Fixing A Swift Memory Leak

I wrote a Mac menu bar app for internal use at Etsy that show the status of a number of internal systems and pops up a notification if any of them change status—for example, if a problem is detected and someone puts a hold on our deployment queue.

Some months ago, a colleague noticed the app was leaking memory. I put off looking at it until recently, when I noticed that my own instance was eating up 3+ GB of memory.

Fortunately, Apple provides some great tools for tracking down leaks. One is Instruments, an incredibly powerful tool that I know thiiis (puts thumb and forefinger a millimeter apart) much about. But I know enough to inspect a process for leaks.

I started Instruments, selected the Leaks profiling template, and attached it to my leaky app, and saw this:

Holy leak, Batman!

Yup, that’s a leak. You can also drill down to see where the leak is coming from:

Drilling down to the leak

So the leak is obviously in the networking code, seemingly in Apple’s part of it.

I should pause here to say a bit about how the app works. It’s pretty simple: every 3 seconds, it fetches an internal status web page, parses out the statuses from an HTML table (writing a JSON endpoint for the status page is on my todo list!), and updates the menu accordingly.

The networking code in the app was pretty simple:

let session = NSURLSession.sharedSession()
let task = session.dataTaskWithURL(url) { data, response, error in
    // check for error, do something with the data

NSURLSession was introduced with in Mavericks and iOS 7 as a replacement for NSURLConnection.

I tried setting the cache sizes to 0, and invalidating the session when I was done, as recommended in this article, which seems to describe the same leak:

let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
configuration.URLCache = NSURLCache(memoryCapacity: 0, diskCapacity: 0, diskPath: nil)
let session = NSURLSession(configuration: configuration)
defer {
let task = session.dataTaskWithURL(url) { data, response, error in
    // ...

But this didn’t help. What did end up working was using an ephemeral session:

let session = NSURLSession(configuration: NSURLSessionConfiguration.ephemeralSessionConfiguration())
let task = session.dataTaskWithURL(url) { data, response, error in
    // ...

With that in place, here’s what Instruments showed me:

No leaks!

Yay! No leaks!

We Don’t Need No Stinkin’ GUI

If you’re more of a command-line gal, there’s another way to investigate leaks built into OS X: the leaks command:

$ leaks EtsyInternalStatus
Process:         EtsyInternalStatus [24147]
Path:            /Applications/
Load Address:    0x105eaf000
Identifier:      com.etsy.EtsyInternalStatus
Version:         1.2.3 (1.2.3)
Code Type:       X86-64
Parent Process:  ??? [1]

Date/Time:       2015-09-02 20:33:26.173 -0700
Launch Time:     2015-09-02 20:30:44.447 -0700
OS Version:      Mac OS X 10.10.5 (14F27)
Report Version:  7
Analysis Tool:   /Applications/
Analysis Tool Version:  Xcode 7.0 (7A192o)

leaks Report Version:  2.0
Process 24147: 25225 nodes malloced for 15639 KB
Process 24147: 615 leaks for 5321984 total leaked bytes.
Leak: 0x10a02f000  size=135168  zone: MallocHelperZone_0x105f65000  length: 5020
<snipped 7800 lines of leaked memory dumps>

Now, the fixed version:

$ leaks EtsyInternalStatus
Process:         EtsyInternalStatus [85066]
Path:            /Applications/
Load Address:    0x102d2f000
Identifier:      com.etsy.EtsyInternalStatus
Version:         1.2.4 (1.2.4)
Code Type:       X86-64
Parent Process:  ??? [1]

Date/Time:       2015-10-10 10:27:29.556 -0700
Launch Time:     2015-10-10 10:16:11.476 -0700
OS Version:      Mac OS X 10.10.5 (14F27)
Report Version:  7
Analysis Tool:   /Applications/
Analysis Tool Version:  Xcode 7.0.1 (7A1001)

leaks Report Version:  2.0
Process 85066: 62741 nodes malloced for 6598 KB
Process 85066: 0 leaks for 0 total leaked bytes.

The eagled-eyed among you might have noticed that about a month elapsed between me running those checks (life intervened), and there was an Xcode release in the meantime. Turns out that when I ran the leak check again on the leaky version of the app, it was fine! I’m guessing that it was a bug in Apple’s networking code after all, and it got fixed in the latest release.

Try out leaks on some of the other apps you have running. You’ll find that leak-free apps are oddly rare, and some are downright eggregious (my beloved Sublime Text text editor, in which I’m writing this post, shows “577841 leaks for 44554352 total leaked bytes.” Oof).

For more information, see Apple’s documentation on Finding Memory Leaks.

On "Working" Outside of Work Hours

Etsy’s culture puts a very strong emphasis on work-life balance. As someone who has mostly worked for startups, this took some adjusting to. The fact that Etsy IRC and email pretty much goes silent outside of normal business hours struck me as odd initially. But, I really appreciate it. Startups are stressful, and they left me feeling that there was always something work-related that I should be doing whenever I had some “free time.”

However, I love my work. I love the people I work with and the projects I work on. I love solving problems. I love coding. I wish I had more time to spend doing it. Having a family puts a massive dent in your free time. Don’t get me wrong, I love my family and am [usually] happy to spend any available time with them, but I so miss having large blocks of time to myself to just get into the zone.

So when I or other colleagues mention coding (or even checking email) after hours, or on vacation, we’re often castigated for doing so. I love that my colleagues are looking out for my well-being, but hey, we’re all individuals. If you have no desire to be anywhere near a computer outside of work hours, that’s awesome. It’s pretty cool to work at a company where that’s not only possible, but encouraged, isn’t it? But please don’t judge me for thinking that my ideal vacation would be go somewhere alone, with my laptop and a fast internet connection, and just code.1

If you’ll excuse me, I’ve got to get to bed. I like to wake up crazy early so I have a couple hours of alone time with my laptop.

  1. There is an argument that a senior engineer working after-hours sets a bad example for more junior engineers, making them feel like they should be doing the same. This is certainly something to be cognizant of, but so long as the prevailing culture makes it clear that this is in no way expected behavior, it should not be an issue.

Two Tools for Packaging Mac Apps

I ran across two apps yesterday that make packaging Mac apps for distribution a breeze:

To easily build a professional DMG-based installer, look no further than DMG Canvas. There’s a free version with some limitations, but honestly $15 is a bargain for such a useful tool.

For more complex installs, you might need to create a .pkg file, in which case grab the free (and ungoogleable) Packages app by Stéphane Sudre.

Write a Mac Menu Bar App in Swift

This past week I taught an Etsy School class on how to write an OS X menu bar (aka status bar) app in Swift, and the walk-through is now public. I hope you find it useful.

If you have any suggestions on improving it, let me know. (Or better yet, submit a PR!)

Sublime Text Guide

I’m a big fan of the Sublime Text text editor, and I’ve become one of its biggest evangelists at Etsy. A few months ago I gave an internal presentation/tutorial session on it, and used those notes to create a guide on our internal wiki. I’ve ported that guide to my blog so I can spread the Good Word to others. Since the source code for my blog is available publicly, I’m also hoping that people will help make it even better.

So head over to my Sublime Text Guide and let me know what you think.