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

1 files changed, 312 insertions, 134 deletions

diff --git a/hugofs/rootmapping_fs.go b/hugofs/rootmapping_fs.go
index a37e21a8b..1efb8ee5f 100644
--- a/hugofs/rootmapping_fs.go
+++ b/hugofs/rootmapping_fs.go
@@ -14,13 +14,20 @@
 package hugofs
 
 import (
+	"errors"
 	"fmt"
+	iofs "io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 
 	"github.com/gohugoio/hugo/common/herrors"
+	"github.com/gohugoio/hugo/common/paths"
+
+	"github.com/bep/overlayfs"
 	"github.com/gohugoio/hugo/hugofs/files"
+	"github.com/gohugoio/hugo/hugofs/glob"
 
 	radix "github.com/armon/go-radix"
 	"github.com/spf13/afero"
@@ -28,17 +35,31 @@ import (
 
 var filepathSeparator = string(filepath.Separator)
 
+var _ ReverseLookupProvder = (*RootMappingFs)(nil)
+
 // NewRootMappingFs creates a new RootMappingFs on top of the provided with
 // root mappings with some optional metadata about the root.
 // Note that From represents a virtual root that maps to the actual filename in To.
 func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 	rootMapToReal := radix.New()
+	realMapToRoot := radix.New()
 	var virtualRoots []RootMapping
 
+	addMapping := func(key string, rm RootMapping, to *radix.Tree) {
+		var mappings []RootMapping
+		v, found := to.Get(key)
+		if found {
+			// There may be more than one language pointing to the same root.
+			mappings = v.([]RootMapping)
+		}
+		mappings = append(mappings, rm)
+		to.Insert(key, mappings)
+	}
+
 	for _, rm := range rms {
 		(&rm).clean()
 
-		fromBase := files.ResolveComponentFolder(rm.From)
+		rm.FromBase = files.ResolveComponentFolder(rm.From)
 
 		if len(rm.To) < 2 {
 			panic(fmt.Sprintf("invalid root mapping; from/to: %s/%s", rm.From, rm.To))
@@ -46,21 +67,80 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 
 		fi, err := fs.Stat(rm.To)
 		if err != nil {
-			if herrors.IsNotExist(err) {
+			if os.IsNotExist(err) {
 				continue
 			}
 			return nil, err
 		}
-		// Extract "blog" from "content/blog"
-		rm.path = strings.TrimPrefix(strings.TrimPrefix(rm.From, fromBase), filepathSeparator)
+
 		if rm.Meta == nil {
 			rm.Meta = NewFileMeta()
 		}
 
-		rm.Meta.SourceRoot = rm.To
-		rm.Meta.BaseDir = rm.ToBasedir
-		rm.Meta.MountRoot = rm.path
+		if !fi.IsDir() {
+			// We do allow single file mounts.
+			// However, the file system logic will be much simpler with just directories.
+			// So, convert this mount into a directory mount with a nameTo filter and renamer.
+			dirFrom, nameFrom := filepath.Split(rm.From)
+			dirTo, nameTo := filepath.Split(rm.To)
+			dirFrom, dirTo = strings.TrimSuffix(dirFrom, filepathSeparator), strings.TrimSuffix(dirTo, filepathSeparator)
+			rm.From = dirFrom
+
+			fi, err = fs.Stat(rm.To)
+			if err != nil {
+				if herrors.IsNotExist(err) {
+					continue
+				}
+				return nil, err
+			}
+
+			rm.fiSingleFile = NewFileMetaInfo(fi, rm.Meta.Copy())
+			rm.To = dirTo
+
+			rm.Meta.Rename = func(name string, toFrom bool) string {
+				if toFrom {
+					if name == nameTo {
+						return nameFrom
+					}
+					return name
+				}
+
+				if name == nameFrom {
+					return nameTo
+				}
+
+				return name
+			}
+			nameToFilename := filepathSeparator + nameTo
+
+			rm.Meta.InclusionFilter = rm.Meta.InclusionFilter.Append(glob.NewFilenameFilterForInclusionFunc(
+				func(filename string) bool {
+					return strings.HasPrefix(nameToFilename, filename)
+				},
+			))
+
+			// Refresh the FileInfo object.
+			fi, err = fs.Stat(rm.To)
+			if err != nil {
+				if herrors.IsNotExist(err) {
+					continue
+				}
+				return nil, err
+			}
+		}
+
+		if rm.FromBase == "" {
+			panic(" rm.FromBase is empty")
+		}
+
+		// Extract "blog" from "content/blog"
+		rm.path = strings.TrimPrefix(strings.TrimPrefix(rm.From, rm.FromBase), filepathSeparator)
+
+		rm.Meta.SourceRoot = fi.(MetaProvider).Meta().Filename
+		rm.Meta.BaseDir = rm.ToBase
 		rm.Meta.Module = rm.Module
+		rm.Meta.ModuleOrdinal = rm.ModuleOrdinal
+		rm.Meta.Component = rm.FromBase
 		rm.Meta.IsProject = rm.IsProject
 
