) {
+	c := qt.New(t)
+
+	zeroZero := doctree.New(
+		doctree.Config[*testValue]{
+			Shifter: &testShifter{},
+		},
+	)
+
+	a := &testValue{ID: "/a"}
+	zeroZero.InsertIntoValuesDimension("/a", a)
+	ab := &testValue{ID: "/a/b"}
+	zeroZero.InsertIntoValuesDimension("/a/b", ab)
+
+	c.Assert(zeroZero.Get("/a"), eq, &testValue{ID: "/a", Lang: 0})
+	s, v := zeroZero.LongestPrefix("/a/b/c", true, nil)
+	c.Assert(v, eq, ab)
+	c.Assert(s, eq, "/a/b")
+
+	// Change language.
+	oneZero := zeroZero.Increment(0)
+	c.Assert(zeroZero.Get("/a"), eq, &testValue{ID: "/a", Lang: 0})
+	c.Assert(oneZero.Get("/a"), eq, &testValue{ID: "/a", Lang: 1})
+}
+
+func TestTreeData(t *testing.T) {
+	c := qt.New(t)
+
+	tree := doctree.New(
+		doctree.Config[*testValue]{
+			Shifter: &testShifter{},
+		},
+	)
+
+	tree.InsertIntoValuesDimension("", &testValue{ID: "HOME"})
+	tree.InsertIntoValuesDimension("/a", &testValue{ID: "/a"})
+	tree.InsertIntoValuesDimension("/a/b", &testValue{ID: "/a/b"})
+	tree.InsertIntoValuesDimension("/b", &testValue{ID: "/b"})
+	tree.InsertIntoValuesDimension("/b/c", &testValue{ID: "/b/c"})
+	tree.InsertIntoValuesDimension("/b/c/d", &testValue{ID: "/b/c/d"})
+
+	var values []string
+
+	ctx := &doctree.WalkContext[*testValue]{}
+
+	w := &doctree.NodeShiftTreeWalker[*testValue]{
+		Tree:        tree,
+		WalkContext: ctx,
+		Handle: func(s string, t *testValue, match doctree.DimensionFlag) (bool, error) {
+			ctx.Data().Insert(s, map[string]any{
+				"id": t.ID,
+			})
+
+			if s != "" {
+				p, v := ctx.Data().LongestPrefix(path.Dir(s))
+				values = append(values, fmt.Sprintf("%s:%s:%v", s, p, v))
+			}
+			return false, nil
+		},
+	}
+
+	w.Walk(context.Background())
+
+	c.Assert(strings.Join(values, "|"), qt.Equals, "/a::map[id:HOME]|/a/b:/a:map[id:/a]|/b::map[id:HOME]|/b/c:/b:map[id:/b]|/b/c/d:/b/c:map[id:/b/c]")
+}
+
+func TestTreeEvents(t *testing.T) {
+	c := qt.New(t)
+
+	tree := doctree.New(
+		doctree.Config[*testValue]{
+			Shifter: &testShifter{echo: true},
+		},
+	)
+
+	tree.InsertIntoValuesDimension("/a", &testValue{ID: "/a", Weight: 2, IsBranch: true})
+	tree.InsertIntoValuesDimension("/a/p1", &testValue{ID: "/a/p1", Weight: 5})
+	tree.InsertIntoValuesDimension("/a/p", &testValue{ID: "/a/p2", Weight: 6})
+	tree.InsertIntoValuesDimension("/a/s1", &testValue{ID: "/a/s1", Weight: 5, IsBranch: true})
+	tree.InsertIntoValuesDimension("/a/s1/p1", &testValue{ID: "/a/s1/p1", Weight: 8})
+	tree.InsertIntoValuesDimension("/a/s1/p1", &testValue{ID: "/a/s1/p2", Weight: 9})
+	tree.InsertIntoValuesDimension("/a/s1/s2", &testValue{ID: "/a/s1/s2", Weight: 6, IsBranch: true})
+	tree.InsertIntoValuesDimension("/a/s1/s2/p1", &testValue{ID: "/a/s1/s2/p1", Weight: 8})
+	tree.InsertIntoValuesDimension("/a/s1/s2/p2", &testValue{ID: "/a/s1/s2/p2", Weight: 7})
+
+	w := &doctree.NodeShiftTreeWalker[*testValue]{
+		Tree:        tree,
+		WalkContext: &doctree.WalkContext[*testValue]{},
+	}
+
+	w.Handle = func(s string, t *testValue, match doctree.DimensionFlag) (bool, error) {
+		if t.IsBranch {
+			w.WalkContext.AddEventListener("weight", s, func(e *doctree.Event[*testValue]) {
+				if e.Source.Weight > t.Weight {
+					t.Weight = e.Source.Weight
+					w.WalkContext.SendEvent(&doctree.Event[*testValue]{Source: t, Path: s, Name: "weight"})
+				}
+				// Reduces the amount of events bubbling up the tree. If the weight for this branch has
+				// increased, that will be announced in its own event.
