os: add OpenInRoot

For #67002

Change-Id: If919ee8a5e3d90e91c7848330762e3254245fba1
Reviewed-on: https://go-review.googlesource.com/c/go/+/629555
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/67002.txt

@@ -1,3 +1,4 @@
+pkg os, func OpenInRoot(string, string) (*File, error) #67002
 pkg os, func OpenRoot(string) (*Root, error) #67002
 pkg os, method (*Root) Close() error #67002
 pkg os, method (*Root) Create(string) (*File, error) #67002

src/os/root.go

@@ -14,6 +14,22 @@ import (
 	"slices"
 )
 
+// OpenInRoot opens the file name in the directory dir.
+// It is equivalent to OpenRoot(dir) followed by opening the file in the root.
+//
+// OpenInRoot returns an error if any component of the name
+// references a location outside of dir.
+//
+// See [Root] for details and limitations.
+func OpenInRoot(dir, name string) (*File, error) {
+	r, err := OpenRoot(dir)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	return r.Open(name)
+}
+
 // Root may be used to only access files within a single directory tree.
 //
 // Methods on Root can only access files and directories beneath a root directory.

src/os/root_test.go

@@ -1157,3 +1157,26 @@ func TestRootRaceRenameDir(t *testing.T) {
 		}
 	}
 }
+
+func TestOpenInRoot(t *testing.T) {
+	dir := makefs(t, []string{
+		"file",
+		"link => ../ROOT/file",
+	})
+	f, err := os.OpenInRoot(dir, "file")
+	if err != nil {
+		t.Fatalf("OpenInRoot(`file`) = %v, want success", err)
+	}
+	f.Close()
+	for _, name := range []string{
+		"link",
+		"../ROOT/file",
+		dir + "/file",
+	} {
+		f, err := os.OpenInRoot(dir, name)
+		if err == nil {
+			f.Close()
+			t.Fatalf("OpenInRoot(%q) = nil, want error", name)
+		}
+	}
+}