 		meta := rm.Meta.Copy()
@@ -72,15 +152,13 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 
 		rm.fi = NewFileMetaInfo(fi, meta)
 
-		key := filepathSeparator + rm.From
-		var mappings []RootMapping
-		v, found := rootMapToReal.Get(key)
-		if found {
-			// There may be more than one language pointing to the same root.
-			mappings = v.([]RootMapping)
+		addMapping(filepathSeparator+rm.From, rm, rootMapToReal)
+		rev := rm.To
+		if !strings.HasPrefix(rev, filepathSeparator) {
+			rev = filepathSeparator + rev
 		}
-		mappings = append(mappings, rm)
-		rootMapToReal.Insert(key, mappings)
+
+		addMapping(rev, rm, realMapToRoot)
 
 		virtualRoots = append(virtualRoots, rm)
 	}
@@ -90,6 +168,7 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 	rfs := &RootMappingFs{
 		Fs:            fs,
 		rootMapToReal: rootMapToReal,
+		realMapToRoot: realMapToRoot,
 	}
 
 	return rfs, nil
@@ -103,9 +182,9 @@ func newRootMappingFsFromFromTo(
 	rms := make([]RootMapping, len(fromTo)/2)
 	for i, j := 0, 0; j < len(fromTo); i, j = i+1, j+2 {
 		rms[i] = RootMapping{
-			From:      fromTo[j],
-			To:        fromTo[j+1],
-			ToBasedir: baseDir,
+			From:   fromTo[j],
+			To:     fromTo[j+1],
+			ToBase: baseDir,
 		}
 	}
 
@@ -114,16 +193,18 @@ func newRootMappingFsFromFromTo(
 
 // RootMapping describes a virtual file or directory mount.
 type RootMapping struct {
-	From      string    // The virtual mount.
-	To        string    // The source directory or file.
-	ToBasedir string    // The base of To. May be empty if an absolute path was provided.
-	Module    string    // The module path/ID.
-	IsProject bool      // Whether this is a mount in the main project.
-	Meta      *FileMeta // File metadata (lang etc.)
-
-	fi   FileMetaInfo
-	path string // The virtual mount point, e.g. "blog".
+	From          string    // The virtual mount.
+	FromBase      string    // The base directory of the virtual mount.
+	To            string    // The source directory or file.
+	ToBase        string    // The base of To. May be empty if an absolute path was provided.
+	Module        string    // The module path/ID.
+	ModuleOrdinal int       // The module ordinal starting with 0 which is the project.
+	IsProject     bool      // Whether this is a mount in the main project.
+	Meta          *FileMeta // File metadata (lang etc.)
 