+				e.StopPropagation()
+			})
+		} else {
+			w.WalkContext.SendEvent(&doctree.Event[*testValue]{Source: t, Path: s, Name: "weight"})
+		}
+
+		return false, nil
+	}
+
+	c.Assert(w.Walk(context.Background()), qt.IsNil)
+	c.Assert(w.WalkContext.HandleEventsAndHooks(), qt.IsNil)
+
+	c.Assert(tree.Get("/a").Weight, eq, 9)
+	c.Assert(tree.Get("/a/s1").Weight, eq, 9)
+	c.Assert(tree.Get("/a/p").Weight, eq, 6)
+	c.Assert(tree.Get("/a/s1/s2").Weight, eq, 8)
+	c.Assert(tree.Get("/a/s1/s2/p2").Weight, eq, 7)
+}
+
+func TestTreeInsert(t *testing.T) {
+	c := qt.New(t)
+
+	tree := doctree.New(
+		doctree.Config[*testValue]{
+			Shifter: &testShifter{},
+		},
+	)
+
+	a := &testValue{ID: "/a"}
+	tree.InsertIntoValuesDimension("/a", a)
+	ab := &testValue{ID: "/a/b"}
+	tree.InsertIntoValuesDimension("/a/b", ab)
+
+	c.Assert(tree.Get("/a"), eq, &testValue{ID: "/a", Lang: 0})
+	c.Assert(tree.Get("/notfound"), qt.IsNil)
+
+	ab2 := &testValue{ID: "/a/b", Lang: 0}
+	v, ok := tree.InsertIntoValuesDimension("/a/b", ab2)
+	c.Assert(ok, qt.IsTrue)
+	c.Assert(v, qt.DeepEquals, ab2)
+
+	tree1 := tree.Increment(0)
+	c.Assert(tree1.Get("/a/b"), qt.DeepEquals, &testValue{ID: "/a/b", Lang: 1})
+}
+
+func TestTreePara(t *testing.T) {
+	c := qt.New(t)
+
+	p := para.New(4)
+	r, _ := p.Start(context.Background())
+
+	tree := doctree.New(
+		doctree.Config[*testValue]{
+			Shifter: &testShifter{},
+		},
+	)
+
+	for i := 0; i < 8; i++ {
+		i := i
+		r.Run(func() error {
+			a := &testValue{ID: "/a"}
+			lock := tree.Lock(true)
+			defer lock()
+			tree.InsertIntoValuesDimension("/a", a)
+			ab := &testValue{ID: "/a/b"}
+			tree.InsertIntoValuesDimension("/a/b", ab)
+
+			key := fmt.Sprintf("/a/b/c/%d", i)
+			val := &testValue{ID: key}
+			tree.InsertIntoValuesDimension(key, val)
+			c.Assert(tree.Get(key), eq, val)
+			// s, _ := tree.LongestPrefix(key, nil)
+			// c.Assert(s, eq, "/a/b")
+
+			return nil
+		})
+	}
+
+	c.Assert(r.Wait(), qt.IsNil)
+}
+
+func TestValidateKey(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(doctree.ValidateKey(""), qt.IsNil)
+	c.Assert(doctree.ValidateKey("/a/b/c"), qt.IsNil)
+	c.Assert(doctree.ValidateKey("/"), qt.IsNotNil)
+	c.Assert(doctree.ValidateKey("a"), qt.IsNotNil)
+	c.Assert(doctree.ValidateKey("abc"), qt.IsNotNil)
+	c.Assert(doctree.ValidateKey("/abc/"), qt.IsNotNil)
+}
+
+type testShifter struct {
+	echo bool
+}
+
+func (s *testShifter) ForEeachInDimension(n *testValue, d int, f func(n *testValue) bool) {
+	if d != doctree.DimensionLanguage.Index() {
+		panic("not implemented")
+	}
+	f(n)
+}
+
+func (s *testShifter) Insert(old, new *testValue) *testValue {
+	return new
+}
+
+func (s *testShifter) InsertInto(old, new *testValue, dimension doctree.Dimension) *testValue {
+	return new
+}
+
+func (s *testShifter) Delete(n *testValue, dimension doctree.Dimension) (bool, bool) {
+	return true, true
+}
+
+func (s *testShifter) Shift(n *testValue, dimension doctree.Dimension, exact bool) (*testValue, bool, doctree.DimensionFlag) {
+	if s.echo {
+		return n, true, doctree.DimensionLanguage
+	}
+	if n.NoCopy {
+		if n.Lang == dimension[0] {
+			return n, true, doctree.DimensionLanguage
+		}
+		return nil, false, doctree.DimensionLanguage
+	}
+	c := *n
+	c.Lang = dimension[0]
+	return &c, true, doctree.DimensionLanguage
+}
+
+func (s *testShifter) All(n *testValue) []*testValue {
+	return []*testValue{n}
+}
+
+type testValue struct {
+	ID   string
+	Lang int
+
+	Weight   int
+	IsBranch bool
+
+	NoCopy bool
+}
+
+func BenchmarkTreeInsert(b *testing.B) {
+	runBench := func(b *testing.B, numElements int) {
