Caius Theory

Now with even more cowbell…

Stop HealthKit causing SIGABRT

You have some crazy idea for an iOS app that uses HealthKit so you fire up Xcode, create a new project & add the HealthKit entitlement. Follow the tutorial to request authorization from the HKHealthKitStore. Hit run to make sure the app compiles and find that it instantly crashes with a SIGABRT in AppDelegate.

Puzzled by this you go over the minimal amount of code you've added and pare it right down to just the HKHealthKitStore.requestAuthorization call which is still causing the SIGABRT as soon as the app tries to boot.

The missing piece of the puzzle is Info.plist needs a key adding to it for the HealthKit authorisation screen. The documentation helpfully forgets to mention this however. Here's some quick simple steps to fix it:

  1. Open Info.plist in Xcode
  2. Click the (+) at the top to add a new key/value to the file
  3. Enter "Privacy - Health Share Usage Description" for the key
  4. Enter a useful message to the user explaining why they should allow access to their healthkit data for your app for the value
  5. Run your app and see the HealthKit authorisation sheet appear

NB: if you want to update/write any data to healthkit, you'll need to add the "Privacy - Health Update Usage Description" key with a description as well.

Remove OS X Disk Password

I recently reinstalled a laptop and in doing so setup full disk encryption in a slightly strange fashion. The basic flow I followed was:

  1. Boot into Recovery mode (hold ⌘-R at boot)
  2. Erase the internal HD as HFS+ (Journaled, Encrypted) and set a disk password
  3. Install OS X onto the internal disk
  4. During setup, use Migration Assistant to copy clone containing previous install data from backup disk

This worked great in the end, once I'd recompiling various utilities I had installed. (Downside of moving from one CPU arch to another - can't just copy all compiled binaries over.)

However, I failed at step 2 above and entered "password" as my disk password since it was only intended to be temporary. Usually OS X's full disk encryption (FileVault 2) allows the machine users to unlock the disk, and not a standalone password. Due to the slightly odd way I setup the machine, I had the option of either using the disk password or my user account's password.

Having hunted around trying to find how to change or remove this disk password and leave only my users password, I finally stumbled across the magic incantations in an apple discussion thread asking How to disable "Disk Password" on boot?.

The magic incantations are as follows:

  1. List all the passwords that can currently unlock the drive

    Make sure there is a second password listed or removing the disk password will lock you out of the disk.

     $ sudo fdesetup list -extended
     ESCROW  UUID                                                    TYPE USER
             28376DDE-B6E1-48BE-A06F-4212067581D6    Disk Passphrase User
             4DBF8CEF-40F7-4F00-902F-A47AA643C656                 OS User caius
  2. Note the UUID of the "Disk Passphrase" entry, and remove that from the list

     sudo fdesetup remove -uuid 28376DDE-B6E1-48BE-A06F-4212067581D6
  3. List the passwords again to make sure the Disk Passphrase entry was removed

     $ sudo fdesetup list -extended
     ESCROW  UUID                                                    TYPE USER
             4DBF8CEF-40F7-4F00-902F-A47AA643C656                 OS User caius

Hey presto, only your user is left being able to unlock the disk.

Find dependencies blocking rails upgrades

The initial pain point when upgrading a rails app is figuring out which of your dependencies are blocking you upgrading the actual rails gem (& immediate dependencies, actionpack, etc.). One way to start this is to update the rails dependency in your Gemfile and run bundle update rails. Then check the error output (it never works first time…) to see which gems are blocking the upgrade. Repeat, rinse until it works.

I figured I'd cheat a little and eyeball the Gemfile.lock to see which gems had an explicit dependency pinning rails (or actionpack, activejob, etc) to a version lower than I want to upgrade to, so I could get an idea of what needs to be upgraded without having to do them all one-by-one.

Then instead of eyeballing Gemfile.lock, I wrote an awk script to pull out the interesting dependencies (ie, anything that depends on rails gems) so I just have to check which versions they depend on by hand.