+	fi           FileMetaInfo
+	fiSingleFile FileMetaInfo // Also set when this mounts represents a single file with a rename func.
+	path         string       // The virtual mount point, e.g. "blog".
 }
 
 type keyRootMappings struct {
@@ -150,9 +231,7 @@ func (r RootMapping) trimFrom(name string) string {
 	return strings.TrimPrefix(name, r.From)
 }
 
-var (
-	_ FilesystemUnwrapper = (*RootMappingFs)(nil)
-)
+var _ FilesystemUnwrapper = (*RootMappingFs)(nil)
 
 // A RootMappingFs maps several roots into one. Note that the root of this filesystem
 // is directories only, and they will be returned in Readdir and Readdirnames
@@ -160,9 +239,10 @@ var (
 type RootMappingFs struct {
 	afero.Fs
 	rootMapToReal *radix.Tree
+	realMapToRoot *radix.Tree
 }
 
-func (fs *RootMappingFs) Dirs(base string) ([]FileMetaInfo, error) {
+func (fs *RootMappingFs) Mounts(base string) ([]FileMetaInfo, error) {
 	base = filepathSeparator + fs.cleanName(base)
 	roots := fs.getRootsWithPrefix(base)
 
@@ -172,17 +252,14 @@ func (fs *RootMappingFs) Dirs(base string) ([]FileMetaInfo, error) {
 
 	fss := make([]FileMetaInfo, len(roots))
 	for i, r := range roots {
-		bfs := afero.NewBasePathFs(fs.Fs, r.To)
-		bfs = decoratePath(bfs, func(name string) string {
-			p := strings.TrimPrefix(name, r.To)
-			if r.path != "" {
-				// Make sure it's mounted to a any sub path, e.g. blog
-				p = filepath.Join(r.path, p)
-			}
-			p = strings.TrimLeft(p, filepathSeparator)
-			return p
-		})
 
+		if r.fiSingleFile != nil {
+			// A single file mount.
+			fss[i] = r.fiSingleFile
+			continue
+		}
+
+		bfs := NewBasePathFs(fs.Fs, r.To)
 		fs := bfs
 		if r.Meta.InclusionFilter != nil {
 			fs = newFilenameFilterFs(fs, r.To, r.Meta.InclusionFilter)
@@ -229,18 +306,9 @@ func (fs RootMappingFs) Filter(f func(m RootMapping) bool) *RootMappingFs {
 	return &fs
 }
 
-// LstatIfPossible returns the os.FileInfo structure describing a given file.
-func (fs *RootMappingFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
-	fis, err := fs.doLstat(name)
-	if err != nil {
-		return nil, false, err
-	}
-	return fis[0], false, nil
-}
-
 // Open opens the named file for reading.
 func (fs *RootMappingFs) Open(name string) (afero.File, error) {
-	fis, err := fs.doLstat(name)
+	fis, err := fs.doStat(name)
 	if err != nil {
 		return nil, err
 	}
@@ -251,8 +319,68 @@ func (fs *RootMappingFs) Open(name string) (afero.File, error) {
 // Stat returns the os.FileInfo structure describing a given file.  If there is
 // an error, it will be of type *os.PathError.
 func (fs *RootMappingFs) Stat(name string) (os.FileInfo, error) {
-	fi, _, err := fs.LstatIfPossible(name)
-	return fi, err
+	fis, err := fs.doStat(name)
+	if err != nil {
+		return nil, err
+	}
+
+	return fis[0], nil
+}
+
+type ComponentPath struct {
+	Component string
+	Path      string
+	Lang      string
+}
+
+func (c ComponentPath) ComponentPathJoined() string {
+	return path.Join(c.Component, c.Path)
+}
+
+type ReverseLookupProvder interface {
+	ReverseLookup(filename string, checkExists bool) ([]ComponentPath, error)
+}
+
+// func (fs *RootMappingFs) ReverseStat(filename string) ([]FileMetaInfo, error)
+func (fs *RootMappingFs) ReverseLookup(in string, checkExists bool) ([]ComponentPath, error) {
+	in = fs.cleanName(in)
+	key := filepathSeparator + in
+
+	s, roots := fs.getRootsReverse(key)
+
+	if len(roots) == 0 {
+		return nil, nil
+	}
+
+	var cps []ComponentPath
+
+	base := strings.TrimPrefix(key, s)
+	dir, name := filepath.Split(base)
+
+	for _, first := range roots {
+		if first.Meta.Rename != nil {
+			name = first.Meta.Rename(name, true)
+		}
+
+		// Now we know that this file _could_ be in this fs.
+		filename := filepathSeparator + filepath.Join(first.path, dir, name)
+
+		if checkExists {
+			// Confirm that it exists.
+			_, err := fs.Stat(first.FromBase + filename)
+			if err != nil {
+				continue
+			}
+		}
+
+		cps = append(cps, ComponentPath{
+			Component: first.FromBase,
+			Path:      paths.ToSlashTrimLeading(filename),
+			Lang:      first.Meta.Lang,
+		})
+	}
+
+	return cps, nil
 }
 