+		for i := 0; i < b.N; i++ {
+			tree := doctree.New(
+				doctree.Config[*testValue]{
+					Shifter: &testShifter{},
+				},
+			)
+
+			for i := 0; i < numElements; i++ {
+				lang := rand.Intn(2)
+				tree.InsertIntoValuesDimension(fmt.Sprintf("/%d", i), &testValue{ID: fmt.Sprintf("/%d", i), Lang: lang, Weight: i, NoCopy: true})
+			}
+		}
+	}
+
+	b.Run("1000", func(b *testing.B) {
+		runBench(b, 1000)
+	})
+
+	b.Run("10000", func(b *testing.B) {
+		runBench(b, 10000)
+	})
+
+	b.Run("100000", func(b *testing.B) {
+		runBench(b, 100000)
+	})
+
+	b.Run("300000", func(b *testing.B) {
+		runBench(b, 300000)
+	})
+}
+
+func BenchmarkWalk(b *testing.B) {
+	const numElements = 1000
+
+	createTree := func() *doctree.NodeShiftTree[*testValue] {
+		tree := doctree.New(
+			doctree.Config[*testValue]{
+				Shifter: &testShifter{},
+			},
+		)
+
+		for i := 0; i < numElements; i++ {
+			lang := rand.Intn(2)
+			tree.InsertIntoValuesDimension(fmt.Sprintf("/%d", i), &testValue{ID: fmt.Sprintf("/%d", i), Lang: lang, Weight: i, NoCopy: true})
+		}
+
+		return tree
+	}
+
+	handle := func(s string, t *testValue, match doctree.DimensionFlag) (bool, error) {
+		return false, nil
+	}
+
+	for _, numElements := range []int{1000, 10000, 100000} {
+
+		b.Run(fmt.Sprintf("Walk one dimension %d", numElements), func(b *testing.B) {
+			tree := createTree()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				w := &doctree.NodeShiftTreeWalker[*testValue]{
+					Tree:   tree,
+					Handle: handle,
+				}
+				if err := w.Walk(context.Background()); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+
+		b.Run(fmt.Sprintf("Walk all dimensions %d", numElements), func(b *testing.B) {
+			base := createTree()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				for d1 := 0; d1 < 1; d1++ {
+					for d2 := 0; d2 < 2; d2++ {
+						tree := base.Shape(d1, d2)
+						w := &doctree.NodeShiftTreeWalker[*testValue]{
+							Tree:   tree,
+							Handle: handle,
+						}
+						if err := w.Walk(context.Background()); err != nil {
+							b.Fatal(err)
+						}
+					}
+				}
+			}
+		})
+
+	}
+}
diff --git a/hugolib/doctree/nodeshifttree.go b/hugolib/doctree/nodeshifttree.go
new file mode 100644
index 000000000..1c1175305
--- /dev/null
+++ b/hugolib/doctree/nodeshifttree.go
@@ -0,0 +1,433 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+import (
+	"context"
+	"fmt"
+	"path"
+	"strings"
+	"sync"
+
+	radix "github.com/armon/go-radix"
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+type (
+	Config[T any] struct {
+		// Shifter handles tree transformations.
+		Shifter Shifter[T]
+	}
+
+	// Shifter handles tree transformations.
+	Shifter[T any] interface {
+		// ForEeachInDimension will call the given function for each value in the given dimension.
+		// If the function returns true, the walk will stop.
+		ForEeachInDimension(n T, d int, f func(T) bool)
+
+		// Insert inserts new into the tree into the dimension it provides.
+		// It may replace old.
+		// It returns a T (can be the same as old).
+		Insert(old, new T) T
+
+		// Insert inserts new into the given dimension.
+		// It may replace old.
+		// It returns a T (can be the same as old).
+		InsertInto(old, new T, dimension Dimension) T
+
+		// Delete deletes T from the given dimension and returns whether the dimension was deleted and if  it's empty after the delete.
+		Delete(v T, dimension Dimension) (bool, bool)
+
+		// Shift shifts T into the given dimension
+		// and returns the shifted T and a bool indicating if the shift was successful and
+		// how accurate a match T is according to its dimensions.
