Michael Kelly

Q is Scary

June 8, 2017 mozilla

q is the hands-down winner of my "Libraries I'm Terrified Of" award. It's a Python library for outputting debugging information while running a program.

On the surface, everything seems fine. It logs everything to /tmp/q (configurable), which you can watch with tail -f. The basic form of q is passing it a variable:

import q

foo = 7

Take a good long look at that code sample, and then answer me this: What is the type of q?

If you said "callable module", you are right. Also, that is not a thing that exists in Python.

Also, check out the output in /tmp/q:

0.0s <module>: foo=7

It knows the variable name. It also knows that it's being called at the module level; if we were in a function, <module> would be replaced with the name of the function.

You can also divide (/) or bitwise OR (|) values with q to log them as well. And you can decorate a function with it to trace the arguments and return value. It also has a method, q.d(), that starts an interactive session.

And it does all this in under 400 lines, the majority of which is either a docstring or code to format the output.


How in the Hell

So first, let's get this callable module stuff out of the way. Here's the last two lines in q.py:

# Install the Q() object in sys.modules so that "import q" gives a callable q.
sys.modules['q'] = Q()

Turns out sys.modules is a dictionary with all the loaded modules, and you can just stuff it with whatever nonsense you like.

The Q class itself is super-fun. Check out the declaration:

# When we insert Q() into sys.modules, all the globals become None, so we
# have to keep everything we use inside the Q class.
class Q(object):
    __doc__ = __doc__  # from the module's __doc__ above

    import ast
    import code
    import inspect
    import os
    import pydoc
    import sys
    import random
    import re
    import time
    import functools

"When we insert Q() into sys.modules, all the globals become None"

What? Why?! I mean I can see how that's not an issue for modules, which are usually the only things inside sys.modules, but still. I tried chasing this down, but the entire sys module is written in C, and that ain't my business.

Most of the other bits inside Q are straightforward by comparison; a few helpers for outputting stuff cleanly, overrides for __truediv__ and __or__ for those weird operator versions of logging, etc. If you've never heard of callable types1 before, that's the reason why an instance of this class can be both called as a function and treated as a value.

So what's __call__ do?

Ghost Magic

def __call__(self, *args):
    """If invoked as a decorator on a function, adds tracing output to the
    function; otherwise immediately prints out the arguments."""
    info = self.inspect.getframeinfo(self.sys._getframe(1), context=9)

    # ... snip ...

Welcome to the inspect module. Turns out, Python has a built-in module that lets you get all sorts of fun info about objects, classes, etc. It also lets you get info about stack frames, which store the state of each subroutine in the chain of subroutine calls that led to running the code that's currently executing.

Here, q is using a CPython-specific function sys._getframe to get a frame object for the code that called q, and then using inspect to get info about that code.

# info.index is the index of the line containing the end of the call
# expression, so this gets a few lines up to the end of the expression.
lines = ['']
if info.code_context:
    lines = info.code_context[:info.index + 1]

# If we see "@q" on a single line, behave like a trace decorator.
for line in lines:
    if line.strip() in ('@q', '@q()') and args:
        return self.trace(args[0])

...and then it just does a text search of the source code to figure out if it was called as a function or as a decorator. Because it can't just guess by the type of the argument being passed (you might want to log a function object), and it can't just return a callable that can be used as a decorator either.

trace is pretty normal, whatever that means. It just logs the intercepted arguments and return value / raised exception.

# Otherwise, search for the beginning of the call expression; once it
# parses, use the expressions in the call to label the debugging
# output.
for i in range(1, len(lines) + 1):
    labels = self.get_call_exprs(''.join(lines[-i:]).replace('\n', ''))
    if labels:
self.show(info.function, args, labels)
return args and args[0]

The last bit pulls out labels from the source code; this is how q knows the name of the variable that you pass in. I'm not going to go line-by-line through get_call_exprs, but it uses the ast module to parse the function call into an Abstract Syntax Tree, and walks through that to find the variable names.

