5 files changed, 68 insertions, 24 deletions

diff --git a/src/os/path.go b/src/os/path.go
index df87887b9b..6ac4cbe20f 100644
--- a/src/os/path.go
+++ b/src/os/path.go
@@ -26,19 +26,25 @@ func MkdirAll(path string, perm FileMode) error {
 	}
 
 	// Slow path: make sure parent exists and then call Mkdir for path.
-	i := len(path)
-	for i > 0 && IsPathSeparator(path[i-1]) { // Skip trailing path separator.
+
+	// Extract the parent folder from path by first removing any trailing
+	// path separator and then scanning backward until finding a path
+	// separator or reaching the beginning of the string.
+	i := len(path) - 1
+	for i >= 0 && IsPathSeparator(path[i]) {
 		i--
 	}
-
-	j := i
-	for j > 0 && !IsPathSeparator(path[j-1]) { // Scan backward over element.
-		j--
+	for i >= 0 && !IsPathSeparator(path[i]) {
+		i--
+	}
+	if i < 0 {
+		i = 0
 	}
 
-	if j > 1 {
-		// Create parent.
-		err = MkdirAll(fixRootDirectory(path[:j-1]), perm)
+	// If there is a parent directory, and it is not the volume name,
+	// recurse to ensure parent directory exists.
+	if parent := path[:i]; len(parent) > len(volumeName(path)) {
+		err = MkdirAll(parent, perm)
 		if err != nil {
 			return err
 		}
diff --git a/src/os/path_plan9.go b/src/os/path_plan9.go
index a54b4b98f1..f1c9dbc048 100644
--- a/src/os/path_plan9.go
+++ b/src/os/path_plan9.go
@@ -14,6 +14,6 @@ func IsPathSeparator(c uint8) bool {
 	return PathSeparator == c
 }
 
-func fixRootDirectory(p string) string {
-	return p
+func volumeName(p string) string {
+	return ""
 }
diff --git a/src/os/path_unix.go b/src/os/path_unix.go
index c975cdb11e..1c80fa91f8 100644
--- a/src/os/path_unix.go
+++ b/src/os/path_unix.go
@@ -70,6 +70,6 @@ func splitPath(path string) (string, string) {
 	return dirname, basename
 }
 
-func fixRootDirectory(p string) string {
-	return p
+func volumeName(p string) string {
+	return ""
 }
diff --git a/src/os/path_windows.go b/src/os/path_windows.go
index 3356908a36..ec9a87274d 100644
--- a/src/os/path_windows.go
+++ b/src/os/path_windows.go
@@ -214,14 +214,3 @@ func fixLongPath(path string) string {
 	}
 	return string(pathbuf[:w])
 }
-
-// fixRootDirectory fixes a reference to a drive's root directory to
-// have the required trailing slash.
-func fixRootDirectory(p string) string {
-	if len(p) == len(`\\?\c:`) {
-		if IsPathSeparator(p[0]) && IsPathSeparator(p[1]) && p[2] == '?' && IsPathSeparator(p[3]) && p[5] == ':' {
-			return p + `\`
-		}
-	}
-	return p
-}
diff --git a/src/os/path_windows_test.go b/src/os/path_windows_test.go
index 2506b4f0d8..4e5e501d1f 100644
--- a/src/os/path_windows_test.go
+++ b/src/os/path_windows_test.go
@@ -5,7 +5,11 @@
 package os_test
 
 import (
+	"fmt"
+	"internal/syscall/windows"
+	"internal/testenv"
 	"os"
+	"path/filepath"
 	"strings"
 	"syscall"
 	"testing"
@@ -106,3 +110,48 @@ func TestOpenRootSlash(t *testing.T) {
 		dir.Close()
 	}
 }
+
+func testMkdirAllAtRoot(t *testing.T, root string) {
+	// Create a unique-enough directory name in root.
+	base := fmt.Sprintf("%s-%d", t.Name(), os.Getpid())
+	path := filepath.Join(root, base)
+	if err := os.MkdirAll(path, 0777); err != nil {
+		t.Fatalf("MkdirAll(%q) failed: %v", path, err)
+	}
+	// Clean up
+	if err := os.RemoveAll(path); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMkdirAllExtendedLengthAtRoot(t *testing.T) {
+	if testenv.Builder() == "" {
+		t.Skipf("skipping non-hermetic test outside of Go builders")
+	}
+
+	const prefix = `\\?\`
+	vol := filepath.VolumeName(t.TempDir()) + `\`
+	if len(vol) < 4 || vol[:4] != prefix {
+		vol = prefix + vol
+	}
+	testMkdirAllAtRoot(t, vol)
+}
+
+func TestMkdirAllVolumeNameAtRoot(t *testing.T) {
+	if testenv.Builder() == "" {
+		t.Skipf("skipping non-hermetic test outside of Go builders")
+	}
+
+	vol, err := syscall.UTF16PtrFromString(filepath.VolumeName(t.TempDir()) + `\`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	const maxVolNameLen = 50
+	var buf [maxVolNameLen]uint16
+	err = windows.GetVolumeNameForVolumeMountPoint(vol, &buf[0], maxVolNameLen)
+	if err != nil {
+		t.Fatal(err)
+	}
+	volName := syscall.UTF16ToString(buf[:])
+	testMkdirAllAtRoot(t, volName)
+}