Add a HTTP cache for remote resources.

Fixes #12502
Closes #11891

32 files changed, 1149 insertions, 235 deletions

diff --git a/cache/filecache/filecache.go b/cache/filecache/filecache.go
index 093d2941c..01c466ca6 100644
--- a/cache/filecache/filecache.go
+++ b/cache/filecache/filecache.go
@@ -1,4 +1,4 @@
-// Copyright 2018 The Hugo Authors. All rights reserved.
+// 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.
@@ -23,6 +23,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/gohugoio/httpcache"
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/hugofs"
 
@@ -182,6 +183,15 @@ func (c *Cache) ReadOrCreate(id string,
 	return
 }
 
+// NamedLock locks the given id. The lock is released when the returned function is called.
+func (c *Cache) NamedLock(id string) func() {
+	id = cleanID(id)
+	c.nlocker.Lock(id)
+	return func() {
+		c.nlocker.Unlock(id)
+	}
+}
+
 // GetOrCreate tries to get the file with the given id from cache. If not found or expired, create will
 // be invoked and the result cached.
 // This method is protected by a named lock using the given id as identifier.
@@ -218,7 +228,23 @@ func (c *Cache) GetOrCreate(id string, create func() (io.ReadCloser, error)) (It
 	var buff bytes.Buffer
 	return info,
 		hugio.ToReadCloser(&buff),
-		afero.WriteReader(c.Fs, id, io.TeeReader(r, &buff))
+		c.writeReader(id, io.TeeReader(r, &buff))
+}
+
+func (c *Cache) writeReader(id string, r io.Reader) error {
+	dir := filepath.Dir(id)
+	if dir != "" {
+		_ = c.Fs.MkdirAll(dir, 0o777)
+	}
+	f, err := c.Fs.Create(id)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, _ = io.Copy(f, r)
+
+	return nil
 }
 
 // GetOrCreateBytes is the same as GetOrCreate, but produces a byte slice.
@@ -253,9 +279,10 @@ func (c *Cache) GetOrCreateBytes(id string, create func() ([]byte, error)) (Item
 		return info, b, nil
 	}
 
-	if err := afero.WriteReader(c.Fs, id, bytes.NewReader(b)); err != nil {
+	if err := c.writeReader(id, bytes.NewReader(b)); err != nil {
 		return info, nil, err
 	}
+
 	return info, b, nil
 }
 
@@ -305,16 +332,8 @@ func (c *Cache) getOrRemove(id string) hugio.ReadSeekCloser {
 		return nil
 	}
 
-	if c.maxAge > 0 {
-		fi, err := c.Fs.Stat(id)
-		if err != nil {
-			return nil
-		}
-
-		if c.isExpired(fi.ModTime()) {
-			c.Fs.Remove(id)
-			return nil
-		}
+	if removed, err := c.removeIfExpired(id); err != nil || removed {
+		return nil
 	}
 
 	f, err := c.Fs.Open(id)
@@ -325,6 +344,49 @@ func (c *Cache) getOrRemove(id string) hugio.ReadSeekCloser {
 	return f
 }
 