It goes without saying that you should never do any of this. Ever. Nothing is sacred when it comes to debugging, though, and q is incredibly useful when you're having trouble getting your program to print anything out sanely.

Also, if you're ever bored on a nice summer evening, check out the list of modules in the Python standard library. It's got everything:

  1. Check out this page and search for "Callable Types" and/or __call__.

Build and Sign WebExtensions with CircleCI

May 4, 2017 mozilla

Once Planet Mozilla updated with my last post, I got a few bug reports and feature requests for mailman-admin-helper, along with a pull request (Thanks, TheOne!). Clearly I'm not the only person who isn't a fan of our mailing list admin.

Before landing anything, I decided to see if I could get automatic builds running so that I wouldn't have to pull a build pull requests myself when I want to test them. What I ended up with, however, does a bit more than that; it also runs lints, and even signs and uploads new releases when I push a new tag.

We use CircleCI on Normandy, so I defaulted to using them for this as well. I'll walk through the sections, but here's the entire circle.yml file I ended up with:

    version: 7.10.0
    - sudo apt-get update; sudo apt-get install jq
    - go get -u github.com/tcnksm/ghr
    - npm install -g web-ext
    - web-ext build
    - mv web-ext-artifacts $CIRCLE_ARTIFACTS
    - web-ext lint --self-hosted
    tag: /v[0-9]+(\.[0-9]+)*/
    owner: Osmose
      - jq --arg tag "${CIRCLE_TAG:1}" '.version = $tag' manifest.json > tmp.json && mv tmp.json manifest.json
      - web-ext sign --api-key $AMO_API_KEY --api-secret $AMO_API_SECRET
      - ghr -u Osmose $CIRCLE_TAG web-ext-artifacts

If you want to adapt this to your own project, you'll want to change the deployment.release.owner field to the Github account hosting your WebExtension, and add the following environment variables to your CircleCI project config (NOT your circle.yml file, which is committed to your repo):

How does it work?

circle.yml files are split into phases. Each phase has a default action that is overridden with the override key.

    version: 7.10.0

The machine phase defines the machine used to run your build. Here we're just making sure that we have a recent version of Node.

    - sudo apt-get update; sudo apt-get install jq
    - go get -u github.com/tcnksm/ghr
    - npm install -g web-ext

The dependencies step is for installing libraries and programs that your build needs. Our build process has three dependencies:

    - web-ext build
    - mv web-ext-artifacts $CIRCLE_ARTIFACTS

The compile step is used to build your project before testing. While we aren't running any tests that need a built add-on, this is a good time to build the add-on and upload it to the $CIRCLE_ARTIFACTS directory, which is saved and made available for download once the build is complete. This makes it easy to pull a ready-to-test build of the add-on from open pull requests.

    - web-ext lint --self-hosted

The test step is for actually running your tests. We don't have automated tests for mailman-admin-helper, but web-ext comes with a handy lint command to help catch common errors.

One thing to note about CircleCI is that any commands that return non-zero return codes will stop the build immediately and mark it as failed, except for commands in the test step. test step commands will mark a build as failed, but will not stop other commands in the test step from running. This is useful for running multiple types of tests or lints because it allows you to see all of your failures instead of exiting early before running all of your tests.

    tag: /v[0-9]+(\.[0-9]+)*/
    owner: Osmose
      - jq --arg tag "${CIRCLE_TAG:1}" '.version = $tag' manifest.json > tmp.json && mv tmp.json manifest.json
      - web-ext sign --api-key $AMO_API_KEY --api-secret $AMO_API_SECRET
      - ghr -u Osmose $CIRCLE_TAG web-ext-artifacts

The deployment section only runs on successful builds, and handles deploying your code. It's made up of multiple named sections, and each section must either have a branch or tag field describing the branches or tags that the section will run for.

In our case, we're using a regex that matches tags named like version numbers prefixed with v, e.g. v0.1.2. We also set the owner to my Github account so that forks will not run the deployment process.

The commands do three things:

  1. Use jq to modify the version key in manifest.json to match the version number from the tag. The v prefix is removed before the replacement.

