From 06f90ebfa6f3e0a744584513e2786988b26569da Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Wed, 08 Jul 2026 11:11:48 -0700
Subject: [PATCH] os: properly handle trailing / in Root.MkdirAll

MkdirAll("dir/") should create "dir", not return an error.

Fixes #80308

Change-Id: I75458c30b6cb94be3a3d368f6024caef6a6a6964
---

diff --git a/src/os/root_noopenat.go b/src/os/root_noopenat.go
index 228b580..a84e952 100644
--- a/src/os/root_noopenat.go
+++ b/src/os/root_noopenat.go
@@ -158,6 +158,9 @@
 	if err := checkPathEscapes(r, name); err == errPathEscapes {
 		return &PathError{Op: "mkdirat", Path: name, Err: err}
 	}
+	if name == "" {
+		return &PathError{Op: "mkdirat", Path: name, Err: syscall.ENOENT}
+	}
 	prefix := r.root.name + string(PathSeparator)
 	if err := MkdirAll(prefix+name, perm); err != nil {
 		if pe, ok := err.(*PathError); ok {
diff --git a/src/os/root_openat.go b/src/os/root_openat.go
index d7c9334..92865e4 100644
--- a/src/os/root_openat.go
+++ b/src/os/root_openat.go
@@ -167,6 +167,12 @@
 					fi, e := r.Stat(fullname)
 					if e == nil && fi.Mode().IsDir() {
 						err = nil
+					} else if e == nil {
+						err = syscall.ENOTDIR
+					} else if !IsNotExist(e) {
+						// EPERM, ELOOP, etc.,
+						// probably more useful than EEXIST.
+						err = e
 					}
 				}
 			}
@@ -177,7 +183,12 @@
 		}
 		return struct{}{}, &PathError{Op: "mkdirat", Err: err}
 	}
-	_, err := doInRoot(r, fullname, 0, openDirFunc, openLastComponentFunc)
+	flags := uint(doInRootCreatingDirectory)
+	switch runtime.GOOS {
+	case "linux", "windows":
+		flags = doInRootNoHandleTerminalSlash // see rootMkdir
+	}
+	_, err := doInRoot(r, fullname, flags, openDirFunc, openLastComponentFunc)
 	if err != nil {
 		if _, ok := err.(*PathError); !ok {
 			err = &PathError{Op: "mkdirat", Path: fullname, Err: err}
diff --git a/src/os/root_test.go b/src/os/root_test.go
index 5e0acf9..33a2f33 100644
--- a/src/os/root_test.go
+++ b/src/os/root_test.go
@@ -585,6 +585,11 @@
 
 func TestRootMkdirAll(t *testing.T) {
 	for _, test := range rootTestCases {
+		if test.name == "directory does not exist" {
+			// Test expects error, mkdirall creates the missing directory.
+			// TestRootMultiMkdirAll covers this case better anyway, just skip.
+			continue
+		}
 		test.run(t, func(t *testing.T, target string, root *os.Root) {
 			wantError := test.wantError
 			if test.ltarget != "" {
@@ -593,7 +598,7 @@
 				wantError = true
 			}
 
-			err := root.Mkdir(test.open, 0o777)
+			err := root.MkdirAll(test.open, 0o777)
 			if errEndsTest(t, err, wantError, "root.MkdirAll(%q)", test.open) {
 				return
 			}
@@ -3133,6 +3138,52 @@
 	})
 }
 
+func TestRootMultiMkdirAllShallow(t *testing.T) {
+	runRootMultiTest(t, func(t *testing.T, test *rootMultiTest) (string, error) {
+		return testRootMultiMkdirAll(t, test, test.targetPath)
+	})
+}
+
+func TestRootMultiMkdirAllDeep(t *testing.T) {
+	runRootMultiTest(t, func(t *testing.T, test *rootMultiTest) (string, error) {
+		targetPath := test.targetPath
+		if len(targetPath) > 0 && os.IsPathSeparator(targetPath[len(targetPath)-1]) {
+			targetPath += "a/b/"
+		} else {
+			targetPath += "/a/b"
+		}
+		return testRootMultiMkdirAll(t, test, targetPath)
+	})
+}
+
+func testRootMultiMkdirAll(t *testing.T, test *rootMultiTest, targetPath string) (string, error) {
+	var mkdirAll = os.MkdirAll
+	if test.root != nil {
+		mkdirAll = test.root.MkdirAll
+	}
+
+	test.setOp("MkdirAll(%q, 0o777)", targetPath)
+	gotErr := mkdirAll(targetPath, 0o777)
+
+	switch {
+	case test.root != nil && test.target.lescapes():
+		// "mkdir ../target", or equivalent escaping path.
+		test.wantError(t, gotErr, os.ErrPathEscapes)
+	case test.root != nil && test.target.escapes():
+		// "mkdir ../target", or equivalent escaping path.
+		test.wantError(t, gotErr, errAny)
+		return "", errSkipRootConsistencyCheck
+	case test.root != nil && test.target.kind == testFileSymlink && test.target.target.kind == testFileAbsent && targetPath != test.targetPath:
+		// A minor inconsistency between Root.MkdirAll and os.MkdirAll:
+		// When an intermediate component of the tree being constructed is a
+		// dangling symlink, Root.MkdirAll will follow the symlink and create
+		// its target directory, while os.MkdirAll will fail with an error.
+		return "", errSkipRootConsistencyCheck
+	default:
+	}
+	return "", gotErr
+}
+
 func TestRootMultiRename(t *testing.T) {
 	if runtime.GOOS == "wasip1" {
 		switch os.Getenv("GOWASIRUNTIME") {