os: add DirFS

It will inevitably be important to be able to pass an operating system
directory to code written to expect an fs.FS.

os.DirFS provides the conversion.

For #41190.

Change-Id: Id1a8fcbe4c7a30de2c47dea0504e9481a88b1b39
Reviewed-on: https://go-review.googlesource.com/c/go/+/243911
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/file.go

@@ -45,6 +45,7 @@ import (
 	"internal/poll"
 	"internal/testlog"
 	"io"
+	"io/fs"
 	"runtime"
 	"syscall"
 	"time"
@@ -608,3 +609,21 @@ func isWindowsNulName(name string) bool {
 	}
 	return true
 }
+
+// DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
+func DirFS(dir string) fs.FS {
+	return dirFS(dir)
+}
+
+type dirFS string
+
+func (dir dirFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
+	}
+	f, err := Open(string(dir) + "/" + name)
+	if err != nil {
+		return nil, err // nil fs.File
+	}
+	return f, nil
+}

src/os/os_test.go

@@ -23,6 +23,7 @@ import (
 	"sync"
 	"syscall"
 	"testing"
+	"testing/fstest"
 	"time"
 )
 
@@ -2671,3 +2672,9 @@ func TestOpenFileKeepsPermissions(t *testing.T) {
 		t.Errorf("Stat after OpenFile is %v, should be writable", fi.Mode())
 	}
 }
+
+func TestDirFS(t *testing.T) {
+	if err := fstest.TestFS(DirFS("./signal"), "signal.go", "internal/pty/pty.go"); err != nil {
+		t.Fatal(err)
+	}
+}