 func (fs *RootMappingFs) hasPrefix(prefix string) bool {
@@ -275,21 +403,22 @@ func (fs *RootMappingFs) getRoot(key string) []RootMapping {
 }
 
 func (fs *RootMappingFs) getRoots(key string) (string, []RootMapping) {
-	s, v, found := fs.rootMapToReal.LongestPrefix(key)
-	if !found || (s == filepathSeparator && key != filepathSeparator) {
+	return fs.getRootsIn(key, fs.rootMapToReal)
+}
+
+func (fs *RootMappingFs) getRootsReverse(key string) (string, []RootMapping) {
+	return fs.getRootsIn(key, fs.realMapToRoot)
+}
+
+func (fs *RootMappingFs) getRootsIn(key string, tree *radix.Tree) (string, []RootMapping) {
+	s, v, found := tree.LongestPrefix(key)
+
+	if !found {
 		return "", nil
 	}
 	return s, v.([]RootMapping)
 }
 
-func (fs *RootMappingFs) debug() {
-	fmt.Println("debug():")
-	fs.rootMapToReal.Walk(func(s string, v any) bool {
-		fmt.Println("Key", s)
-		return false
-	})
-}
-
 func (fs *RootMappingFs) getRootsWithPrefix(prefix string) []RootMapping {
 	var roots []RootMapping
 	fs.rootMapToReal.WalkPrefix(prefix, func(b string, v any) bool {
@@ -316,61 +445,63 @@ func (fs *RootMappingFs) getAncestors(prefix string) []keyRootMappings {
 }
 
 func (fs *RootMappingFs) newUnionFile(fis ...FileMetaInfo) (afero.File, error) {
-	meta := fis[0].Meta()
-	f, err := meta.Open()
-	if err != nil {
-		return nil, err
-	}
 	if len(fis) == 1 {
-		return f, nil
+		return fis[0].Meta().Open()
 	}
 
-	rf := &rootMappingFile{File: f, fs: fs, name: meta.Name, meta: meta}
-	if len(fis) == 1 {
-		return rf, err
-	}
-
-	next, err := fs.newUnionFile(fis[1:]...)
-	if err != nil {
-		return nil, err
+	openers := make([]func() (afero.File, error), len(fis))
+	for i := len(fis) - 1; i >= 0; i-- {
+		fi := fis[i]
+		openers[i] = func() (afero.File, error) {
+			meta := fi.Meta()
+			f, err := meta.Open()
+			if err != nil {
+				return nil, err
+			}
+			return &rootMappingDir{DirOnlyOps: f, fs: fs, name: meta.Name, meta: meta}, nil
+		}
 	}
 
