os: add Root.ReadFile and Root.WriteFile

For #73126

Change-Id: Ie69cc274e7b59f958c239520318b89ff0141e26b
Reviewed-on: https://go-review.googlesource.com/c/go/+/674315
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>

api/next/73126.txt

@@ -0,0 +1,2 @@
+pkg os, method (*Root) ReadFile(string) ([]uint8, error) #73126
+pkg os, method (*Root) WriteFile(string, []uint8, fs.FileMode) error #73126

doc/next/6-stdlib/99-minor/os/67002.md

@@ -6,7 +6,9 @@ The [os.Root] type supports the following additional methods:
   * [os.Root.Lchown]
   * [os.Root.Link]
   * [os.Root.MkdirAll]
+  * [os.Root.ReadFile]
   * [os.Root.Readlink]
   * [os.Root.RemoveAll]
   * [os.Root.Rename]
   * [os.Root.Symlink]
+  * [os.Root.WriteFile]

doc/next/6-stdlib/99-minor/os/73126.md

@@ -0,0 +1 @@
+<!-- go.dev/issue/73126 is documented as part of 67002 -->

src/os/root.go

@@ -248,6 +248,31 @@ func (r *Root) Symlink(oldname, newname string) error {
 	return rootSymlink(r, oldname, newname)
 }
 
+// ReadFile reads the named file in the root and returns its contents.
+// See [ReadFile] for more details.
+func (r *Root) ReadFile(name string) ([]byte, error) {
+	f, err := r.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return readFileContents(statOrZero(f), f.Read)
+}
+
+// WriteFile writes data to the named file in the root, creating it if necessary.
+// See [WriteFile] for more details.
+func (r *Root) WriteFile(name string, data []byte, perm FileMode) error {
+	f, err := r.OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	_, err = f.Write(data)
+	if err1 := f.Close(); err == nil {
+		err = err1
+	}
+	return err
+}
+
 func (r *Root) logOpen(name string) {
 	if log := testlog.Logger(); log != nil {
 		// This won't be right if r's name has changed since it was opened,

src/os/root_test.go

@@ -1899,3 +1899,23 @@ func TestRootRemoveDot(t *testing.T) {
 		t.Error(`root.Remove(All)?(".") removed the root`)
 	}
 }
+
+func TestRootWriteReadFile(t *testing.T) {
+	dir := t.TempDir()
+	root, err := os.OpenRoot(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer root.Close()
+
+	name := "filename"
+	want := []byte("file contents")
+	if err := root.WriteFile(name, want, 0o666); err != nil {
+		t.Fatalf("root.WriteFile(%q, %q, 0o666) = %v; want nil", name, want, err)
+	}
+
+	got, err := root.ReadFile(name)
+	if err != nil {
+		t.Fatalf("root.ReadFile(%q) = %q, %v; want %q, nil", name, got, err, want)
+	}
+}