path/filepath: fix Windows-specific Clean bug

Fixes #27791
Change-Id: I762fa663379086c24cb4ddc8233a2c0a82b1238e
Reviewed-on: https://go-review.googlesource.com/137055
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/path/filepath/path.go

@@ -96,14 +96,19 @@ func Clean(path string) string {
 		}
 		return originalPath + "."
 	}
+
+	n := len(path)
+	if volLen > 2 && n == 1 && os.IsPathSeparator(path[0]) {
+		// UNC volume name with trailing slash.
+		return FromSlash(originalPath[:volLen])
+	}
 	rooted := os.IsPathSeparator(path[0])
 
 	// Invariants:
 	//	reading from path; r is index of next byte to process.
-	//	writing to buf; w is index of next byte to write.
-	//	dotdot is index in buf where .. must stop, either because
+	//	writing to out; w is index of next byte to write.
+	//	dotdot is index in out where .. must stop, either because
 	//		it is the leading slash or it is a leading ../../.. prefix.
-	n := len(path)
 	out := lazybuf{path: path, volAndPath: originalPath, volLen: volLen}
 	r, dotdot := 0, 0
 	if rooted {

src/path/filepath/path_test.go

@@ -92,6 +92,9 @@ var wincleantests = []PathTest{
 	{`//host/share/foo/../baz`, `\\host\share\baz`},
 	{`\\a\b\..\c`, `\\a\b\c`},
 	{`\\a\b`, `\\a\b`},
+	{`\\a\b\`, `\\a\b`},
+	{`\\folder\share\foo`, `\\folder\share\foo`},
+	{`\\folder\share\foo\`, `\\folder\share\foo`},
 }
 
 func TestClean(t *testing.T) {