+		Shift(v T, dimension Dimension, exact bool) (T, bool, DimensionFlag)
+	}
+)
+
+// NodeShiftTree is the root of a tree that can be shaped using the Shape method.
+// Note that multipled shapes of the same tree is meant to be used concurrently,
+// so use the applicable locking when needed.
+type NodeShiftTree[T any] struct {
+	tree *radix.Tree
+
+	// E.g. [language, role].
+	dims    Dimension
+	shifter Shifter[T]
+
+	mu *sync.RWMutex
+}
+
+func New[T any](cfg Config[T]) *NodeShiftTree[T] {
+	if cfg.Shifter == nil {
+		panic("Shifter is required")
+	}
+
+	return &NodeShiftTree[T]{
+		mu:      &sync.RWMutex{},
+		shifter: cfg.Shifter,
+		tree:    radix.New(),
+	}
+}
+
+func (r *NodeShiftTree[T]) Delete(key string) {
+	r.delete(key)
+}
+
+func (r *NodeShiftTree[T]) DeleteAll(key string) {
+	r.tree.WalkPrefix(key, func(key string, value any) bool {
+		v, ok := r.tree.Delete(key)
+		if ok {
+			resource.MarkStale(v)
+		}
+		return false
+	})
+}
+
+func (r *NodeShiftTree[T]) DeletePrefix(prefix string) int {
+	count := 0
+	var keys []string
+	r.tree.WalkPrefix(prefix, func(key string, value any) bool {
+		keys = append(keys, key)
+		return false
+	})
+	for _, key := range keys {
+		if ok := r.delete(key); ok {
+			count++
+		}
+	}
+	return count
+}
+
+func (r *NodeShiftTree[T]) delete(key string) bool {
+	var wasDeleted bool
+	if v, ok := r.tree.Get(key); ok {
+		var isEmpty bool
+		wasDeleted, isEmpty = r.shifter.Delete(v.(T), r.dims)
+		if isEmpty {
+			r.tree.Delete(key)
+		}
+	}
+	return wasDeleted
+}
+
+func (t *NodeShiftTree[T]) DeletePrefixAll(prefix string) int {
+	count := 0
+
+	t.tree.WalkPrefix(prefix, func(key string, value any) bool {
+		if v, ok := t.tree.Delete(key); ok {
+			resource.MarkStale(v)
+			count++
+		}
+		return false
+	})
+
+	return count
+}
+
+// Increment the value of dimension d by 1.
+func (t *NodeShiftTree[T]) Increment(d int) *NodeShiftTree[T] {
+	return t.Shape(d, t.dims[d]+1)
+}
+
+func (r *NodeShiftTree[T]) InsertIntoCurrentDimension(s string, v T) (T, bool) {
+	s = mustValidateKey(cleanKey(s))
+	if vv, ok := r.tree.Get(s); ok {
+		v = r.shifter.InsertInto(vv.(T), v, r.dims)
+	}
+	r.tree.Insert(s, v)
+	return v, true
+}
+
+func (r *NodeShiftTree[T]) InsertIntoValuesDimension(s string, v T) (T, bool) {
+	s = mustValidateKey(cleanKey(s))
+	if vv, ok := r.tree.Get(s); ok {
+		v = r.shifter.Insert(vv.(T), v)
+	}
+	r.tree.Insert(s, v)
+	return v, true
+}
+
+func (r *NodeShiftTree[T]) InsertRawWithLock(s string, v any) (any, bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.tree.Insert(s, v)
+}
+
+func (r *NodeShiftTree[T]) InsertWithLock(s string, v T) (T, bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.InsertIntoValuesDimension(s, v)
+}
+
+func (t *NodeShiftTree[T]) Len() int {
+	return t.tree.Len()
+}
+
+func (t *NodeShiftTree[T]) CanLock() bool {
+	ok := t.mu.TryLock()
+	if ok {
+		t.mu.Unlock()
+	}
+	return ok
+}
+
+// Lock locks the data store for read or read/write access until commit is invoked.
+// Note that Root is not thread-safe outside of this transaction construct.
+func (t *NodeShiftTree[T]) Lock(writable bool) (commit func()) {
+	if writable {
+		t.mu.Lock()
+	} else {
+		t.mu.RLock()
+	}
+	return func() {
+		if writable {
+			t.mu.Unlock()
+		} else {
+			t.mu.RUnlock()
+		}
+	}
+}
+
+// LongestPrefix finds the longest prefix of s that exists in the tree that also matches the predicate (if set).
+// Set exact to true to only match exact in the current dimension (e.g. language).
+func (r *NodeShiftTree[T]) LongestPrefix(s string, exact bool, predicate func(v T) bool) (string, T) {
+	for {
+		longestPrefix, v, found := r.tree.LongestPrefix(s)
+
+		if found {
+			if t, ok, _ := r.shift(v.(T), exact); ok && (predicate == nil || predicate(t)) {
+				return longestPrefix, t
+			}
+		}
+
+		if s == "" || s == "/" {
+			var t T
+			return "", t
+		}
+
+		// Walk up to find a node in the correct dimension.
