TIL: Browsers ignore Expires header on reload

Michał ‘mina86’ Nazarewicz | 26 lutego 2019

This may have been obvious, but I’ve just learned that browsers ignore Expires header when the user manually reloads the page (as in by pressing F5 or choosing Reload option).

I’ve run into this when testing how Firefox treats pages which ‘never’ expire. To my surprise, the browser made requests for files it had a fresh copy of in its cache. To see behaviour much more representative of the experience of a returning user, one should select the address bar (Alt+D does the trick) and then press Return to navigate to the current page again. Hitting Reload is more akin, though not exactly the same, to the first visit.

Of course, all of the above applies to the max-age directive of the Cache-Control header as well.

Moral of the story? Make sure you test the actual real-life scenarios before making any decisions.

Setting up Tor hidden service

Michał ‘mina86’ Nazarewicz | 17 lutego 2019

Anyone can think of myriad reasons to run a Tor hidden service. Surely many unsavoury endeavours spring to mind but of course, there are as many noble ones. There are also various pragmatic causes like circumventing lousy NATs. Me? I just wanted to play around with my router.

Configuring a hidden service is actually straightforward so to make things more interesting, this article will cover configuring a hidden service on a Turris Omnia router with the help of Linux Containers to maximise isolation of components. While some steps will be Omnia-specific, most translate easily to other systems, so this post may be applicable regardless of the distribution used.

Java: String↔char[]

Michał ‘mina86’ Nazarewicz | 9 lutego 2019

Do you recall when I decided to abuse Go’s run-time and play with string[]byte conversion? Fun times… I wonder if we could do the same to Java?

To remind ourselves of the ‘problem’, strings in Java are immutable but because Java has no concept of ownership or const keyword (can we move the industry to Rust already?) to make true on that promise, Java run-time has to make a defensive copy each time a new string is created or when string’s characters are returned.

Alas, do not despair! There is another way (exception handling elided for brevity):

private static Field getValueField() {
	final Field field = String.class.getDeclaredField("value");
	/* Test that it works. */
	final char[] chars = new char[]{'F', 'o', 'o'};
	final String string = new String();
	field.set(string, chars);
	if (string.equals("Foo") && field.get(string) == chars) {
		return field;
	throw new UnsupportedOperationException(
		"UnsafeString not supported by the run-time");

private final static Field valueField = getValueField();

public static String fromChars(final char[] chars) {
	final String string = new String();
	valueField.set(string, chars);
	return string;

public static char[] toChars(final String string) {
	return (char[]) valueField.get(string);

However. There is a twist…

Calculating sRGB↔XYZ matrix

Michał ‘mina86’ Nazarewicz | 3 lutego 2019

I’ve recently found myself in need of an sRGB↔XYZ transformation matrix expressed to the maximum possible precision. Sources on the Internet typically limit the precision to just a few decimal places so I've decided to do the calculations by myself.

What we’re looking for is a 3-by-3 matrix \(M\) which, when multiplied by red, green and blue coördinates of a colour, produces its XYZ coördinates. In other words, a change of basis matrix from a space whose basis vectors are sRGB’s primary colours: $$ M = \begin{bmatrix} X_r & X_g & Y_b \\ Y_r & Y_g & Y_b \\ Z_r & Z_g & Z_b \end{bmatrix} $$