util/vizerror: add Wrapf function that combines Errorf with Wrap

This makes it convenient to wrap an error with a vizerror that needs to use a format string.

Updates tailscale/corp#27066

Signed-off-by: Percy Wegmann <percy@tailscale.com>

util/vizerror/vizerror.go

@@ -75,6 +75,18 @@ func WrapWithMessage(wrapped error, publicMsg string) error {
 	}
 }
 
+// Wrapf wraps the given error with a Vizerror built using the specified publicMsgFormat and values.
+// It always returns a vizerror.Error.
+//
+// Warning: avoid using an error as one of the format arguments, as this will cause the text
+// of that error to be displayed to the end user (which is probably not what you want).
+func Wrapf(wrapped error, publicMsgFormat string, a ...any) error {
+	return Error{
+		publicErr: fmt.Errorf(publicMsgFormat, a...),
+		wrapped:   wrapped,
+	}
+}
+
 // As returns the first vizerror.Error in err's chain.
 func As(err error) (e Error, ok bool) {
 	ok = errors.As(err, &e)

util/vizerror/vizerror_test.go

@@ -64,3 +64,14 @@ func TestWrapWithMessage(t *testing.T) {
 		t.Errorf("Unwrap = %q, want %q", errors.Unwrap(err), wrapped)
 	}
 }
+
+func TestWrapf(t *testing.T) {
+	wrapped := errors.New("wrapped")
+	err := Wrapf(wrapped, "safe: %s", "for sure")
+	if err.Error() != "safe: for sure" {
+		t.Errorf(`Wrapf(wrapped, "safe: for sure").Error() = %q, want %q`, err.Error(), "safe: for sure")
+	}
+	if errors.Unwrap(err) != wrapped {
+		t.Errorf("Unwrap = %q, want %q", errors.Unwrap(err), wrapped)
+	}
+}