This Week’s Program: Nov 9 - Nov 13

It’s Friday the 13th 👻! This was a weak week, all centering around one line of code:

(.addShutdownHook (Runtime/getRuntime) (Thread. (fn [] (println "Bye!"))))

What this code does, why it does what it does, and why it didn’t work as expected is what consumed this week’s work. On the whole, I really didn’t write as much code as I would have liked, but I learned a lot.

For this week’s newsletter: POSIX Signals, the JVM, and Trampolines.

sonic-sketches: eb53a0d0b2409ebf68dbe8941e8325a359ace8b7

Last week, I alluded to struggles with understanding how the Overtone process worked and to make sure it was playing nicely on the command line. Some Googling and digging through Overtone source code ate up my time this week.

First, there’s this little gem of a document: Troubleshooting Guide for Java SE 6 with HotSpot VM: Integrating Signal and Exception Handling. I read this. I read it again. I just read it again when preparing this email. I still just can’t even… Everytime Solaris is mentioned my heart just pounds kind of fast and my eyes get blurry. If you can even please do send me a summary.

When you’re in your terminal and you send Ctrl-C to the foreground process, this sends the SIGINT signal (The command kill -2 does the same). In Ruby, we have Signal::trap that makes it really straightforward to handle clean shutdowns. The fork and exec (and trap) pattern is well-tread. Approaching the JVM, I really didn’t have a great sense of how it handled inbound POSIX signals.

Buried in that Oracle document is this little bit of code:

java.lang.Runtime.addShutdownHook

So that’s how I learned how Java handles signals designed to terminate processes. The Runtime.addShutdownHook method is Java’s mechanism for adding cleanup work to a doomed process. The method accepts a Thread. In Clojure’s Java interop syntax, it looks like the above code.

This code didn’t work. When I sent Ctrl-C, I did not see “Bye!”.

Trampoline

Why didn’t Ctrl-C work as originally thought? I was running this program with Leiningen through the lein run command. When this is run, Leiningen starts two JVM processes: one for lein itself, and the other for my code. When I was sending the SIGINT signal, it was being handled by the lein process.

So I ran the rambunctiously named: lein trampoline run. “Bye!”

Read this article about the inner workings of Leiningen’s Trampoline. It’s fascinating, I swear.

sonic-sketches: b9c3989517ef3e756909221771dfcc2a7a5c1a84

Later, I removed this code. I originally assumed it was necessary. Turns out that Overtone already has a clean shutdown mechanism. When I (use 'overtone.live) it boots up an internal SuperCollider server. I thought that I would need to handle its graceful shutdown myself, but the wise folks at Overtone already handled that. Deep in the bowels of Overtone’s SuperCollider client is defonce _shutdown-hook. Clever.

Next week I’ll hopefully get to make some sounds.

Bye!
- Mark