  2. Use web-ext to build and sign the WebExtension, using API keys stored in environment variables. This creates an XPI file in the web-ext-artifacts directory.

  3. Use ghr to upload the contents of web-ext-artifacts (which should just by the signed XPI) to the tag on Github. This uses the GITHUB_TOKEN environment variable for authentication.

The end result is that, whenever a new tag is pushed to the repository, CircleCI adds a signed XPI to the release page on Github automatically, without any human intervention. Convenient!

Feel free to steal this for your own WebExtension, or share any comments or suggestions either in the comments or directly on the mailman-admin-helper repository. Thanks for reading!

mailman-admin-helper: Mildly Easier Mailman Spam Management

April 30, 2017 mozilla

Mozilla hosts a few Mailman instances1, and I run a few mailing lists on them. Our interface for managing incoming spam is... okay.

Mailman default admindb interface

The form inputs are tiny. And it takes, like, 3 clicks to discard and blacklist spam per-sender. And, because I only learned about the options for filtering by spam headers within the past month, I had to use this interface on a daily basis for years.

Finally, about a year or so ago, I got fed up and wrote a bookmarklet that auto-clicked every form element needed to discard and blacklist every email on the page. Since it's rare for the lists I moderate to get legitimate emails that are marked for moderation, I didn't need anything more complex.

However, we recently updated our Mailman pages to use CSP, specifically the script-src none directive. Because the pages no longer accept any URL as valid for script execution, my bookmarklet stopped working. I searched online for workarounds and didn't find anything informative2.

Luckily, I happen to have experience making WebExtensions that inject content scripts into web pages. It's as simple as creating a manifest.json file:

  "manifest_version": 2,
  "name": "mailman-admin-helper",
  "version": "0.1.1",
  "applications": {
    "gecko": {
      "id": "mailman-admin-helper@mkelly.me"

  "description": "Adds useful shortcuts to Mozilla Mailman admin.",

  "content_scripts": [
      "matches": [
      "js": ["index.js"],
      "css": ["index.css"]

The content_scripts key is where the magic happens. List some domains, write some JavaScript and CSS, and you're done! The web-ext tool makes testing, building, and signing the extension pretty painless.

An hour or two later, and I had finished my new WebExtension, mailman-admin-helper. After it is installed, the admin interface is greatly simplified:

Mailman admindb interface as modified by the mailman-admin-helper extension

The block of checkboxes and radio buttons has been replaced by 4 buttons that immediately make their changes and refresh the page when clicked. And if you need to inspect and modify an individual email, you can still click through the email subject to get to the normal moderation page.

Granted, it cuts out a lot of functionality, but this extension is mostly meant for myself to use. Pull requests are welcome, though, in case anyone wants to add functionality that they commonly use.

Big thanks to the Add-ons team and community for making WebExtensions super-easy to use!

  1. I'm not entirely sure why we have two, but it's cool.

  2. I did find bug 866522, which discusses the reason bookmarklets don't work with CSP, as well as some proposed fixes to Firefox and the (in my opinion, correct) wisdom that bookmarklets are a dead-end anyway.


April 25, 2017 mozilla

Recently I found myself trying to comprehend an unfamiliar piece of code. In this case, it was content-UITour.js, a file that handles the interaction between unprivileged webpages and UITour.jsm.

UITour allows webpages to highlight buttons in the toolbar, open menu panels, and perform other tasks involved in giving Firefox users a tour of the user interface. The event-based API allows us to iterate quickly on the onboarding experience for Firefox by controlling it via easily-updated webpages. Only a small set of Mozilla-owned domains are allowed access to the UITour API.

Top-level View

My first step when trying to grok unfamiliar JavaScript is to check out everything at the top-level of the file. If we take content-UITour.js and remove some comments, imports, and constants, we get:

var UITourListener = {
  handleEvent(event) {
    /* ... */

  /* ... */

addEventListener("mozUITour", UITourListener, false, true);

Webpages that want to use UITour emit synthetic events with the name "mozUITour". In the snippet above, UITourListener is the object that receives these events. Normally, event listeners are functions, but they can also be EventListeners, which are simply objects with a handleEvent function.

