From e10b05d1fe44991bfd1d841e4f9c398c48cfe910 Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Thu, 9 Nov 2023 09:53:44 -0800
Subject: [PATCH] [release-branch.go1.20] path/filepath: consider \\?\c: as a
 volume on Windows

While fixing several bugs in path handling on Windows,
beginning with \\?\.

Prior to #540277, VolumeName considered the first path component
after the \\?\ prefix to be part of the volume name.
After, it considered only the \\? prefix to be the volume name.

Restore the previous behavior.

Fixes #64040
Updates #64028

Change-Id: I6523789e61776342800bd607fb3f29d496257e68
Reviewed-on: https://go-review.googlesource.com/c/go/+/541175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
(cherry picked from commit eda42f7c60adab26ed1a340414c726c4bf46b1f7)
---
 src/path/filepath/path_test.go    | 15 ++++++++++-----
 src/path/filepath/path_windows.go | 20 +++++++-------------
 2 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/src/path/filepath/path_test.go b/src/path/filepath/path_test.go
index 0a2286648f5bab..3077ffb6c7c357 100644
--- a/src/path/filepath/path_test.go
+++ b/src/path/filepath/path_test.go
@@ -108,6 +108,8 @@ var wincleantests = []PathTest{
 	{`//abc`, `\\abc`},
 	{`///abc`, `\\\abc`},
 	{`//abc//`, `\\abc\\`},
+	{`\\?\C:\`, `\\?\C:\`},
+	{`\\?\C:\a`, `\\?\C:\a`},
 
 	// Don't allow cleaning to move an element with a colon to the start of the path.
 	{`a/../c:`, `.\c:`},
@@ -1472,10 +1474,13 @@ var volumenametests = []VolumeNameTest{
 	{`//.`, `\\.`},
 	{`//./`, `\\.\`},
 	{`//./NUL`, `\\.\NUL`},
-	{`//?/`, `\\?`},
+	{`//?`, `\\?`},
+	{`//?/`, `\\?\`},
+	{`//?/NUL`, `\\?\NUL`},
+	{`/??`, `\??`},
+	{`/??/`, `\??\`},
+	{`/??/NUL`, `\??\NUL`},
 	{`//./a/b`, `\\.\a`},
-	{`//?/`, `\\?`},
-	{`//?/`, `\\?`},
 	{`//./C:`, `\\.\C:`},
 	{`//./C:/`, `\\.\C:`},
 	{`//./C:/a/b/c`, `\\.\C:`},
@@ -1484,8 +1489,8 @@ var volumenametests = []VolumeNameTest{
 	{`//./UNC/host\`, `\\.\UNC\host\`},
 	{`//./UNC`, `\\.\UNC`},
 	{`//./UNC/`, `\\.\UNC\`},
-	{`\\?\x`, `\\?`},
-	{`\??\x`, `\??`},
+	{`\\?\x`, `\\?\x`},
+	{`\??\x`, `\??\x`},
 }
 
 func TestVolumeName(t *testing.T) {
diff --git a/src/path/filepath/path_windows.go b/src/path/filepath/path_windows.go
index c490424f20c968..eacab0e5ced678 100644
--- a/src/path/filepath/path_windows.go
+++ b/src/path/filepath/path_windows.go
@@ -102,12 +102,14 @@ func volumeNameLen(path string) int {
 		// \\.\unc\a\b\..\c into \\.\unc\a\c.
 		return uncLen(path, len(`\\.\UNC\`))
 
-	case pathHasPrefixFold(path, `\\.`):
-		// Path starts with \\., and is a Local Device path.
+	case pathHasPrefixFold(path, `\\.`) ||
+		pathHasPrefixFold(path, `\\?`) || pathHasPrefixFold(path, `\??`):
+		// Path starts with \\.\, and is a Local Device path; or
+		// path starts with \\?\ or \??\ and is a Root Local Device path.
 		//
-		// We currently treat the next component after the \\.\ prefix
-		// as part of the volume name, although there doesn't seem to be
-		// a principled reason to do this.
+		// We treat the next component after the \\.\ prefix as
+		// part of the volume name, which means Clean(`\\?\c:\`)
+		// won't remove the trailing \. (See #64028.)
 		if len(path) == 3 {
 			return 3 // exactly \\.
 		}
@@ -117,14 +119,6 @@ func volumeNameLen(path string) int {
 		}
 		return len(path) - len(rest) - 1
 
-	case pathHasPrefixFold(path, `\\?`) || pathHasPrefixFold(path, `\??`):
-		// Path starts with \\?\ or \??\, and is a Root Local Device path.
-		//
-		// While Windows usually treats / and \ as equivalent,
-		// /??/ does not seem to be recognized as a Root Local Device path.
-		// We treat it as one anyway here to be safe.
-		return 3
-
 	case len(path) >= 2 && isSlash(path[1]):
 		// Path starts with \\, and is a UNC path.
 		return uncLen(path, 2)