+		s = path.Dir(s)
+
+	}
+}
+
+// LongestPrefixAll returns the longest prefix considering all tree dimensions.
+func (r *NodeShiftTree[T]) LongestPrefixAll(s string) (string, bool) {
+	s, _, found := r.tree.LongestPrefix(s)
+	return s, found
+}
+
+func (r *NodeShiftTree[T]) GetRaw(s string) (T, bool) {
+	v, ok := r.tree.Get(s)
+	if !ok {
+		var t T
+		return t, false
+	}
+	return v.(T), true
+}
+
+func (r *NodeShiftTree[T]) WalkPrefixRaw(prefix string, walker func(key string, value T) bool) {
+	walker2 := func(key string, value any) bool {
+		return walker(key, value.(T))
+	}
+	r.tree.WalkPrefix(prefix, walker2)
+}
+
+// Shape the tree for dimension d to value v.
+func (t *NodeShiftTree[T]) Shape(d, v int) *NodeShiftTree[T] {
+	x := t.clone()
+	x.dims[d] = v
+	return x
+}
+
+func (t *NodeShiftTree[T]) String() string {
+	return fmt.Sprintf("Root{%v}", t.dims)
+}
+
+func (r *NodeShiftTree[T]) Get(s string) T {
+	t, _ := r.get(s)
+	return t
+}
+
+func (r *NodeShiftTree[T]) ForEeachInDimension(s string, d int, f func(T) bool) {
+	s = cleanKey(s)
+	v, ok := r.tree.Get(s)
+	if !ok {
+		return
+	}
+	r.shifter.ForEeachInDimension(v.(T), d, f)
+}
+
+type WalkFunc[T any] func(string, T) (bool, error)
+
+type NodeShiftTreeWalker[T any] struct {
+	// The tree to walk.
+	Tree *NodeShiftTree[T]
+
+	// Handle will be called for each node in the main tree.
+	// If the callback returns true, the walk will stop.
+	// The callback can optionally return a callback for the nested tree.
+	Handle func(s string, v T, exact DimensionFlag) (terminate bool, err error)
+
+	// Optional prefix filter.
+	Prefix string
+
+	// Enable read or write locking if needed.
+	LockType LockType
+
+	// When set, no dimension shifting will be performed.
+	NoShift bool
+
+	// Don't fall back to alternative dimensions (e.g. language).
+	Exact bool
+
+	// Used in development only.
+	Debug bool
+
+	// Optional context.
+	// Note that this is copied to the nested walkers using Extend.
+	// This means that walkers can pass data (down) and events (up) to
+	// the related walkers.
+	WalkContext *WalkContext[T]
+
+	// Local state.
+	// This is scoped to the current walker and not copied to the nested walkers.
+	skipPrefixes []string
+}
+
+// Extend returns a new NodeShiftTreeWalker with the same configuration as the
+// and the same WalkContext as the original.
+// Any local state is reset.
+func (r NodeShiftTreeWalker[T]) Extend() *NodeShiftTreeWalker[T] {
+	r.resetLocalState()
+	return &r
+}
+
+// SkipPrefix adds a prefix to be skipped in the walk.
+func (r *NodeShiftTreeWalker[T]) SkipPrefix(prefix ...string) {
+	r.skipPrefixes = append(r.skipPrefixes, prefix...)
+}
+
+// ShouldSkip returns whether the given key should be skipped in the walk.