According to Mossop's comment, content-UITour.js is loaded in browser.js. A search for firefox loadFrameScript brings up two useful pages:

It looks like content-UITour.js is loaded for each tab with a webpage open, but it can do some more privileged stuff than a normal webpage. Also, the global object seems to be window, referring to the browser window containing the webpage, since events from the webpage are bubbling up to it. Neat!

Events from Webpages

So what about handleEvent?

handleEvent(event) {
  if (!Services.prefs.getBoolPref("browser.uitour.enabled")) {
  if (!this.ensureTrustedOrigin()) {
  addMessageListener("UITour:SendPageCallback", this);
  addMessageListener("UITour:SendPageNotification", this);
  sendAsyncMessage("UITour:onPageEvent", {
    detail: event.detail,
    type: event.type,
    pageVisibilityState: content.document.visibilityState,

If UITour itself is disabled, or if the origin of the webpage we're registered on isn't trustworthy, events are thrown away. Otherwise, we register UITourListener as a message listener, and send a message of our own.

I remember seeing addMessageListener and sendAsyncMessage on the browser message manager documentation; they look like a fairly standard event system. But where are these events coming from, and where are they going to?

In lieu of any better leads, our best bet is to search DXR for "UITour:onPageEvent", which leads to nsBrowserGlue.js. Luckily for us, I've actually heard of this file before: it's a grab-bag for things that need to happen to set up Firefox that don't fit anywhere else. For our purposes, it's enough to know that stuff in here gets run once when the browser starts.

The lines in question:

// Listen for UITour messages.
// Do it here instead of the UITour module itself so that the UITour module is lazy loaded
// when the first message is received.
var globalMM = Cc["@mozilla.org/globalmessagemanager;1"].getService(Ci.nsIMessageListenerManager);
globalMM.addMessageListener("UITour:onPageEvent", function(aMessage) {
  UITour.onPageEvent(aMessage, aMessage.data);

Oh, I remember reading about the global message manager! It covers every frame. This seems to be where all the events coming up from individual frames get gathered and passed to UITour. That UITour variable is coming from a clever lazy-import block at the top:

/* ... */
["UITour", "resource:///modules/UITour.jsm"],
/* ... */
].forEach(([name, resource]) => XPCOMUtils.defineLazyModuleGetter(this, name, resource));

In other words, UITour refers to the module in UITour.jsm, but it isn't loaded until we receive our first event, which helps make Firefox startup snappier.

For our purposes, we're not terribly interested in what UITour does with these messages, as long as we know how they're getting there. We are, however, interested in the messages that we're listening for: "UITour:SendPageCallback" and "UITour:SendPageNotification". Another DXR search tells me that those are in UITour.jsm. A skim of the results shows that these messages are used for things like notifying the webpage when an operation is finished, or returning information that was requested by the webpage.

To summarize:

The rest of the content-UITour.js is split between origin verification and sending events back down to the webpage.

Verifying Webpage URLs

Next, let's take a look at ensureTrustedOrigin:

ensureTrustedOrigin() {
  if (content.top != content)
    return false;

  let uri = content.document.documentURIObject;

  if (uri.schemeIs("chrome"))
    return true;

  if (!this.isSafeScheme(uri))
    return false;

  let permission = Services.perms.testPermission(uri, UITOUR_PERMISSION);
  if (permission == Services.perms.ALLOW_ACTION)
    return true;

  return this.isTestingOrigin(uri);

MDN tells us that content is the Window object for the primary content window; in other words, the webpage. top, on the other hand, is the topmost window in the window hierarchy (relevant for webpages that get loaded in iframes). Thus, the first check is to make sure we're not in some sort of frame. Without this, a webpage could control when UITour executes things by loading a whitelisted origin in an iframe1.

documentURIObject lets us check the origin of the loaded webpage. chrome:// URIs get passed immediately, since they're already privileged. The next three checks are more interesting:


isSafeScheme(aURI) {
  let allowedSchemes = new Set(["https", "about"]);
  if (!Services.prefs.getBoolPref("browser.uitour.requireSecure"))

  if (!allowedSchemes.has(aURI.scheme))
    return false;

  return true;

This function checks the URI scheme to see if it's considered "safe" enough to use UITour functions. By default, https:// and about: pages are allowed. http:// pages are also allowed if the browser.uitour.requireSecure preference is false (it defaults to true).


