os: make MkdirAll support path in extended-length form

Calling MkdirAll on paths in extended-length form (\\?\-prefixed)
failed.

MkdirAll calls itself recursively with parent directory of given path in
its parameter. It finds parent directory by looking for delimiter in
the path, and taking the left part. When path is in extended-length form,
it finds empty path at the end.

Here is a sample of path in extended-length form:
\\?\c:\foo\bar

This change fixes that by passing trailing path separator to MkdirAll (so
it works for path like \\?\c:\).

Fixes #22230

Change-Id: I363660b262588c5382ea829773d3b6005ab8df3c
Reviewed-on: https://go-review.googlesource.com/86295
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/path.go

@@ -39,8 +39,10 @@ func MkdirAll(path string, perm FileMode) error {
 	}
 
 	if j > 1 {
-		// Create parent
-		err = MkdirAll(path[0:j-1], perm)
+		// Create parent.
+		// Pass trailing path separator to MkdirAll, so our
+		// algorithm works for paths, like \\?\c:\
+		err = MkdirAll(path[0:j], perm)
 		if err != nil {
 			return err
 		}

src/os/path_windows_test.go

@@ -5,8 +5,10 @@
 package os_test
 
 import (
+	"io/ioutil"
 	"os"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -44,3 +46,31 @@ func TestFixLongPath(t *testing.T) {
 		}
 	}
 }
+
+func TestMkdirAllExtendedLength(t *testing.T) {
+	tmpDir, err := ioutil.TempDir("", "TestMkdirAllExtendedLength")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	const prefix = `\\?\`
+	if len(tmpDir) < 4 || tmpDir[:4] != prefix {
+		fullPath, err := syscall.FullPath(tmpDir)
+		if err != nil {
+			t.Fatalf("FullPath(%q) fails: %v", tmpDir, err)
+		}
+		tmpDir = prefix + fullPath
+	}
+	path := tmpDir + `\dir\`
+	err = os.MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll(%q) failed: %v", path, err)
+	}
+
+	path = path + `.\dir2`
+	err = os.MkdirAll(path, 0777)
+	if err == nil {
+		t.Fatalf("MkdirAll(%q) should have failed, but did not", path)
+	}
+}