+func (r *NodeShiftTreeWalker[T]) ShouldSkip(s string) bool {
+	for _, prefix := range r.skipPrefixes {
+		if strings.HasPrefix(s, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+func (r *NodeShiftTreeWalker[T]) Walk(ctx context.Context) error {
+	if r.Tree == nil {
+		panic("Tree is required")
+	}
+	r.resetLocalState()
+
+	if r.LockType > LockTypeNone {
+		commit1 := r.Tree.Lock(r.LockType == LockTypeWrite)
+		defer commit1()
+	}
+
+	main := r.Tree
+
+	var err error
+	fnMain := func(s string, v interface{}) bool {
+		if r.ShouldSkip(s) {
+			return false
+		}
+
+		t, ok, exact := r.toT(r.Tree, v)
+		if !ok {
+			return false
+		}
+
+		var terminate bool
+		terminate, err = r.Handle(s, t, exact)
+		if terminate || err != nil {
+			return true
+		}
+		return false
+	}
+
+	if r.Prefix != "" {
+		main.tree.WalkPrefix(r.Prefix, fnMain)
+	} else {
+		main.tree.Walk(fnMain)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (r *NodeShiftTreeWalker[T]) resetLocalState() {
+	r.skipPrefixes = nil
+}
+
+func (r *NodeShiftTreeWalker[T]) toT(tree *NodeShiftTree[T], v any) (t T, ok bool, exact DimensionFlag) {
+	if r.NoShift {
+		t = v.(T)
+		ok = true
+	} else {
+		t, ok, exact = tree.shift(v.(T), r.Exact)
+	}
+	return
+}
+
+func (r *NodeShiftTree[T]) Has(s string) bool {
+	_, ok := r.get(s)
+	return ok
+}
+
+func (t NodeShiftTree[T]) clone() *NodeShiftTree[T] {
+	return &t
+}
+
+func (r *NodeShiftTree[T]) shift(t T, exact bool) (T, bool, DimensionFlag) {
+	return r.shifter.Shift(t, r.dims, exact)
+}
+
+func (r *NodeShiftTree[T]) get(s string) (T, bool) {
+	s = cleanKey(s)
+	v, ok := r.tree.Get(s)
+	if !ok {
+		var t T
+		return t, false
+	}
+	t, ok, _ := r.shift(v.(T), true)
+	return t, ok
+}
+
+type WalkConfig[T any] struct {
+	// Optional prefix filter.
+	Prefix string
+
+	// Callback will be called for each node in the tree.
+	// If the callback returns true, the walk will stop.
+	Callback func(ctx *WalkContext[T], s string, t T) (bool, error)
+
+	// Enable read or write locking if needed.
+	LockType LockType
+
+	// When set, no dimension shifting will be performed.
+	NoShift bool
+
+	// Exact will only match exact in the current dimension (e.g. language),
+	// and will not look for alternatives.
+	Exact bool
+}
diff --git a/hugolib/doctree/simpletree.go b/hugolib/doctree/simpletree.go
new file mode 100644
index 000000000..811a7ff80
--- /dev/null
+++ b/hugolib/doctree/simpletree.go
@@ -0,0 +1,91 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+import (
+	"sync"
+
+	radix "github.com/armon/go-radix"
+)
+
+// Tree is a radix tree that holds T.
+type Tree[T any] interface {
+	Get(s string) T
+	LongestPrefix(s string) (string, T)
+	Insert(s string, v T) T
+	WalkPrefix(lockType LockType, s string, f func(s string, v T) (bool, error)) error
+}
+
+// NewSimpleTree creates a new SimpleTree.
+func NewSimpleTree[T any]() *SimpleTree[T] {
+	return &SimpleTree[T]{tree: radix.New()}
+}
+
+// SimpleTree is a thread safe radix tree that holds T.
+type SimpleTree[T any] struct {
+	mu   sync.RWMutex
+	tree *radix.Tree
+	zero T
+}
+
+func (tree *SimpleTree[T]) Get(s string) T {
+	tree.mu.RLock()
+	defer tree.mu.RUnlock()
+
+	if v, ok := tree.tree.Get(s); ok {
+		return v.(T)
+	}
+	return tree.zero
+}
+
+func (tree *SimpleTree[T]) LongestPrefix(s string) (string, T) {
+	tree.mu.RLock()
+	defer tree.mu.RUnlock()
+
+	if s, v, ok := tree.tree.LongestPrefix(s); ok {
+		return s, v.(T)
+	}
+	return "", tree.zero
+}
+
+func (tree *SimpleTree[T]) Insert(s string, v T) T {
+	tree.mu.Lock()
+	defer tree.mu.Unlock()
+
+	tree.tree.Insert(s, v)
+	return v
+}
+
+func (tree *SimpleTree[T]) WalkPrefix(lockType LockType, s string, f func(s string, v T) (bool, error)) error {
+	switch lockType {
+	case LockTypeNone:
+	case LockTypeRead:
+		tree.mu.RLock()
+		defer tree.mu.RUnlock()
+	case LockTypeWrite:
+		tree.mu.Lock()
+		defer tree.mu.Unlock()
+	}
+	var err error
+	tree.tree.WalkPrefix(s, func(s string, v any) bool {
+		var b bool
+		b, err = f(s, v.(T))
+		if err != nil {
+			return true
+		}
+		return b
+	})
+
+	return err
+}
diff --git a/hugolib/doctree/support.go b/hugolib/doctree/support.go
new file mode 100644
index 000000000..8083df127
--- /dev/null
+++ b/hugolib/doctree/support.go
@@ -0,0 +1,251 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+
+	radix "github.com/armon/go-radix"
+)
+
+var _ MutableTrees = MutableTrees{}
+
+const (
+	LockTypeNone LockType = iota
+	LockTypeRead
+	LockTypeWrite
+)
+
+// AddEventListener adds an event listener to the tree.
+// Note that the handler func may not add listeners.
