1 files changed, 228 insertions, 0 deletions
diff --git a/src/testing/fstest/mapfs.go b/src/testing/fstest/mapfs.go
new file mode 100644
index 0000000000..10e56f5b3c
--- /dev/null
+++ b/src/testing/fstest/mapfs.go
@@ -0,0 +1,228 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"io"
+	"io/fs"
+	"path"
+	"sort"
+	"strings"
+	"time"
+)
+
+// A MapFS is a simple in-memory file system for use in tests,
+// represented as a map from path names (arguments to Open)
+// to information about the files or directories they represent.
+//
+// The map need not include parent directories for files contained
+// in the map; those will be synthesized if needed.
+// But a directory can still be included by setting the MapFile.Mode's ModeDir bit;
+// this may be necessary for detailed control over the directory's FileInfo
+// or to create an empty directory.
+//
+// File system operations read directly from the map,
+// so that the file system can be changed by editing the map as needed.
+// An implication is that file system operations must not run concurrently
+// with changes to the map, which would be a race.
+// Another implication is that opening or reading a directory requires
+// iterating over the entire map, so a MapFS should typically be used with not more
+// than a few hundred entries or directory reads.
+type MapFS map[string]*MapFile
+
+// A MapFile describes a single file in a MapFS.
+type MapFile struct {
+	Data    []byte      // file content
+	Mode    fs.FileMode // FileInfo.Mode
+	ModTime time.Time   // FileInfo.ModTime
+	Sys     interface{} // FileInfo.Sys
+}
+
+var _ fs.FS = MapFS(nil)
+var _ fs.File = (*openMapFile)(nil)
+
+// Open opens the named file.
+func (fsys MapFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	file := fsys[name]
+	if file != nil && file.Mode&fs.ModeDir == 0 {
+		// Ordinary file
+		return &openMapFile{name, mapFileInfo{path.Base(name), file}, 0}, nil
+	}
+
+	// Directory, possibly synthesized.
+	// Note that file can be nil here: the map need not contain explicit parent directories for all its files.
+	// But file can also be non-nil, in case the user wants to set metadata for the directory explicitly.
+	// Either way, we need to construct the list of children of this directory.
+	var list []mapFileInfo
+	var elem string
+	var need = make(map[string]bool)
+	if name == "." {
+		elem = "."
+		for fname, f := range fsys {
+			i := strings.Index(fname, "/")
+			if i < 0 {
+				list = append(list, mapFileInfo{fname, f})
+			} else {
+				need[fname[:i]] = true
+			}
+		}
+	} else {
+		elem = name[strings.LastIndex(name, "/")+1:]
+		prefix := name + "/"
+		for fname, f := range fsys {
+			if strings.HasPrefix(fname, prefix) {
+				felem := fname[len(prefix):]
+				i := strings.Index(felem, "/")
+				if i < 0 {
+					list = append(list, mapFileInfo{felem, f})
+				} else {
+					need[fname[len(prefix):len(prefix)+i]] = true
+				}
+			}
+		}
+		// If the directory name is not in the map,
+		// and there are no children of the name in the map,
+		// then the directory is treated as not existing.
+		if file == nil && list == nil && len(need) == 0 {
+			return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+		}
+	}
+	for _, fi := range list {
+		delete(need, fi.name)
+	}
+	for name := range need {
+		list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir}})
+	}
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].name < list[j].name
+	})
+
+	if file == nil {
+		file = &MapFile{Mode: fs.ModeDir}
+	}
+	return &mapDir{name, mapFileInfo{elem, file}, list, 0}, nil
+}
+
+// fsOnly is a wrapper that hides all but the fs.FS methods,
+// to avoid an infinite recursion when implementing special
+// methods in terms of helpers that would use them.
+// (In general, implementing these methods using the package fs helpers
+// is redundant and unnecessary, but having the methods may make
+// MapFS exercise more code paths when used in tests.)
+type fsOnly struct{ fs.FS }
+
+func (fsys MapFS) ReadFile(name string) ([]byte, error) {
+	return fs.ReadFile(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Stat(name string) (fs.FileInfo, error) {
+	return fs.Stat(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return fs.ReadDir(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Glob(pattern string) ([]string, error) {
+	return fs.Glob(fsOnly{fsys}, pattern)
+}
+
+// A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
+type mapFileInfo struct {
+	name string
+	f    *MapFile
+}
+
+func (i *mapFileInfo) Name() string               { return i.name }
+func (i *mapFileInfo) Size() int64                { return int64(len(i.f.Data)) }
+func (i *mapFileInfo) Mode() fs.FileMode          { return i.f.Mode }
+func (i *mapFileInfo) Type() fs.FileMode          { return i.f.Mode.Type() }
+func (i *mapFileInfo) ModTime() time.Time         { return i.f.ModTime }
+func (i *mapFileInfo) IsDir() bool                { return i.f.Mode&fs.ModeDir != 0 }
+func (i *mapFileInfo) Sys() interface{}           { return i.f.Sys }
+func (i *mapFileInfo) Info() (fs.FileInfo, error) { return i, nil }
+
+// An openMapFile is a regular (non-directory) fs.File open for reading.
+type openMapFile struct {
+	path string
+	mapFileInfo
+	offset int64
+}
+
+func (f *openMapFile) Stat() (fs.FileInfo, error) { return &f.mapFileInfo, nil }
+
+func (f *openMapFile) Close() error { return nil }
+
+func (f *openMapFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.f.Data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openMapFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.f.Data))
+	}
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openMapFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// A mapDir is a directory fs.File (so also an fs.ReadDirFile) open for reading.
+type mapDir struct {
+	path string
+	mapFileInfo
+	entry  []mapFileInfo
+	offset int
+}
+
+func (d *mapDir) Stat() (fs.FileInfo, error) { return &d.mapFileInfo, nil }
+func (d *mapDir) Close() error               { return nil }
+func (d *mapDir) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.path, Err: fs.ErrInvalid}
+}
+
+func (d *mapDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.entry) - d.offset
+	if count > 0 && n > count {
+		n = count
+	}
+	if n == 0 && count > 0 {
+		return nil, io.EOF
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.entry[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}