+func (c *Cache) getBytesAndRemoveIfExpired(id string) ([]byte, bool) {
+	if c.maxAge == 0 {
+		// No caching.
+		return nil, false
+	}
+
+	f, err := c.Fs.Open(id)
+	if err != nil {
+		return nil, false
+	}
+	defer f.Close()
+
+	b, err := io.ReadAll(f)
+	if err != nil {
+		return nil, false
+	}
+
+	removed, err := c.removeIfExpired(id)
+	if err != nil {
+		return nil, false
+	}
+
+	return b, removed
+}
+
+func (c *Cache) removeIfExpired(id string) (bool, error) {
+	if c.maxAge <= 0 {
+		return false, nil
+	}
+
+	fi, err := c.Fs.Stat(id)
+	if err != nil {
+		return false, err
+	}
+
+	if c.isExpired(fi.ModTime()) {
+		c.Fs.Remove(id)
+		return true, nil
+	}
+
+	return false, nil
+}
+
 func (c *Cache) isExpired(modTime time.Time) bool {
 	if c.maxAge < 0 {
 		return false
@@ -398,3 +460,37 @@ func NewCaches(p *helpers.PathSpec) (Caches, error) {
 func cleanID(name string) string {
 	return strings.TrimPrefix(filepath.Clean(name), helpers.FilePathSeparator)
 }
+
+// AsHTTPCache returns an httpcache.Cache implementation for this file cache.
+// Note that none of the methods are protected by named locks, so you need to make sure
+// to do that in your own code.
+func (c *Cache) AsHTTPCache() httpcache.Cache {
+	return &httpCache{c: c}
+}
+
+type httpCache struct {
+	c *Cache
+}
+
+func (h *httpCache) Get(id string) (resp []byte, ok bool) {
+	id = cleanID(id)
+	b, removed := h.c.getBytesAndRemoveIfExpired(id)
+
+	return b, !removed
+}
+
+func (h *httpCache) Set(id string, resp []byte) {
+	if h.c.maxAge == 0 {
+		return
+	}
+
+	id = cleanID(id)
+
+	if err := h.c.writeReader(id, bytes.NewReader(resp)); err != nil {
+		panic(err)
+	}
+}
+
+func (h *httpCache) Delete(key string) {
+	h.c.Fs.Remove(key)
+}
diff --git a/cache/httpcache/httpcache.go b/cache/httpcache/httpcache.go
new file mode 100644
index 000000000..ff360001f
--- /dev/null
+++ b/cache/httpcache/httpcache.go
@@ -0,0 +1,208 @@
+// 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 httpcache
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/gobwas/glob"
+	"github.com/gohugoio/hugo/common/predicate"
+	"github.com/gohugoio/hugo/config"
+	"github.com/mitchellh/mapstructure"
+)
+
+// DefaultConfig holds the default configuration for the HTTP cache.
+var DefaultConfig = Config{
+	Cache: Cache{
+		For: GlobMatcher{
+			Excludes: []string{"**"},
+		},
+	},
+	Polls: []PollConfig{
+		{
+			For: GlobMatcher{
+				Includes: []string{"**"},
+			},
+			Disable: true,
+		},
+	},
+}
+
+// Config holds the configuration for the HTTP cache.
+type Config struct {
+	// Configures the HTTP cache behaviour (RFC 9111).
+	// When this is not enabled for a resource, Hugo will go straight to the file cache.
+	Cache Cache
+
+	// Polls holds a list of configurations for polling remote resources to detect changes in watch mode.
+	// This can be disabled for some resources, typically if they are known to not change.
+	Polls []PollConfig
+}
+
+type Cache struct {
+	// Enable HTTP cache behaviour (RFC 9111) for these rsources.
+	For GlobMatcher
+}
+
+func (c *Config) Compile() (ConfigCompiled, error) {
+	var cc ConfigCompiled
+
+	p, err := c.Cache.For.CompilePredicate()
+	if err != nil {
+		return cc, err
+	}
+
+	cc.For = p
+
+	for _, pc := range c.Polls {
+
+		p, err := pc.For.CompilePredicate()
+		if err != nil {
+			return cc, err
+		}
+
+		cc.PollConfigs = append(cc.PollConfigs, PollConfigCompiled{
+			For:    p,
+			Config: pc,
+		})
+	}
+
+	return cc, nil
+}
+
+// PollConfig holds the configuration for polling remote resources to detect changes in watch mode.
+// TODO1 make sure this enabled only in watch mode.
+type PollConfig struct {
+	// What remote resources to apply this configuration to.
+	For GlobMatcher
+
+	// Disable polling for this configuration.
+	Disable bool
+
+	// Low is the lower bound for the polling interval.
+	// This is the starting point when the resource has recently changed,
+	// if that resource stops changing, the polling interval will gradually increase towards High.
+	Low time.Duration
+
+	// High is the upper bound for the polling interval.
+	// This is the interval used when the resource is stable.
+	High time.Duration
+}
+
+func (c PollConfig) MarshalJSON() (b []byte, err error) {
+	// Marshal the durations as strings.
+	type Alias PollConfig
+	return json.Marshal(&struct {
+		Low  string
+		High string
+		Alias
+	}{
+		Low:   c.Low.String(),
+		High:  c.High.String(),
+		Alias: (Alias)(c),
+	})
+}
+
+type GlobMatcher struct {
+	// Excludes holds a list of glob patterns that will be excluded.
+	Excludes []string
+
+	// Includes holds a list of glob patterns that will be included.
+	Includes []string