The next check is against the permissions system. The Services.jsm documentation says that Services.perms refers to an instance of the nsIPermissionManager interface. The check itself is easy to understand, but what's missing is how these permissions get added in the first place. A fresh Firefox profile has some sites already whitelisted for UITour, but where does that whitelist come from?

This is where DXR really shines. If we look at nsIPermissionManager.idl and click the name of the interface, a dropdown appears with several options. The "Find subclasses" option performs a search for "derived:nsIPermissionManager", which leads to the header file for nsPermissionManager.

We're looking for where the default permission values come from, so an in-page search for the word "default" eventually lands on a function named ImportDefaults. Clicking that name and selecting "Jump to definition" lands us inside nsPermissionManager.cpp, and the very first line of the function is:

nsCString defaultsURL = mozilla::Preferences::GetCString(kDefaultsUrlPrefName);

An in-page search for kDefaultsUrlPrefName leads to:

// Default permissions are read from a URL - this is the preference we read
// to find that URL. If not set, don't use any default permissions.
static const char kDefaultsUrlPrefName[] = "permissions.manager.defaultsUrl";

On my Firefox profile, the "permissions.manager.defaultsUrl" preference is set to resource://app/defaults/permissions:

# This file has default permissions for the permission manager.
# The file-format is strict:
# * matchtype \t type \t permission \t host
# * "origin" should be used for matchtype, "host" is supported for legacy reasons
# * type is a string that identifies the type of permission (e.g. "cookie")
# * permission is an integer between 1 and 15
# See nsPermissionManager.cpp for more...

# UITour
origin    uitour    1    https://www.mozilla.org
origin    uitour    1    https://self-repair.mozilla.org
origin    uitour    1    https://support.mozilla.org
origin    uitour    1    https://addons.mozilla.org
origin    uitour    1    https://discovery.addons.mozilla.org
origin    uitour    1    about:home

# ...

Found it! A quick DXR search reveals that this file is in /browser/app/permissions in the tree. I'm not entirely sure where that defaults bit in the URL is coming from, but whatever.

With this, we can confirm that the permissions check is where most valid uses of UITour are passed, and that this permissions file is where the whitelist of allowed domains lives.


The last check in ensureTrustedOrigin falls back to isTestingOrigin:

isTestingOrigin(aURI) {
  if (Services.prefs.getPrefType(PREF_TEST_WHITELIST) != Services.prefs.PREF_STRING) {
    return false;

  // Add any testing origins (comma-seperated) to the whitelist for the session.
  for (let origin of Services.prefs.getCharPref(PREF_TEST_WHITELIST).split(",")) {
    try {
      let testingURI = Services.io.newURI(origin);
      if (aURI.prePath == testingURI.prePath) {
        return true;
    } catch (ex) {
  return false;

Remember those boring constants we ignored earlier? Here's one of them in action! Specifically, it's PREF_TEST_WHITELIST, which is set to "browser.uitour.testingOrigins".

This function appears to parse the preference as a comma-separated list of URIs. It fails early if the preference isn't a string, then splits the string and loops over each entry, converting them to URI objects.

The nsIURI documentation notes that prePath is everything in the URI before the path, including the protocol, hostname, port, etc. Using prePath, the function iterates over each URI in the preference and checks it against the URI of the webpage. If it matches, then the page is considered safe!

(And if anything fails when parsing URIs, errors are reported to the console using reportError and discarded.)

As the preference name implies, this is useful for developers who want to test a webpage that uses UITour without having to set up their local development environment to fake being one of the whitelisted origins.