+func (ctx *WalkContext[T]) AddEventListener(event, path string, handler func(*Event[T])) {
+	if ctx.eventHandlers == nil {
+		ctx.eventHandlers = make(eventHandlers[T])
+	}
+	if ctx.eventHandlers[event] == nil {
+		ctx.eventHandlers[event] = make([]func(*Event[T]), 0)
+	}
+
+	// We want to match all above the path, so we need to exclude any similar named siblings.
+	if !strings.HasSuffix(path, "/") {
+		path += "/"
+	}
+
+	ctx.eventHandlers[event] = append(
+		ctx.eventHandlers[event], func(e *Event[T]) {
+			// Propagate events up the tree only.
+			if strings.HasPrefix(e.Path, path) {
+				handler(e)
+			}
+		},
+	)
+}
+
+// AddPostHook adds a post hook to the tree.
+// This will be run after the tree has been walked.
+func (ctx *WalkContext[T]) AddPostHook(handler func() error) {
+	ctx.HooksPost = append(ctx.HooksPost, handler)
+}
+
+func (ctx *WalkContext[T]) Data() *SimpleTree[any] {
+	ctx.dataInit.Do(func() {
+		ctx.data = &SimpleTree[any]{
+			tree: radix.New(),
+		}
+	})
+	return ctx.data
+}
+
+// SendEvent sends an event up the tree.
+func (ctx *WalkContext[T]) SendEvent(event *Event[T]) {
+	ctx.events = append(ctx.events, event)
+}
+
+// StopPropagation stops the propagation of the event.
+func (e *Event[T]) StopPropagation() {
+	e.stopPropagation = true
+}
+
+// ValidateKey returns an error if the key is not valid.
+func ValidateKey(key string) error {
+	if key == "" {
+		// Root node.
+		return nil
+	}
+
+	if len(key) < 2 {
+		return fmt.Errorf("too short key: %q", key)
+	}
+
+	if key[0] != '/' {
+		return fmt.Errorf("key must start with '/': %q", key)
+	}
+
+	if key[len(key)-1] == '/' {
+		return fmt.Errorf("key must not end with '/': %q", key)
+	}
+
+	return nil
+}
+
+// Event is used to communicate events in the tree.
+type Event[T any] struct {
+	Name            string
+	Path            string
+	Source          T
+	stopPropagation bool
+}
+
+type LockType int
+
+// MutableTree is a tree that can be modified.
+type MutableTree interface {
+	Delete(key string)
+	DeleteAll(key string)
+	DeletePrefix(prefix string) int
+	DeletePrefixAll(prefix string) int
+	Lock(writable bool) (commit func())
+	CanLock() bool // Used for troubleshooting only.
+}
+
+// WalkableTree is a tree that can be walked.
+type WalkableTree[T any] interface {
+	WalkPrefixRaw(prefix string, walker func(key string, value T) bool)
+}
+
+var _ WalkableTree[any] = (*WalkableTrees[any])(nil)
+
+type WalkableTrees[T any] []WalkableTree[T]
+
+func (t WalkableTrees[T]) WalkPrefixRaw(prefix string, walker func(key string, value T) bool) {
+	for _, tree := range t {
+		tree.WalkPrefixRaw(prefix, walker)
+	}
+}
+
+var _ MutableTree = MutableTrees(nil)
+
+type MutableTrees []MutableTree
+
+func (t MutableTrees) Delete(key string) {
+	for _, tree := range t {
+		tree.Delete(key)
+	}
+}
+
+func (t MutableTrees) DeleteAll(key string) {
+	for _, tree := range t {
+		tree.DeleteAll(key)
+	}
+}
+
+func (t MutableTrees) DeletePrefix(prefix string) int {
+	var count int
+	for _, tree := range t {
+		count += tree.DeletePrefix(prefix)
+	}
+	return count
+}
+
+func (t MutableTrees) DeletePrefixAll(prefix string) int {
+	var count int
+	for _, tree := range t {
+		count += tree.DeletePrefixAll(prefix)
+	}
+	return count
+}
+
+func (t MutableTrees) Lock(writable bool) (commit func()) {
+	commits := make([]func(), len(t))
+	for i, tree := range t {
+		commits[i] = tree.Lock(writable)
+	}
+	return func() {
+		for _, commit := range commits {
+			commit()
+		}
+	}
+}
+
+func (t MutableTrees) CanLock() bool {
+	for _, tree := range t {
+		if !tree.CanLock() {
+			return false
+		}
+	}
+	return true
+}
+
+// WalkContext is passed to the Walk callback.