-	uf := &afero.UnionFile{Base: rf, Layer: next}
-
-	uf.Merger = func(lofi, bofi []os.FileInfo) ([]os.FileInfo, error) {
+	merge := func(lofi, bofi []iofs.DirEntry) []iofs.DirEntry {
 		// Ignore duplicate directory entries
-		seen := make(map[string]bool)
-		var result []os.FileInfo
-
-		for _, fis := range [][]os.FileInfo{bofi, lofi} {
-			for _, fi := range fis {
-
-				if fi.IsDir() && seen[fi.Name()] {
+		for _, fi1 := range bofi {
+			var found bool
+			for _, fi2 := range lofi {
+				if !fi2.IsDir() {
 					continue
 				}
-
-				if fi.IsDir() {
-					seen[fi.Name()] = true
+				if fi1.Name() == fi2.Name() {
+					found = true
+					break
 				}
-
-				result = append(result, fi)
+			}
+			if !found {
+				lofi = append(lofi, fi1)
 			}
 		}
 
-		return result, nil
+		return lofi
 	}
 
-	return uf, nil
+	info := func() (os.FileInfo, error) {
+		return fis[0], nil
+	}
+
+	return overlayfs.OpenDir(merge, info, openers...)
 }
 
 func (fs *RootMappingFs) cleanName(name string) string {
-	return strings.Trim(filepath.Clean(name), filepathSeparator)
+	name = strings.Trim(filepath.Clean(name), filepathSeparator)
+	if name == "." {
+		name = ""
+	}
+	return name
 }
 
-func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error) {
-	prefix = filepathSeparator + fs.cleanName(prefix)
+func (rfs *RootMappingFs) collectDirEntries(prefix string) ([]iofs.DirEntry, error) {
+	prefix = filepathSeparator + rfs.cleanName(prefix)
 
-	var fis []os.FileInfo
+	var fis []iofs.DirEntry
 
 	seen := make(map[string]bool) // Prevent duplicate directories
 	level := strings.Count(prefix, filepathSeparator)
@@ -380,15 +511,17 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 		if err != nil {
 			return err
 		}
-		direntries, err := f.Readdir(-1)
+		direntries, err := f.(iofs.ReadDirFile).ReadDir(-1)
 		if err != nil {
 			f.Close()
 			return err
 		}
 
 		for _, fi := range direntries {
+
 			meta := fi.(FileMetaInfo).Meta()
 			meta.Merge(rm.Meta)
+
 			if !rm.Meta.InclusionFilter.Match(strings.TrimPrefix(meta.Filename, meta.SourceRoot), fi.IsDir()) {
 				continue
 			}
@@ -400,11 +533,14 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 				}
 				seen[name] = true
 				opener := func() (afero.File, error) {
-					return fs.Open(filepath.Join(rm.From, name))
+					return rfs.Open(filepath.Join(rm.From, name))
 				}
 				fi = newDirNameOnlyFileInfo(name, meta, opener)
+			} else if rm.Meta.Rename != nil {
+				if n := rm.Meta.Rename(fi.Name(), true); n != fi.Name() {
+					fi.(MetaProvider).Meta().Name = n
+				}
 			}
-
 			fis = append(fis, fi)
 		}
 
@@ -414,7 +550,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 	}
 
 	// First add any real files/directories.
-	rms := fs.getRoot(prefix)
+	rms := rfs.getRoot(prefix)
 	for _, rm := range rms {
 		if err := collectDir(rm, rm.fi); err != nil {
 			return nil, err
@@ -423,7 +559,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 
 	// Next add any file mounts inside the given directory.
 	prefixInside := prefix + filepathSeparator
-	fs.rootMapToReal.WalkPrefix(prefixInside, func(s string, v any) bool {
+	rfs.rootMapToReal.WalkPrefix(prefixInside, func(s string, v any) bool {
 		if (strings.Count(s, filepathSeparator) - level) != 1 {
 			// This directory is not part of the current, but we
 			// need to include the first name part to make it
@@ -437,7 +573,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 			}
 			seen[name] = true
 			opener := func() (afero.File, error) {
-				return fs.Open(path)
+				return rfs.Open(path)
 			}
 
 			fi := newDirNameOnlyFileInfo(name, nil, opener)
@@ -460,7 +596,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 			seen[name] = true
 
 			opener := func() (afero.File, error) {
-				return fs.Open(rm.From)
+				return rfs.Open(rm.From)
 			}
 
 			fi := newDirNameOnlyFileInfo(name, rm.Meta, opener)
@@ -473,7 +609,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 	})
 