+}
+
+type ConfigCompiled struct {
+	For         predicate.P[string]
+	PollConfigs []PollConfigCompiled
+}
+
+func (c *ConfigCompiled) PollConfigFor(s string) PollConfigCompiled {
+	for _, pc := range c.PollConfigs {
+		if pc.For(s) {
+			return pc
+		}
+	}
+	return PollConfigCompiled{}
+}
+
+func (c *ConfigCompiled) IsPollingDisabled() bool {
+	for _, pc := range c.PollConfigs {
+		if !pc.Config.Disable {
+			return false
+		}
+	}
+	return true
+}
+
+type PollConfigCompiled struct {
+	For    predicate.P[string]
+	Config PollConfig
+}
+
+func (p PollConfigCompiled) IsZero() bool {
+	return p.For == nil
+}
+
+func (gm *GlobMatcher) CompilePredicate() (func(string) bool, error) {
+	var p predicate.P[string]
+	for _, include := range gm.Includes {
+		g, err := glob.Compile(include, '/')
+		if err != nil {
+			return nil, err
+		}
+		fn := func(s string) bool {
+			return g.Match(s)
+		}
+		p = p.Or(fn)
+	}
+
+	for _, exclude := range gm.Excludes {
+		g, err := glob.Compile(exclude, '/')
+		if err != nil {
+			return nil, err
+		}
+		fn := func(s string) bool {
+			return !g.Match(s)
+		}
+		p = p.And(fn)
+	}
+
+	return p, nil
+}
+
+func DecodeConfig(bcfg config.BaseConfig, m map[string]any) (Config, error) {
+	if len(m) == 0 {
+		return DefaultConfig, nil
+	}
+
+	var c Config
+
+	dc := &mapstructure.DecoderConfig{
+		Result:           &c,
+		DecodeHook:       mapstructure.StringToTimeDurationHookFunc(),
+		WeaklyTypedInput: true,
+	}
+
+	decoder, err := mapstructure.NewDecoder(dc)
+	if err != nil {
+		return c, err
+	}
+
+	if err := decoder.Decode(m); err != nil {
+		return c, err
+	}
+
+	return c, nil
+}
diff --git a/cache/httpcache/httpcache_integration_test.go b/cache/httpcache/httpcache_integration_test.go
new file mode 100644
index 000000000..d3337c023
--- /dev/null
+++ b/cache/httpcache/httpcache_integration_test.go
@@ -0,0 +1,64 @@
+// 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 httpcache_test
+
+import (
+	"testing"
+	"time"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestConfigCustom(t *testing.T) {
+	files := `
+-- hugo.toml --
+[httpcache]
+[httpcache.cache.for]
+includes = ["**gohugo.io**"]
+[[httpcache.polls]]
+low = "5s"
+high = "32s"
+[httpcache.polls.for]
+includes = ["**gohugo.io**"]
+		
+	
+`
+
+	b := hugolib.Test(t, files)
+
+	httpcacheConf := b.H.Configs.Base.HTTPCache
+	compiled := b.H.Configs.Base.C.HTTPCache
+
+	b.Assert(httpcacheConf.Cache.For.Includes, qt.DeepEquals, []string{"**gohugo.io**"})
+	b.Assert(httpcacheConf.Cache.For.Excludes, qt.IsNil)
+
+	pc := compiled.PollConfigFor("https://gohugo.io/foo.jpg")
+	b.Assert(pc.Config.Low, qt.Equals, 5*time.Second)
+	b.Assert(pc.Config.High, qt.Equals, 32*time.Second)
+	b.Assert(compiled.PollConfigFor("https://example.com/foo.jpg").IsZero(), qt.IsTrue)
+}
+
+func TestConfigDefault(t *testing.T) {
+	files := `
+-- hugo.toml --
+`
+	b := hugolib.Test(t, files)
+
+	compiled := b.H.Configs.Base.C.HTTPCache
+
+	b.Assert(compiled.For("https://gohugo.io/posts.json"), qt.IsFalse)
+	b.Assert(compiled.For("https://gohugo.io/foo.jpg"), qt.IsFalse)
+	b.Assert(compiled.PollConfigFor("https://gohugo.io/foo.jpg").Config.Disable, qt.IsTrue)
+}
diff --git a/cache/httpcache/httpcache_test.go b/cache/httpcache/httpcache_test.go
new file mode 100644
index 000000000..e3659f97b
--- /dev/null
+++ b/cache/httpcache/httpcache_test.go
@@ -0,0 +1,42 @@
+// 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 httpcache
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestGlobMatcher(t *testing.T) {
+	c := qt.New(t)
+
+	g := GlobMatcher{
+		Includes: []string{"**/*.jpg", "**.png", "**/bar/**"},
+		Excludes: []string{"**/foo.jpg", "**.css"},
+	}
+
+	p, err := g.CompilePredicate()
+	c.Assert(err, qt.IsNil)
+
+	c.Assert(p("foo.jpg"), qt.IsFalse)
+	c.Assert(p("foo.png"), qt.IsTrue)
+	c.Assert(p("foo/bar.jpg"), qt.IsTrue)
+	c.Assert(p("foo/bar.png"), qt.IsTrue)
+	c.Assert(p("foo/bar/foo.jpg"), qt.IsFalse)
+	c.Assert(p("foo/bar/foo.css"), qt.IsFalse)
+	c.Assert(p("foo.css"), qt.IsFalse)
+	c.Assert(p("foo/bar/foo.css"), qt.IsFalse)
+	c.Assert(p("foo/bar/foo.xml"), qt.IsTrue)
+}
diff --git a/commands/commandeer.go b/commands/commandeer.go
index 59fe32f74..f18a95bb9 100644
--- a/commands/commandeer.go
+++ b/commands/commandeer.go
@@ -48,6 +48,7 @@ import (
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/hugolib"
+	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/resources/kinds"
 	"github.com/spf13/afero"
 	"github.com/spf13/cobra"
@@ -103,6 +104,9 @@ type rootCommand struct {
 	commonConfigs *lazycache.Cache[int32, *commonConfig]
 	hugoSites     *lazycache.Cache[int32, *hugolib.HugoSites]
 