# Reads a Gemfile.lock and outputs all dependencies that depend on rails

  parent = 0
  parent_printed = 0
  rails_gems = "^(rail(s|ties)|action(mailer|pack|view)|active(job|model|record|support))$"

# We only want the specs from the GEM section
NR == 1, $1 ~ /GEM/ { next }
$1 == "" { exit }

# Skip parent gems we don't care about (rails itself…)
$0 ~ /^ {4}[^ ]/ &&
$1 ~ rails_gems {
  parent = 0
  parent_printed = 0

# Parent gems that aren't part of rails core
# Store the name to be printed if we match below
$0 ~ /^ {4}[^ ]/ {
  parent = $0
  parent_printed = 0

# If the nested gem (6 space prefix) matches rails-names and we have a parent value
# set then we print them out - making sure to only print the parent once
$0 ~ /^ {6}[^ ]/ &&
$1 ~ rails_gems &&
parent != 0 {
  if (parent_printed == 0) {
    parent_printed = 1
    print parent

  print $0

Run it against your Gemfile.lock for the app you're upgrading:

awk -f rails5.awk Gemfile.lock

And you'll get output like this, to run through and see if any of the dependencies are pinning to lower versions than you need.

    coffee-rails (4.0.1)
      railties (>= 4.0.0, < 5.0)
    factory_girl (4.4.0)
      activesupport (>= 3.0.0)
    factory_girl_rails (4.4.1)
      railties (>= 3.0.0)
    globalid (0.3.7)
      activesupport (>= 4.1.0)
    google-api-client (0.8.6)
      activesupport (>= 3.2)
    jquery-rails (3.1.4)
      railties (>= 3.0, < 5.0)
    jquery-ui-rails (5.0.5)
      railties (>= 3.2.16)
    rails-deprecated_sanitizer (1.0.3)
      activesupport (>= 4.2.0.alpha)
    rails-dom-testing (1.0.7)
      activesupport (>= 4.2.0.beta, < 5.0)
    rspec-rails (3.4.2)
      actionpack (>= 3.0, < 4.3)
      activesupport (>= 3.0, < 4.3)
      railties (>= 3.0, < 4.3)
    sass-rails (4.0.5)
      railties (>= 4.0.0, < 5.0)
    sprockets-rails (2.3.3)
      actionpack (>= 3.0)
      activesupport (>= 3.0)

In this case, I'm trying to take this app to rails 5.0, so all the ones specifying < 5 and < 4.3 need upgrading beforehand.

SmartOS Recovery mount /usbkey

Recently I managed to hose a box in a perfectly self-inflicted storm of idiocy. Imagine a SmartOS server with the following issues:

Needless to say, this caused a tiny issue in the server doing what it's supposed to. Luckily I had access to a KVM remote console for the box and the following worked.

I brought the machine up, choosing the second option for recovery at the grub menu. Waited for a login prompt, then logged in with root/root.

Realised quite quickly that /usbkey must be persisted on the zones zfs pool otherwise the configuration would be lost after shutdown, so imported the correct pool, created a directory to mount into and then mounted the zfs share.

zpool import zones
mkdir /usbkey
mount -F zfs zones/usbkey /usbkey

Much Wenlock Triathlon 2016

500m Pool Swim: 00:11:34

Felt good setting off, within a couple of lengths felt like I had no energy. Didn’t have enough to eat/drink during the morning before starting, really makes a huge difference once you set off. Must remember to replace my goggles at some point too, the strap comes loose without warning.

Transition 1: 00:03:06

Rough time for transition. Legs felt good as soon as I was out the pool. Got my kit on without too much trouble, although taking a t-shirt style cycling top instead of a full zip jacket was a mistake—forgot I’d be putting it on wet. Had a gel in transition in the hope it would give me some energy.

19km Cycle: 00:48:03

Lovely start to the cycle route, massive downhill on recently resurfaced roads for the most part. Best part of the cycle ride, with the rest of the route consisting of climbing back up to the start. Couple of nasty steep hills I had to walk sections of, for all I fitted lower gears they still weren't low enough given my lack of bike fitness currently.

Transition 2: 00:06:57

Bike racked easily, shoes swapped, gel necked and a couple of Lucozade bottles clutched and off to run. Took on another gel.

7km Run: 01:04:37

Needed all the fluid I carried, didn't take anything else during the run and felt no less energetic than the other disciplines. Legs didn't really hurt, just felt like lead and had no power. Route was basically a mini trail run then up a tarmac road to the turning point then head back via trail run to finish. Adopted a walk/run approach.

Total: 02:14:17

Certainly the hardest triathlon I've done, and I'm probably at my least fittest compared to any of the others I've done to boot. The location was lovely, the weather was pretty decent (sunny but not too hot). Don't feel like I was beaten by it, but definitely haven't given it my best. One to redo next year and train towards.

Setup DHCP interface in FreeBSD

Given a FreeBSD instance without a configured network interface that you'd like to configure, first check what the name of the interface you want to configure is with ifconfig. (Mine is em0 in this instance.)

Then we need to add the configuration telling services that we want to use DHCP for this interface, and setting up our default router (use your IP, not mine!) too:

cat >> rc.conf <<CONF

And then we need to start dhclient on the given interface:

service dhclient start em0

Hey presto, you should see dhclient finding a DHCP server and being handed an IP address for em0.

