2012/05/23

Don't be too lazy

Today I fixed a bug in my C library rewrite of Tickit.

The manifestation of the bug was that Tickit::Console didn't work properly via the C/XS version of Tickit, but worked fine on the Perl version. It wasn't receiving keyboard input at all. Yet I know basic keyboard input works fine on all the simple demos with the C library version, so something more subtle was up.

On close inspection it turned out that libtickit was being too lazy with constructing its libtermkey instance for handling keyboard input. The previous code arrangement was that constructing the instance was deferred as late as possible, until one of the input-handling methods was actually called. This meant that setting the filehandle could be done as a normal accessor and not as a constructor argument. (C functions lack the neatness of Perl's named argument style, so my usual style is minimal constructors and lots of mutation accessors).

This worked fine for simple cases, because after setup the demo programs all wait on a call to tickit_term_input_wait(), and the first thing that does is create the TermKey instance, which sets up the terminal for non-canonical input mode and disables local echo. This ensures input arrives a key at a time.

However, since the Tickit::Console example runs via Tickit::Async, the way it runs is to wait in a poll() loop waiting on readability on STDIN. When STDIN becomes readable does it call tickit_term_input_readable(), and only then does it create the TermKey instance that actually sets up the terminal.

The upshot here is that until that entire first line of input is received, the terminal isn't actually set up to the correct mode in the first place.

Fixing this bug was a simple matter of making the underlying tickit_term_set_input_handle() mutator eagerly allocate the TermKey instance immediately, so that the terminal is already set up in the correct mode once the Tickit::Term constructor returns. That way it works correctly in both synchronous and asynchronous code.

In summary - when creating lazy functionality in a program, make sure that your laziness doesn't cause you to neglect to set something up that someone else was relying on. Be lazy, but don't be too lazy.

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