From c315862c99fcca83389b99715a712a6c8e1ad65e Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Mon, 18 Nov 2024 18:09:17 -0800
Subject: [PATCH] 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 +
 src/os/root.go      | 16 ++++++++++++++++
 src/os/root_test.go | 23 +++++++++++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/api/next/67002.txt b/api/next/67002.txt
index 67c47969f4f..72fdec83708 100644
--- a/api/next/67002.txt
+++ b/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
diff --git a/src/os/root.go b/src/os/root.go
index c7d9b5b0712..d9fc6358a50 100644
--- a/src/os/root.go
+++ b/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.
diff --git a/src/os/root_test.go b/src/os/root_test.go
index 1cff474b939..288b4060cd8 100644
--- a/src/os/root_test.go
+++ b/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)
+		}
+	}
+}