Work around --gc failure on Windows <= 10

This applies two related fixes/improvements:

* The --gc now keeps empty `_resources/_gen/images` etc folders, even if empty. This should have been the behaviour
from the start.
* Also, if removal of an empty dir on Windows fails with the "used by another process" error, just ignore it for now.

Fixes #10781

cache/filecache/filecache_pruner.go

@@ -17,6 +17,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime"
+	"strings"
 
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/hugofs"
@@ -66,6 +68,7 @@ func (c *Cache) Prune(force bool) (int, error) {
 
 		if info.IsDir() {
 			f, err := c.Fs.Open(name)
+
 			if err != nil {
 				// This cache dir may not exist.
 				return nil
@@ -74,7 +77,24 @@ func (c *Cache) Prune(force bool) (int, error) {
 			_, err = f.Readdirnames(1)
 			if err == io.EOF {
 				// Empty dir.
-				err = c.Fs.Remove(name)
+				if name == "." {
+					// e.g. /_gen/images -- keep it even if empty.
+					err = nil
+				} else {
+					err = c.Fs.Remove(name)
+					if err != nil {
+						if runtime.GOOS == "windows" {
+							if strings.Contains(err.Error(), "used by another process") {
+								// See https://github.com/gohugoio/hugo/issues/10781
+								// This is a known issue on Windows with Go 1.20.
+								// There's not much we can do about it.
+								// So just return nil.
+								err = nil
+
+							}
+						}
+					}
+				}
 			}
 
 			if err != nil && !herrors.IsNotExist(err) {

cache/filecache/integration_test.go

@@ -0,0 +1,102 @@
+// Copyright 2023 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 filecache_test
+
+import (
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+// See issue #10781. That issue wouldn't have been triggered if we kept
+// the empty root directories (e.g. _resources/gen/images).
+// It's still an upstream Go issue that we also need to handle, but
+// this is a test for the first part.
+func TestPruneShouldPreserveEmptyCacheRoots(t *testing.T) {
+	files := `
+-- hugo.toml --
+baseURL = "https://example.com"
+-- content/_index.md --
+---
+title: "Home"
+---
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{T: t, TxtarString: files, RunGC: true, NeedsOsFS: true},
+	).Build()
+
+	_, err := b.H.BaseFs.ResourcesCache.Stat(filepath.Join("_gen", "images"))
+
+	b.Assert(err, qt.IsNil)
+
+}
+
+func TestPruneImages(t *testing.T) {
+	files := `
+-- hugo.toml --
+baseURL = "https://example.com"
+[caches]
+[caches.images]
+maxAge = "200ms"
+dir = ":resourceDir/_gen"
+-- content/_index.md --
+---
+title: "Home"
+---
+-- assets/a/pixel.png --
+iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==
+-- layouts/index.html --
+{{ $img := resources.GetMatch "**.png" }}
+{{ $img = $img.Resize "3x3" }}
+{{ $img.RelPermalink }}
+
+
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{T: t, TxtarString: files, RunGC: true, NeedsOsFS: true},
+	).Build()
+
+	b.Assert(b.GCCount, qt.Equals, 0)
+
+	imagesCacheDir := filepath.Join("_gen", "images")
+	_, err := b.H.BaseFs.ResourcesCache.Stat(imagesCacheDir)
+
+	b.Assert(err, qt.IsNil)
+
+	// TODO(bep) we need a way to test full rebuilds.
+	// For now, just sleep a little so the cache elements expires.
+	time.Sleep(300 * time.Millisecond)
+
+	b.RenameFile("assets/a/pixel.png", "assets/b/pixel2.png").Build()
+	b.Assert(b.GCCount, qt.Equals, 1)
+	// Build it again to GC the empty a dir.
+	b.Build()
+	if runtime.GOOS != "windows" {
+		// See issue #58860 -- this sometimes fails on Windows,
+		// but the empty directory will be removed on the next run.
+		_, err = b.H.BaseFs.ResourcesCache.Stat(filepath.Join(imagesCacheDir, "a"))
+		b.Assert(err, qt.Not(qt.IsNil))
+	}
+	_, err = b.H.BaseFs.ResourcesCache.Stat(imagesCacheDir)
+	b.Assert(err, qt.IsNil)
+
+}

hugolib/integrationtest_builder.go

@@ -84,6 +84,7 @@ type IntegrationTestBuilder struct {
 	renamedFiles []string
 
 	buildCount int
+	GCCount    int
 	counters   *testCounters
 	logBuff    lockingBuffer
 
@@ -193,6 +194,9 @@ func (s *IntegrationTestBuilder) Build() *IntegrationTestBuilder {
 	if s.Cfg.Verbose || err != nil {
 		fmt.Println(s.logBuff.String())
 	}
+	if s.Cfg.RunGC {
+		s.GCCount, err = s.H.GC()
+	}
 	s.Assert(err, qt.IsNil)
 	return s
 }
@@ -263,6 +267,7 @@ func (s *IntegrationTestBuilder) RenameFile(old, new string) *IntegrationTestBui
 	absNewFilename := s.absFilename(new)
 	s.renamedFiles = append(s.renamedFiles, absOldFilename)
 	s.createdFiles = append(s.createdFiles, absNewFilename)
+	s.Assert(s.fs.Source.MkdirAll(filepath.Dir(absNewFilename), 0777), qt.IsNil)
 	s.Assert(s.fs.Source.Rename(absOldFilename, absNewFilename), qt.IsNil)
 	return s
 }
@@ -488,6 +493,9 @@ type IntegrationTestConfig struct {
 	// Whether it needs the real file system (e.g. for js.Build tests).
 	NeedsOsFS bool
 
+	// Whether to run GC after each build.
+	RunGC bool
+
 	// Do not remove the temp dir after the test.
 	PrintAndKeepTempDir bool