path/filepath: convert drive letter to upper case in windows EvalSymlinks

Fixes #3347.

R=golang-dev, aram, r, rsc
CC=golang-dev
https://golang.org/cl/5918043

src/pkg/path/filepath/path_test.go

@@ -846,3 +846,26 @@ func TestVolumeName(t *testing.T) {
 		}
 	}
 }
+
+func TestDriveLetterInEvalSymlinks(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		return
+	}
+	wd, _ := os.Getwd()
+	if len(wd) < 3 {
+		t.Errorf("Current directory path %q is too short", wd)
+	}
+	lp := strings.ToLower(wd)
+	up := strings.ToUpper(wd)
+	flp, err := filepath.EvalSymlinks(lp)
+	if err != nil {
+		t.Fatalf("EvalSymlinks(%q) failed: %q", lp, err)
+	}
+	fup, err := filepath.EvalSymlinks(up)
+	if err != nil {
+		t.Fatalf("EvalSymlinks(%q) failed: %q", up, err)
+	}
+	if flp != fup {
+		t.Errorf("Results of EvalSymlinks do not match: %q and %q", flp, fup)
+	}
+}

src/pkg/path/filepath/symlink_windows.go

@@ -23,5 +23,13 @@ func evalSymlinks(path string) (string, error) {
 		}
 	}
 	b = b[:n]
-	return Clean(syscall.UTF16ToString(b)), nil
+	s := syscall.UTF16ToString(b)
+	// syscall.GetLongPathName does not change the case of the drive letter,
+	// but the result of EvalSymlinks must be unique, so we have
+	// EvalSymlinks(`c:\a`) == EvalSymlinks(`C:\a`).
+	// Make drive letter upper case. This matches what os.Getwd returns.
+	if len(s) >= 2 && s[1] == ':' && 'a' <= s[0] && s[0] <= 'z' {
+		s = string(s[0]+'A'-'a') + s[1:]
+	}
+	return Clean(s), nil
 }