Add a HTTP cache for remote resources.

Fixes #12502
Closes #11891

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)