The Power Of The Test Case

Thanks to Tim Waugh, before we even started, we knew one of the stumbling
blocks for this terminal services thing was going to be VNC’s ZRLE encoding.
You see, in order to make the hot-desking thing work you need to be able
to transfer a VNC client between two VNC servers without the client knowing
about it.

What makes the ZRLE encoding difficult is that it compresses the pixel
stream using zlib and this introduces state into the VNC stream. A rectangle
of encoded pixel data now depends on the pixels that went before. So when,
you transfer a client between servers, the new server either needs to know
the state of the zlib stream or you need to allow the new server to start
with a clean slate somehow.

A glance at the zlib docs
and the Z_FULL_FLUSH/Z_FINISH deflate() flags jumped out
of me. Sweet, this is going to be trivial, says he. Like a good engineer,
I started writing a
test case
to prove the idea. The solution wasn’t as obvious as I’d hoped,
though, and I found myself knee deep in docs, rfcs and the inflate()/
deflate() code. Unfortunately, I had to actually understand
what the hell was going on. The horror!

Plugged the code into Xvnc and tried it out. Broken. Dramatically broken. Debug,
tweak, test. Still broken. And on it went. Eventually, hours later, I went back
to my test case and discovered that it actually wasn’t working. It was sneakily
pretending to be working. But it wasn’t. Trivial fix and all is well.

You can save so much time by figuring stuff out in a small test case rather
than trying to figure it out in the context of a huge mass of code with a million
and one other things happening. I know this to be true, and as time goes on
I’m learning just how true it is. But you still find yourself launching headlong
into problems thinking “it looks trivial, in the same time I’d write a test case
I’ll have the thing fixed” and only hours and hours later do you get a clue
and start writing a test case. I get the feeling Owen always reaches straight
for a test case. Maybe that’s his secret :-)

Anyway, I’ve moved on to adding TLS/SSL support Xvnc and vncviewer now. And,
yes, I’m going to go through all this “state flushing” pain again. Fun
all the way, I tell ya … Now, where’s my test case?

On the plus side, I’ve had to take loads of breaks from this stuff today,
and I’ve been cooking. Had a gorgeous
ratatouille for lunch
and some chilli
is bubbling away as I type.

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