Sendings Messages Back to the Webpage

The other remaining logic in content-UITour.js handles messages sent back to the content process from UITour.jsm:

receiveMessage(aMessage) {
  switch (aMessage.name) {
    case "UITour:SendPageCallback":
      this.sendPageEvent("Response", aMessage.data);
    case "UITour:SendPageNotification":
      this.sendPageEvent("Notification", aMessage.data);

You may remember the Message manager overview, which links to documentation for several functions, including addMessageListener. We passed in UITourListener as the listener, which the documentation says should implement the nsIMessageListener interface. Thus, UITourListener.receiveMessage is called whenever messages are received from UITour.jsm.

The function itself is simple; it defers to sendPageEvent with slightly different parameters depending on the incoming message.

sendPageEvent(type, detail) {
  if (!this.ensureTrustedOrigin()) {

  let doc = content.document;
  let eventName = "mozUITour" + type;
  let event = new doc.defaultView.CustomEvent(eventName, {
    bubbles: true,
    detail: Cu.cloneInto(detail, doc.defaultView)

sendPageEvent starts off with another trusted origin check, to avoid sending results from UITour to untrusted webpages. Next, it creates a custom event to dispatch onto the document element of the webpage. Webpages register an event listener on the root document element to receive data returned from UITour.

defaultView returns the window object for the document in question.

Describing cloneInto could take up an entire post on its own. In short, cloneInto is being used here to copy the object from UITour in the chrome process (a privileged context) for use in the webpage (an unprivileged context). Without this, the webpage would not be able to access the detail value at all.

And That's It!

It takes effort, but I've found that deep-dives like this are a great way to both understand a single piece of code, and to learn from the style of the code's author(s). Hopefully ya'll will find this useful as well!

  1. While this isn't a security issue on its own, it gives some level of control to an attacker, which generally should be avoided where possible.

Caching Async Operations via Promises

August 22, 2016 mozilla

I was working on a bug in Normandy the other day and remembered a fun little trick for caching asynchronous operations in JavaScript.

The bug in question involved two asynchronous actions happening within a function. First, we made an AJAX request to the server to get an "Action" object. Next, we took an attribute of the action, the implementation_url, and injected a <script> tag into the page with the src attribute set to the URL. The JavaScript being injected would then call a global function and pass it a class function, which was the value we wanted to return.

The bug was that if we called the function multiple times with the same action, the function would make multiple requests to the same URL, even though we really only needed to download data for each Action once. The solution was to cache the responses, but instead of caching the responses directly, I found it was cleaner to cache the Promise returned when making the request instead:

export function fetchAction(recipe) {
  const cache = fetchAction._cache;

  if (!(recipe.action in cache)) {
    cache[recipe.action] = fetch(`/api/v1/action/${recipe.action}/`)
      .then(response => response.json());

  return cache[recipe.action];
fetchAction._cache = {};

Another neat trick in the code above is storing the cache as a property on the function itself; it helps avoid polluting the namespace of the module, and also allows callers to clear the cache if they wish to force a re-fetch (although if you actually needed that, it'd be better to add a parameter to the function instead).

After I got this working, I puzzled for a bit on how to achieve something similar for the <script> tag injection. Unlike an AJAX request, the only thing I had to work with was an onload handler for the tag. Eventually I realized that nothing was stopping me from wrapping the <script> tag injection in a Promise and caching it in exactly the same way:

export function loadActionImplementation(action) {
  const cache = loadActionImplementation._cache;

  if (!(action.name in cache)) {
    cache[action.name] = new Promise((resolve, reject) => {
      const script = document.createElement('script');
      script.src = action.implementation_url;
      script.onload = () => {
        if (!(action.name in registeredActions)) {
          reject(new Error(`Could not find action with name ${action.name}.`));
        } else {

  return cache[action.name];
loadActionImplementation._cache = {};

From a nitpicking standpoint, I'm not entirely happy with this function:

But these are minor, and the patch got merged, so I guess it's good enough.