 	// Finally add any ancestor dirs with files in this directory.
-	ancestors := fs.getAncestors(prefix)
+	ancestors := rfs.getAncestors(prefix)
 	for _, root := range ancestors {
 		subdir := strings.TrimPrefix(prefix, root.key)
 		for _, rm := range root.roots {
@@ -491,7 +627,7 @@ func (fs *RootMappingFs) collectDirEntries(prefix string) ([]os.FileInfo, error)
 	return fis, nil
 }
 
-func (fs *RootMappingFs) doLstat(name string) ([]FileMetaInfo, error) {
+func (fs *RootMappingFs) doStat(name string) ([]FileMetaInfo, error) {
 	name = fs.cleanName(name)
 	key := filepathSeparator + name
 
@@ -504,7 +640,7 @@ func (fs *RootMappingFs) doLstat(name string) ([]FileMetaInfo, error) {
 			return []FileMetaInfo{newDirNameOnlyFileInfo(name, nil, fs.virtualDirOpener(name))}, nil
 		}
 
-		// Find any real files or directories with this key.
+		// Find any real directories with this key.
 		_, roots := fs.getRoots(key)
 		if roots == nil {
 			return nil, &os.PathError{Op: "LStat", Path: name, Err: os.ErrNotExist}
@@ -515,7 +651,7 @@ func (fs *RootMappingFs) doLstat(name string) ([]FileMetaInfo, error) {
 
 		for _, rm := range roots {
 			var fi FileMetaInfo
-			fi, _, err = fs.statRoot(rm, name)
+			fi, err = fs.statRoot(rm, name)
 			if err == nil {
 				fis = append(fis, fi)
 			}
@@ -565,33 +701,52 @@ func (fs *RootMappingFs) doLstat(name string) ([]FileMetaInfo, error) {
 	return []FileMetaInfo{roots[0].fi}, nil
 }
 
-func (fs *RootMappingFs) statRoot(root RootMapping, name string) (FileMetaInfo, bool, error) {
-	if !root.Meta.InclusionFilter.Match(root.trimFrom(name), root.fi.IsDir()) {
-		return nil, false, os.ErrNotExist
+func (fs *RootMappingFs) statRoot(root RootMapping, filename string) (FileMetaInfo, error) {
+	dir, name := filepath.Split(filename)
+	if root.Meta.Rename != nil {
+		if n := root.Meta.Rename(name, false); n != name {
+			filename = filepath.Join(dir, n)
+		}
+	}
+
+	if !root.Meta.InclusionFilter.Match(root.trimFrom(filename), root.fi.IsDir()) {
+		return nil, os.ErrNotExist
 	}
-	filename := root.filename(name)
 
-	fi, b, err := lstatIfPossible(fs.Fs, filename)
+	filename = root.filename(filename)
+	fi, err := fs.Fs.Stat(filename)
 	if err != nil {
-		return nil, b, err
+		return nil, err
 	}
 
 	var opener func() (afero.File, error)
 	if fi.IsDir() {
-		// Make sure metadata gets applied in Readdir.
+		// Make sure metadata gets applied in ReadDir.
 		opener = fs.realDirOpener(filename, root.Meta)
 	} else {
+		if root.Meta.Rename != nil {
+			if n := root.Meta.Rename(fi.Name(), true); n != fi.Name() {
+				meta := fi.(MetaProvider).Meta()
+
+				meta.Name = n
+
+			}
+		}
+
 		// Opens the real file directly.
 		opener = func() (afero.File, error) {
 			return fs.Fs.Open(filename)
 		}
+
 	}
 