+type WalkContext[T any] struct {
+	data          *SimpleTree[any]
+	dataInit      sync.Once
+	eventHandlers eventHandlers[T]
+	events        []*Event[T]
+
+	HooksPost []func() error
+}
+
+type eventHandlers[T any] map[string][]func(*Event[T])
+
+func cleanKey(key string) string {
+	if key == "/" {
+		// The path to the home page is logically "/",
+		// but for technical reasons, it's stored as "".
+		// This allows us to treat the home page as a section,
+		// and a prefix search for "/" will return the home page's descendants.
+		return ""
+	}
+	return key
+}
+
+func (ctx *WalkContext[T]) HandleEvents() error {
+	for len(ctx.events) > 0 {
+		event := ctx.events[0]
+		ctx.events = ctx.events[1:]
+
+		// Loop the event handlers in reverse order so
+		// that events created by the handlers themselves will
+		// be picked up further up the tree.
+		for i := len(ctx.eventHandlers[event.Name]) - 1; i >= 0; i-- {
+			ctx.eventHandlers[event.Name][i](event)
+			if event.stopPropagation {
+				break
+			}
+		}
+	}
+	return nil
+}
+
+func (ctx *WalkContext[T]) HandleEventsAndHooks() error {
+	if err := ctx.HandleEvents(); err != nil {
+		return err
+	}
+
+	for _, hook := range ctx.HooksPost {
+		if err := hook(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func mustValidateKey(key string) string {
+	if err := ValidateKey(key); err != nil {
+		panic(err)
+	}
+	return key
+}
diff --git a/hugolib/doctree/treeshifttree.go b/hugolib/doctree/treeshifttree.go
new file mode 100644
index 000000000..f8a6d360b
--- /dev/null
+++ b/hugolib/doctree/treeshifttree.go
@@ -0,0 +1,101 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree
+
+var _ Tree[string] = (*TreeShiftTree[string])(nil)
+
+type TreeShiftTree[T any] struct {
+	// This tree is shiftable in one dimension.
+	d int
+
+	// The value of the current dimension.
+	v int
+
+	// Will be of length equal to the length of the dimension.
+	trees []*SimpleTree[T]
+}
+
+func NewTreeShiftTree[T any](d, length int) *TreeShiftTree[T] {
+	if length <= 0 {
+		panic("length must be > 0")
+	}
+	trees := make([]*SimpleTree[T], length)
+	for i := 0; i < length; i++ {
+		trees[i] = NewSimpleTree[T]()
+	}
+	return &TreeShiftTree[T]{d: d, trees: trees}
+}
+
+func (t TreeShiftTree[T]) Shape(d, v int) *TreeShiftTree[T] {
+	if d != t.d {
+		panic("dimension mismatch")
+	}
+	if v >= len(t.trees) {
+		panic("value out of range")
+	}
+	t.v = v
+	return &t
+}
+
+func (t *TreeShiftTree[T]) Get(s string) T {
+	return t.trees[t.v].Get(s)
+}
+
+func (t *TreeShiftTree[T]) LongestPrefix(s string) (string, T) {
+	return t.trees[t.v].LongestPrefix(s)
+}
+
+func (t *TreeShiftTree[T]) Insert(s string, v T) T {
+	return t.trees[t.v].Insert(s, v)
+}
+
+func (t *TreeShiftTree[T]) WalkPrefix(lockType LockType, s string, f func(s string, v T) (bool, error)) error {
+	return t.trees[t.v].WalkPrefix(lockType, s, f)
+}
+
+func (t *TreeShiftTree[T]) Delete(key string) {
+	for _, tt := range t.trees {
+		tt.tree.Delete(key)
+	}
+}
+
+func (t *TreeShiftTree[T]) DeletePrefix(prefix string) int {
+	var count int
+	for _, tt := range t.trees {
+		count += tt.tree.DeletePrefix(prefix)
+	}
+	return count
+}
+
+func (t *TreeShiftTree[T]) Lock(writable bool) (commit func()) {
+	if writable {
+		for _, tt := range t.trees {
+			tt.mu.Lock()
+		}
+		return func() {
+			for _, tt := range t.trees {
+				tt.mu.Unlock()
+			}
+		}
+	}
+
+	for _, tt := range t.trees {
+		tt.mu.RLock()
+	}
+	return func() {
+		for _, tt := range t.trees {
+			tt.mu.RUnlock()
+		}
+	}
+}
diff --git a/hugolib/doctree/treeshifttree_test.go b/hugolib/doctree/treeshifttree_test.go
new file mode 100644
index 000000000..c39ff38aa
--- /dev/null
+++ b/hugolib/doctree/treeshifttree_test.go
@@ -0,0 +1,28 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package doctree_test
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/hugolib/doctree"
+)
+
+func TestTreeShiftTree(t *testing.T) {
+	c := qt.New(t)
+
+	tree := doctree.NewTreeShiftTree[string](0, 10)
+	c.Assert(tree, qt.IsNotNil)
+}