+	// changesFromBuild received from Hugo in watch mode.
+	changesFromBuild chan []identity.Identity
+
 	commands []simplecobra.Commander
 
 	// Flags
@@ -304,7 +308,7 @@ func (r *rootCommand) ConfigFromProvider(key int32, cfg config.Provider) (*commo
 
 func (r *rootCommand) HugFromConfig(conf *commonConfig) (*hugolib.HugoSites, error) {
 	h, _, err := r.hugoSites.GetOrCreate(r.configVersionID.Load(), func(key int32) (*hugolib.HugoSites, error) {
-		depsCfg := deps.DepsCfg{Configs: conf.configs, Fs: conf.fs, LogOut: r.logger.Out(), LogLevel: r.logger.Level()}
+		depsCfg := r.newDepsConfig(conf)
 		return hugolib.NewHugoSites(depsCfg)
 	})
 	return h, err
@@ -316,12 +320,16 @@ func (r *rootCommand) Hugo(cfg config.Provider) (*hugolib.HugoSites, error) {
 		if err != nil {
 			return nil, err
 		}
-		depsCfg := deps.DepsCfg{Configs: conf.configs, Fs: conf.fs, LogOut: r.logger.Out(), LogLevel: r.logger.Level()}
+		depsCfg := r.newDepsConfig(conf)
 		return hugolib.NewHugoSites(depsCfg)
 	})
 	return h, err
 }
 
+func (r *rootCommand) newDepsConfig(conf *commonConfig) deps.DepsCfg {
+	return deps.DepsCfg{Configs: conf.configs, Fs: conf.fs, LogOut: r.logger.Out(), LogLevel: r.logger.Level(), ChangesFromBuild: r.changesFromBuild}
+}
+
 func (r *rootCommand) Name() string {
 	return "hugo"
 }
@@ -408,6 +416,8 @@ func (r *rootCommand) PreRun(cd, runner *simplecobra.Commandeer) error {
 		return err
 	}
 
+	r.changesFromBuild = make(chan []identity.Identity, 10)
+
 	r.commonConfigs = lazycache.New(lazycache.Options[int32, *commonConfig]{MaxEntries: 5})
 	// We don't want to keep stale HugoSites in memory longer than needed.
 	r.hugoSites = lazycache.New(lazycache.Options[int32, *hugolib.HugoSites]{
diff --git a/commands/hugobuilder.go b/commands/hugobuilder.go
index 32b7e1de8..99bd8a04a 100644
--- a/commands/hugobuilder.go
+++ b/commands/hugobuilder.go
@@ -43,6 +43,7 @@ import (
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/hugolib"
 	"github.com/gohugoio/hugo/hugolib/filesystems"
+	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/livereload"
 	"github.com/gohugoio/hugo/resources/page"
 	"github.com/gohugoio/hugo/watcher"
@@ -343,6 +344,24 @@ func (c *hugoBuilder) newWatcher(pollIntervalStr string, dirList ...string) (*wa
 	go func() {
 		for {
 			select {
+			case changes := <-c.r.changesFromBuild:
+				unlock, err := h.LockBuild()
+				if err != nil {
+					c.r.logger.Errorln("Failed to acquire a build lock: %s", err)
+					return
+				}
+				c.changeDetector.PrepareNew()
+				err = c.rebuildSitesForChanges(changes)
+				if err != nil {
+					c.r.logger.Errorln("Error while watching:", err)
+				}
+				if c.s != nil && c.s.doLiveReload {
+					if c.changeDetector == nil || len(c.changeDetector.changed()) > 0 {
+						livereload.ForceRefresh()
+					}
+				}
+				unlock()
+
 			case evs := <-watcher.Events:
 				unlock, err := h.LockBuild()
 				if err != nil {
@@ -1019,6 +1038,19 @@ func (c *hugoBuilder) rebuildSites(events []fsnotify.Event) error {
 	return h.Build(hugolib.BuildCfg{NoBuildLock: true, RecentlyVisited: c.visitedURLs, ErrRecovery: c.errState.wasErr()}, events...)
 }
 