-	return decorateFileInfo(fi, fs.Fs, opener, "", "", root.Meta), b, nil
+	fim := decorateFileInfo(fi, opener, "", root.Meta)
+
+	return fim, nil
 }
 
 func (fs *RootMappingFs) virtualDirOpener(name string) func() (afero.File, error) {
-	return func() (afero.File, error) { return &rootMappingFile{name: name, fs: fs}, nil }
+	return func() (afero.File, error) { return &rootMappingDir{name: name, fs: fs}, nil }
 }
 
 func (fs *RootMappingFs) realDirOpener(name string, meta *FileMeta) func() (afero.File, error) {
@@ -600,39 +755,41 @@ func (fs *RootMappingFs) realDirOpener(name string, meta *FileMeta) func() (afer
 		if err != nil {
 			return nil, err
 		}
-		return &rootMappingFile{name: name, meta: meta, fs: fs, File: f}, nil
+		return &rootMappingDir{name: name, meta: meta, fs: fs, DirOnlyOps: f}, nil
 	}
 }
 
-type rootMappingFile struct {
-	afero.File
+var _ iofs.ReadDirFile = (*rootMappingDir)(nil)
+
+type rootMappingDir struct {
+	*noOpRegularFileOps
+	DirOnlyOps
 	fs   *RootMappingFs
 	name string
 	meta *FileMeta
 }
 
-func (f *rootMappingFile) Close() error {
-	if f.File == nil {
+func (f *rootMappingDir) Close() error {
+	if f.DirOnlyOps == nil {
 		return nil
 	}
-	return f.File.Close()
+	return f.DirOnlyOps.Close()
 }
 
-func (f *rootMappingFile) Name() string {
+func (f *rootMappingDir) Name() string {
 	return f.name
 }
 
-func (f *rootMappingFile) Readdir(count int) ([]os.FileInfo, error) {
-	if f.File != nil {
-
-		fis, err := f.File.Readdir(count)
+func (f *rootMappingDir) ReadDir(count int) ([]iofs.DirEntry, error) {
+	if f.DirOnlyOps != nil {
+		fis, err := f.DirOnlyOps.(iofs.ReadDirFile).ReadDir(count)
 		if err != nil {
 			return nil, err
 		}
 
-		var result []os.FileInfo
+		var result []iofs.DirEntry
 		for _, fi := range fis {
-			fim := decorateFileInfo(fi, f.fs, nil, "", "", f.meta)
+			fim := decorateFileInfo(fi, nil, "", f.meta)
 			meta := fim.Meta()
 			if f.meta.InclusionFilter.Match(strings.TrimPrefix(meta.Filename, meta.SourceRoot), fim.IsDir()) {
 				result = append(result, fim)
@@ -644,10 +801,31 @@ func (f *rootMappingFile) Readdir(count int) ([]os.FileInfo, error) {
 	return f.fs.collectDirEntries(f.name)
 }
 
-func (f *rootMappingFile) Readdirnames(count int) ([]string, error) {
-	dirs, err := f.Readdir(count)
+// Sentinal error to signal that a file is a directory.
+var errIsDir = errors.New("isDir")
+
+func (f *rootMappingDir) Stat() (iofs.FileInfo, error) {
+	return nil, errIsDir
+}
+
+func (f *rootMappingDir) Readdir(count int) ([]os.FileInfo, error) {
+	panic("not supported: use ReadDir")
+}
+
+// Note that Readdirnames preserves the order of the underlying filesystem(s),
+// which is usually directory order.
+func (f *rootMappingDir) Readdirnames(count int) ([]string, error) {
+	dirs, err := f.ReadDir(count)
 	if err != nil {
 		return nil, err
 	}
-	return fileInfosToNames(dirs), nil
+	return dirEntriesToNames(dirs), nil
+}
+
+func dirEntriesToNames(fis []iofs.DirEntry) []string {
+	names := make([]string, len(fis))
+	for i, d := range fis {
+		names[i] = d.Name()
+	}
+	return names
 }