Unix Signals with Clojure (1.10-beta5), JDK11, and Kubernetes (1.12)

Kubernetes has a beta feature where you can share the process namespaces for a pod, allowing you to send, say, SIGHUP to another container in the pod, assuming you set the relevant permissions in the pod manifest.

Clojure and JDK11 allow you to catch signals, through sun.misc.Signal, (despite years of threatening to take it away).

So you can catch SIGHUP with the following:

(defn set-signal-handler!
  "Catch HUP, INT, or USR1, etc. and run a one argument handler that
  accepts a sun.misc.Signal"
  [sig f]
  (sun.misc.Signal/handle
   (sun.misc.Signal. sig)
   (proxy [sun.misc.SignalHandler] []
     (handle [received-signal]
       (f received-signal)))))

Just for reference, you can get your pid via

 (.pid (java.lang.ProcessHandle/current))

If you need to send a signal from Clojure, I'd suggest shelling out, as to the best of my knowledge, the JVM only lets you send SIGTERM and SIGKILL:

(clojure.java.shell/sh "/bin/kill" "-s" "SIGINT" "<some-pid>")

or

(clojure.java.shell/sh "/usr/bin/pkill" "-HUP" "some-process-name")

Here is the relevant Kubernetes information with the pod configuration, which basically amounts to setting shareProcessNamespace: true at the top of the pod spec, and adding the following to the container that will be sending the signal:

securityContext:
      capabilities:
        add:
        - SYS_PTRACE

Also, if you've come this far, you should take a look at https://github.com/pyr/signal

Note that java has a flag that seems to be related.  I found it after I wrote the original post.

-XX:+AllowUserSignalHandlers

Enables installation of signal handlers by the application. By default, this option is disabled and the application isn’t allowed to install signal handlers.

Referenced Javadocs:
java.lang.ProcessHandle

Friend, oauth2, JWT, and google.

A quick placeholder for showing how to put them together, without added extra dependencies.

View gist

I'll discuss it shortly.

Towards a better dir function in Clojure. (pt. 1)

Coming from Python, for the most part, I felt right at home in the Clojure REPL. However, one of my trustiest old tricks in the Python interpreter didn't work nearly as well in Clojure.

I'm referring, of course, to the "what the hell was that called again?" fixer: dir.

In Python, dir will get you the fns/classes in a namespace, but the first major obstacle in clojure is the lack of support for namespace aliases.

Let's fix that, shall we?

Looking at the source for the dir macro, we find:

user> (source dir)
(defmacro dir
  "Prints a sorted directory of public vars in a namespace"
  [nsname]
  `(doseq [v# (dir-fn '~nsname)]
     (println v#)))

Ok, this is really no more than a bit of sugar, to save us having to quote the namespace and to print the vars on their own line.  We'll have to dig deeper for some meat.

user> (source dir-fn)
(defn dir-fn
  "Returns a sorted seq of symbols naming public vars in
  a namespace"
  [ns]
  (sort (map first (ns-publics (the-ns ns)))))

One thing I really love about Lisps: many times when I look under the hood, I'm expecting a monstrous Rube Goldberg machine, but what I find is some beautifully simple thing, that will still be pretty much the same thing in 50 years.  (Go back and read some of the early algorithmic lisp code, it comes very easy.)

The first part of dir that bugs me is you must specify the full namespace, even if you've aliased it to something much simpler.

(ns yournamespace
  (:require [clojure.string as str]
            [clojure.repl :refer :all]))

(dir str)
=> Exception No namespace: str found  clojure.core/the-ns (core.clj:3830)

user> (dir clojure.string)
blank?
capitalize
escape
...

You can get aliases via ns-aliases, so...

(defn alias-dir-fn [nsname]
  (-> (ns-aliases *ns*)
      (get nsname) ; look up the alias or nil trying
      (or nsname)
      dir-fn))

(defmacro alias-dir
  "Prints a sorted directory of public vars in a namespace or an alias
  namespace."
  [nsname]
  `(doseq [v# (alias-dir-fn '~nsname)]
     (println v#)))

Now we can...

user> (alias-dir str)
blank?
capitalize
escape
...

But I'm not done with dir yet.  Stay tuned for part 2.

Edit: I've filed a ticket for this functionality to be added to Clojure

Using memoization to change a creation function into get-or-create.

N.B. This should work in any language with first class functions, memoization, and immutable values.

tl;dr:  In the past I'd always thought of memoization as a way to save the computer work. It hadn't occurred to me that it could also save me work.

I was re-reading "The Joy of Clojure" and came across a gem I'd missed the first time. Listing 14.12 is described as "A function to create or retrieve a unique Agent for a given player name".