+func (c *hugoBuilder) rebuildSitesForChanges(ids []identity.Identity) error {
+	c.errState.setBuildErr(nil)
+	h, err := c.hugo()
+	if err != nil {
+		return err
+	}
+	whatChanged := &hugolib.WhatChanged{}
+	whatChanged.Add(ids...)
+	err = h.Build(hugolib.BuildCfg{NoBuildLock: true, WhatChanged: whatChanged, RecentlyVisited: c.visitedURLs, ErrRecovery: c.errState.wasErr()})
+	c.errState.setBuildErr(err)
+	return err
+}
+
 func (c *hugoBuilder) reloadConfig() error {
 	c.r.Reset()
 	c.r.configVersionID.Add(1)
diff --git a/common/maps/maps.go b/common/maps/maps.go
index 2686baad6..f9171ebf2 100644
--- a/common/maps/maps.go
+++ b/common/maps/maps.go
@@ -112,17 +112,17 @@ func ToSliceStringMap(in any) ([]map[string]any, error) {
 }
 
 // LookupEqualFold finds key in m with case insensitive equality checks.
-func LookupEqualFold[T any | string](m map[string]T, key string) (T, bool) {
+func LookupEqualFold[T any | string](m map[string]T, key string) (T, string, bool) {
 	if v, found := m[key]; found {
-		return v, true
+		return v, key, true
 	}
 	for k, v := range m {
 		if strings.EqualFold(k, key) {
-			return v, true
+			return v, k, true
 		}
 	}
 	var s T
-	return s, false
+	return s, "", false
 }
 
 // MergeShallow merges src into dst, but only if the key does not already exist in dst.
diff --git a/common/maps/maps_test.go b/common/maps/maps_test.go
index 098098388..b4f9c5a3d 100644
--- a/common/maps/maps_test.go
+++ b/common/maps/maps_test.go
@@ -180,16 +180,18 @@ func TestLookupEqualFold(t *testing.T) {
 		"B": "bv",
 	}
 
-	v, found := LookupEqualFold(m1, "b")
+	v, k, found := LookupEqualFold(m1, "b")
 	c.Assert(found, qt.IsTrue)
 	c.Assert(v, qt.Equals, "bv")
+	c.Assert(k, qt.Equals, "B")
 
 	m2 := map[string]string{
 		"a": "av",
 		"B": "bv",
 	}
 
-	v, found = LookupEqualFold(m2, "b")
+	v, k, found = LookupEqualFold(m2, "b")
 	c.Assert(found, qt.IsTrue)
+	c.Assert(k, qt.Equals, "B")
 	c.Assert(v, qt.Equals, "bv")
 }
diff --git a/common/predicate/predicate.go b/common/predicate/predicate.go
index f9cb1bb2b..f71536474 100644
--- a/common/predicate/predicate.go
+++ b/common/predicate/predicate.go
@@ -24,6 +24,9 @@ func (p P[T]) And(ps ...P[T]) P[T] {
 				return false
 			}
 		}
+		if p == nil {
+			return true
+		}
 		return p(v)
 	}
 }
@@ -36,6 +39,9 @@ func (p P[T]) Or(ps ...P[T]) P[T] {
 				return true
 			}
 		}
+		if p == nil {
+			return false
+		}
 		return p(v)
 	}
 }
diff --git a/common/tasks/tasks.go b/common/tasks/tasks.go
new file mode 100644
index 000000000..1f7e061f9
--- /dev/null
+++ b/common/tasks/tasks.go
@@ -0,0 +1,153 @@
+// 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