The Gecko monoculture

I remember a time, not so very long ago, when Gecko powered 4 or 5 non-Mozilla browsers, some of them on exotic platforms, as well as GPS devices, wysiwyg editors, geographic platforms, email clients, image editors, eBook readers, documentation browsers, the UX of virus scanners, etc, as well as a host of innovative and exotic add-ons. In these days, Gecko was considered, among other things, one of the best cross-platform development toolkits available.

The year is now 2016 and, if you look around, you’ll be hard-pressed to find Gecko used outside of Firefoxen (alright, and Thunderbird and Bluegriffon). Did Google or Apple or Microsoft do that? Not at all. I don’t know how many in the Mozilla community remember this, but this was part of a Mozilla strategy. In this post, I’d like to discuss this strategy, its rationale, and the lessons that we may draw from it.

Dreaming the Internet of Things

One of these days, using the Cloud of OpaqueCompany ™, I will be able to set the colour of my lightbulbs by talking to my TV. Somewhere along the way, my house will become a little bit more energy hungry and a little bit more dependent on the Cloud of OpaqueCompany(tm) . That’s the promise of the Internet of Things. Isn’t that neat? Isn’t that exciting?

Not really. At least, not for me. But, for some reason, whenever I read about that Internet of Things, it is about expensive gadgets that, to me, sounds like Christmas commercials:  marginally useful, designed by marketers for spoilt westerners to be consumed then forgotten before the next Christmas shopping spree.

But this doesn’t have to be.

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I have spent a little time scratching the surface and trying to determine whether there was something more to this Internet of Things, beside the shopping list. I came back convinced that, once you forget the marketing, this Internet of Things can become a revolution as big as the Personal Computer or the World Wide Web – at least if we let it fall into the right hands.

Say you are the owner or manager of a small commerce, say a restaurant. Chances are that you need a burglar alarm, either because you fear that you are going to be burglarised, or because your insurance requires one. You have two solutions. Either you go to a store and buy some off-the-shelf product, or you contract a company, draw a list of requirements and pay for a custom setup. In either case, you are a consumer, and you are stuck with what you paid for. But needs change. Perhaps the insurance policies now requires you to have an alarm that can call the police automatically. Perhaps neighbours complained about the noise of the alarm and you need to turn it into a silent alarm that rings your cellphone. Perhaps the insurance has changed their policy and now requires you to take pictures of the burglary. Perhaps you have had work done and the small window in the bathroom is now large enough that it could be used to break in. Or water damage has destroyed one of your sensors and you need to replace it, but the model doesn’t exist anymore. Or you are tired of triggering the alarm when you take out the garbage and need to refine the policy. Of your product was linked to a subscription, to call the police on your behalf, but the provider has stopped this service. In any of these cases, you are probably stuck. Because your needs have made you a consumer and you are served only as long as there is a market for your specific need.

Now, consider an alternate universe, in which you just need to walk or drive to the nearest store, buy a few off-the-shelf motion detectors, for the price of a few dollars and simply attach them in your restaurant, where you see fit. They use open standards, so you can install an app to get them to work together, or even better, use your cellphone to script them visually into doing what you need. Do you need to add one or ten, or replace them with different models, or add door-lock sensors? It’s just as easy. Do you need to add a camera? Well, place it and use your cellphone to add that camera to your script. Use your cellphone again and customise the effect, to call the police, or ring your cellphone, or deactivate a single alarm between 11pm and 11.30pm, because that’s when you take out the trash. And if your product is linked to a subscription, because it uses open standards, you can switch provider as needed. In this universe, the Internet of Things has put you in control – not a Cloud, not a silo – and drastically cut your costs and your dependencies.

A few months ago, Mozilla has started pivoting from SmartPhones to the Web of Things – that’s the name we give to Internet of Things done right, with open standards, you in charge, rather than silos and Opaque Cloud ™. I can make no promise that we are going to succeed, but I believe in the huge potential of this Web of Things.

By the way, it doesn’t stop at restaurants. The exact same open standards can help you guard against fires in your house or humidity in your server room. Or crowdsourcing flood detection in cities exposed to flash floods or automating experiments in a physics lab. Or watching your heartbeat or listening to your snores. Or determining which part of the village farm needs to be irrigated in priority or which part of the sewers need most attention.

Some of these problems already have commercial solutions. But what about your next problem, the one that hasn’t attracted the attention of any company large enough to produce devices specifically for you?

Here is to the Web of Things. Let’s make sure that it falls into the right hands.