SoundCloud RSS Feeds

SoundCloud appears to have gained popularity in recent times for hosting podcasts on. As a consumer of their service they're pretty good at everything except having a visible RSS feed on a profile page for a show! If I want to listen to a show in my podcast app of choice, an RSS feed is the easiest way for me to achieve that.

Turns out SoundCloud do have RSS feeds, they're just well hidden and unfindable from the profile page itself. Thankfully, you can construct the URL for it from information on the profile page, and here's a bookmarklet that will do it for you:


Or as a handy link to copy to your bookmarks bar. Simply click/run that when on a SoundCloud profile page and you'll be taken to the RSS Feed URL.

exec(3) in Go

Using exec(3) from Go is simple enough, once you figure out to look in the syscall package and how to pass arguments to the new command.

As a simple example, I'm going to exec /bin/echo with a hardcoded string from the go binary. The program built here is in the gecho (Gecko, geddit?) git repo, which each stage as a commit.

In our main function lets setup some variables we're going to need for arguments to syscall.Exec:

func main() {
  cmdPath := "/bin/echo"
  cmdArgs := []string{"Hello", "World"}
  cmdEnv := []string{}    

(We could use os.Environ() for cmdEnv to take the ENV from the go binary, but we don't require anything from the environmnt here so it doesn't matter that we aren't.)

Now we have the arguments for syscall.Exec, lets add that in and see what happens:

err := syscall.Exec(cmdPath, cmdArgs, cmdEnv)
if err != nil {

And running the file (go run gecho.go compiles & runs for us) gives the following output:


Err, say what now? Where's "Hello" gone?!

Took me a while to figure this out when I originally ran into this. The answer is staring us right in the face if we go look at the syscall.Exec docs. Lets have a look at the function signature, argument names and all:

func Exec(argv0 string, argv []string, envv []string) (err error)

Hmm. The first argument is argv0 (and a string), rather than binaryPath or something similar. The second argument is then argv and an array of strings.

At this point I remember that the first element of argv in other runtimes is the name of the binary or command invoked - $0 in a bash script is the name of the script for example.

The answer is simple. cmdArgs in our script should have /bin/echo as the first element, and then we pass cmdArgs[0], cmdArgs as the first two arguments to syscall.Exec. Lets give that a go:

func main() {
  cmdArgs := []string{"/bin/echo", "Hello", "World"}
  cmdEnv := []string{}

  err := syscall.Exec(cmdArgs[0], cmdArgs, cmdEnv)
  if err != nil {

And running it (go run gecho.go remember) gives the expected output:

Hello World

Excellent. Now I just need to remember argv contains the command name as argv[0] and we're golden.

There is also the os/exec package in the stdlib, which is intended for executing other binaries as child processes from what I can tell. Tellingly, when you create a exec.Cmd struct with exec.Command() you give it the name as first argument, and args as following arguments. Then it has the following snippet in the documentation:

The returned Cmd's Args field is constructed from the command name followed by the elements of arg, so arg should not include the command name itself. For example, Command("echo", "hello")

So cmd := exec.Command("echo", "hello"); cmd.Args would return []string{"echo", "hello"} - which is recognisable as what we have to pass to syscall.Exec!

Changing hostname in SmartOS Zone

Given a non-global zone in SmartOS that we want to change the hostname of, we need to make sure to edit the following files to change it:

A quick way to do that is with sed (renaming "fred" to "beth" here):

sed -e 's/fred/beth/g' -i /etc/hosts /etc/nodename

Then shutdown & start the zone (from my testing a restart doesn't apply it).

Install Rubinius on OS X

Using ruby-install, homebrew building it for use with chruby, here's how I install Rubinius under Yosemite (works for Mavericks as well.)

  1. Make sure llvm is installed

     $ brew install llvm
  2. Prepend the homebrew-installed llvm tools to your path

     $ export PATH="$(brew --prefix llvm)/bin:$PATH"
     # Or, for ZSH
     $ path=( $(brew --prefix llvm)/bin $path )
  3. Install rubinius, v2.3.0 at the time of writing

     $ ruby-install rbx 2.3.0
  4. Open a fresh shell once that's built, and you should be able to switch to rbx!

     $ chruby rbx
     $ ruby -v
     rubinius 2.3.0 (2.1.0 9d61df5d 2014-10-31 3.5.0 JI) [x86_64-darwin14.0.0]

There is also a homebrew tap for rubinius which should also work instead of the above. I couldn't get it working on one of my laptops though, which is why I was installing by hand using the above instead. The tap is at and explains install steps.