(def agent-for-player
  (memoize
    (fn [player-name]
      (-> (agent [])
          (set-error-handler! #(println "ERROR: " %1 %2))
          (set-error-mode! :fail)))))

;; The above doesn't quite work for me, set-error-handler!
;; doesn't seem to return the agent. Doesn't make the pattern
;; less compelling, though.

The authors comment that this allows you to maintain player-name as an index into a table of agents without having to explicitly manager and lookup agents.

There are two caveats to this approach: 1) player-name must be immutable, and 2) You really need to understand the memoization mechanism.  clojure.core/memoize, for instance, will keep a internal map of args/response until the end of time. You could use http://clojure.github.io/core.memoize/ to modify the strategy if you so choose.

The place where I'd try this first is in what I call "micro-logs". Frequently as I'm working, I want to log some data to a side channel, and this pattern saves having to manage this manually and cluttering up my code.

(def get-or-create-micro-log
  (memoize
   (fn [file]
     (io/make-parents file)
     (.createNewFile file)
     (-> (io/writer file :append true)
         (agent
          :error-mode :fail
          :error-handler #(println "ERROR: " %1 %2))))))

(defn microlog
  "Useful micro-pattern to send off a write to various
  files via agents without having to maintain a lookup
  table. Symlinks can get you into trouble; at a 
  minimum they will duplicate the agent."
  [lg line]
  (let [a (-> (io/file lg)
              (.getAbsoluteFile)
              (get-or-create-micro-log))
        output (str (str/trim-newline line) "\n")]
    (send-off a
          (fn [writer]
            (doto writer
              (.write output)
              (.flush))))))

(defn microlog-all
  [lg all-data]
  (doseq [d all-data]
    (microlog lg d)))

Edits: Used io/make-parents instead of File calls. Changed send to send-off, since this is I/O. Cleaned up creation of agent.

Om examples: Scrubbing Calculator component

I'm starting to get the hang of Om.  While I've used cljs & jayq for a while, understanding the right way to use om + core.async has taken a bit of doing.  They're both great libraries, and have fairly small APIs, but I've been hungry for fairly simple examples.  In that vein, here are a couple, the second inspired by Bret Victor's Scrubbing Calculator.

Update: git repo with examples

https://github.com/jwhitlark/scrubber-example

Prelude: simple clock

Simple enough, derived from the animation example in om examples.

Scrubbing Int, Approach one: all local state

Two utility functions:

My first attempt entirely used internal state.  Not terribly useful, but it set the stage.

It works entirely in the span element.  Click and drag changes the value. If you drag out of the element before you release the mouse button, the state becomes inconsistent.  The snippet below will let you tune the sensitivity if you use it in the :onMouseMove handler.

Now

Approach two: Moving out the state

So here I've switched things to use an external atom for the value, and moved most of the logic into event listeners under IWillMount.  This works correctly even if you drag off the span element.  It's pretty much everything I set out to do with this.

The only drawback is the event listeners should be connected in the handler for onMouseDown, and removed when "mouseup" is received. I'll update it when I've figured out how to remove existing listeners.

Afterward: Stuff you should be looking at

• devcards are awesome, a great way to prototype this kind of stuff
• Prismatic/om-tools
• om-sweet-om-high-functional-frontend-engineering-with-clojurescript-and-react/
• Isomorphic Clojurescipt, and his example omelette

DataScript might be a great idea for Datomic

I was looking at DataScript, and the simple implementation gave me (I think) a lot of insight into Datomic.  If Tonsky's post is correct, and there is a core to Datomic that could be used as an in-memory Datomic-like thing, which could be used as easily as, say, core.logic, and that I could look under the hood to see how a simple case works, I would be in a much better place.  A big part of the problem is the catch-22 for learning datalog, easy to toy with, don't know where to go from there.

I'm not against spending money.  A few months back I went to an all-day Datomic class, and thought it well worth the $300 or so I spent on it.  I wouldn't be against talking people into paying for Datomic, but I'm not familiar with it to know when I should.  The main problem is that I haven't spent enough time playing with it to get a good feel for where it fits best.  Spinning up a separate server process just doesn't really work for that kind of playing.

Sort of like how I understood garbage collection, but once I implemented a toy lisp, I *got* it, and understood it's tradeoffs in a way I never had before.

I don't know what Datomic's sales numbers look like, but this seems like it would be a good strategy to increase interest/familiarity with the protocols and ideas involved, which would presumably increase the sales pipeline.

.emacs style Clojure rc files

The following will let you run clojure code in the current namespace, from a dotfile in your home directory.

(require '[clojure.java.io :as io])

(load-string (slurp (io/file (System/getenv "HOME") *rc-file*)))