os: make SameFile handle paths like c:a.txt properly

Fixes #8490.

LGTM=r, rsc
R=golang-codereviews, rsc, bradfitz, r
CC=golang-codereviews
https://golang.org/cl/127740043

src/pkg/os/file_windows.go

@@ -117,8 +117,10 @@ func openDir(name string) (file *File, err error) {
 	}
 	d.path = name
 	if !isAbs(d.path) {
-		cwd, _ := Getwd()
-		d.path = cwd + `\` + d.path
+		d.path, e = syscall.FullPath(d.path)
+		if e != nil {
+			return nil, e
+		}
 	}
 	f := newFile(r, name)
 	f.dirinfo = d

src/pkg/os/os_windows_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"syscall"
+	"testing"
 )
 
 func init() {
@@ -25,3 +26,56 @@ func init() {
 		supportsSymlinks = false
 	}
 }
+
+func TestSameWindowsFile(t *testing.T) {
+	temp, err := ioutil.TempDir("", "TestSameWindowsFile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(temp)
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Chdir(temp)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(wd)
+
+	f, err := os.Create("a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+
+	ia1, err := os.Stat("a")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	path, err := filepath.Abs("a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	ia2, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !os.SameFile(ia1, ia2) {
+		t.Errorf("files should be same")
+	}
+
+	p := filepath.VolumeName(path) + filepath.Base(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ia3, err := os.Stat(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !os.SameFile(ia1, ia3) {
+		t.Errorf("files should be same")
+	}
+}

src/pkg/os/stat_windows.go

@@ -87,8 +87,10 @@ func Lstat(name string) (fi FileInfo, err error) {
 	}
 	fs.path = name
 	if !isAbs(fs.path) {
-		cwd, _ := Getwd()
-		fs.path = cwd + `\` + fs.path
+		fs.path, e = syscall.FullPath(fs.path)
+		if e != nil {
+			return nil, e
+		}
 	}
 	return fs, nil
 }

src/pkg/syscall/exec_windows.go

@@ -129,9 +129,8 @@ func SetNonblock(fd Handle, nonblocking bool) (err error) {
 	return nil
 }
 
-// getFullPath retrieves the full path of the specified file.
-// Just a wrapper for Windows GetFullPathName api.
-func getFullPath(name string) (path string, err error) {
+// FullPath retrieves the full path of the specified file.
+func FullPath(name string) (path string, err error) {
 	p, err := UTF16PtrFromString(name)
 	if err != nil {
 		return "", err
@@ -160,7 +159,7 @@ func isSlash(c uint8) bool {
 }
 
 func normalizeDir(dir string) (name string, err error) {
-	ndir, err := getFullPath(dir)
+	ndir, err := FullPath(dir)
 	if err != nil {
 		return "", err
 	}
@@ -199,9 +198,9 @@ func joinExeDirAndFName(dir, p string) (name string, err error) {
 				return "", err
 			}
 			if volToUpper(int(p[0])) == volToUpper(int(d[0])) {
-				return getFullPath(d + "\\" + p[2:])
+				return FullPath(d + "\\" + p[2:])
 			} else {
-				return getFullPath(p)
+				return FullPath(p)
 			}
 		}
 	} else {
@@ -211,9 +210,9 @@ func joinExeDirAndFName(dir, p string) (name string, err error) {
 			return "", err
 		}
 		if isSlash(p[0]) {
-			return getFullPath(d[:2] + p)
+			return FullPath(d[:2] + p)
 		} else {
-			return getFullPath(d + "\\" + p)
+			return FullPath(d + "\\" + p)
 		}
 	}
 	// we shouldn't be here