Designing the Firefox Performance Stats Monitor, part 1: Measuring time without killing battery or performance

For a few versions, Firefox Nightly has been monitoring the performance of add-ons, thanks to the Performance Stats API. While we are waiting for the greenlight to let it graduate to Firefox Aurora, as well as investigating a few lingering false-positives, and while v2 is approaching steadily, it is time for a brain dump on this toolbox and its design.

The initial objective of this monitor is to be able to flag both add-ons and webpages that cause noticeable slowdowns, so as to let users disable/close whatever is making their use of Firefox miserable. We also envision more advanced uses that could let us find out if features of webpages cause slowdowns on specific OS/hardware combinations.

What have I done since last July?

School year 2014-2015 is ending. It’s time for a brief report.

Re-dreaming Firefox (1): Firefox Agents

Gerv’s recent post on the Jeeves Test got me thinking of the Firefox of my dreams. So I decided to write down a few ideas on how I would like to experience the web. Today: Firefox Agents. Let me emphasise that the features described in this blog post do not exist.

Marcel uses Firefox every day, for quite a number of things.

• He uses Firefox for fun, for watching videos and playing online games. For this purpose, he has installed a few tools for finding and downloading videos. Also, one of his main search engines is YouTube. Suggested movies? Sure, as long as they are fun.
• He uses Firefox for social networks. He follows his friends, he searches on Facebook, or Twitter, or Google+. If anything looks fun, or useful, he’d like to be informed.
• He uses Firefox for managing his bank accounts, his taxes, his health insurance. For this purpose, he has paranoid security settings – to avoid phishing, he can only browse to a few whitelisted websites – and no add-ons. He may be interested in getting information from these few websites, and in security updates, but that’s about it. Also, since Firefox handles all his passwords, it must itself be protected by a password.
• He uses Firefox to read his Gmail account. And to read his other Gmail account. And he doesn’t want to leak privacy information by doing so on the same Firefox that he’s using for browsing.
• Oh, and he may also be using Firefox for browsing websites that are sensitive for any kind of reason, whether he’s hunting for gifts for his close family, dating online, chatting with hackers, discussing politics, helping NGOs in sensitive parts of the globe, visiting BitTorrent trackers, consulting a physician through some online service, or, well, anything else that requires privacy. He’d like to perform such browsing with additional anonymity guarantees. This also means locking Firefox with a password.
• Sometimes, his children or friends borrow his computer and use Firefox, too.

Of course, since Marcel brings his own device at (or from) work, that’s the same Firefox that he’s using for all of these tasks, and he’s probably even doing several of these tasks at the same time.

So, Marcel has a set of contradictory requirements, not to mention that each of his uses of Firefox needs to pass a distinct Jeeves Test. How do we keep him happy nevertheless?

Introducing Firefox Agents

In the rest of this post, I will be calling each of these uses of Firefox an Agent (if we ever implement this feature, it will, of course, be called Persona). Each Agent matches one way you use Firefox. While Firefox may be delivered with a predefined set of Agents, users can easily create new Agents. In the example, Marcel has his “Fun Agent”, his “Social Agent”, his “Work Agent”, etc.

Each Agent is unique and separate:

• Each Agent has its own icon on Marcel’s menu/desktop/tablet/phone and task list.
• Each Agent has its own visual identity, to make sure that work-related stuff doesn’t end up accidentally in the Fun Agent.
• Each Agent has its own set of preferences, bookmarks, remembered passwords, cookies, cache, and add-ons.
• Each website may be connected to a given Agent, so that links received through Gmail or through Thunderbird, for instance, automatically open with the right Agent.

As a consequence, any technology that can come bundled with Firefox to, for instance, provide search suggestions or any other kind of website suggestions is tied to an Agent. For instance, Marcel’s browsing a dating site, or shopping for shoes, or having religious activities will not be visible to any of his colleagues looking above his shoulder at his Work Agent, nor will it be tied to either of Marcel’s Gmail accounts. This greatly increases the chances of suggestion technologies passing the Jeeves Test.

Agents are also connected:

• A menu in each Agent, as well as a keyboard shortcut, lets users quickly open/switch to other Agents.
• When an Agent follows a link to a website that belongs to another Agent, the relevant Agent opens automatically.
• Bookmarks may be pushed, on demand, from one Agent to another one.
• Passwords may be pulled, on demand, from one Agent to another one.

How far are we from Agents?

Technologically speaking, Firefox Agents almost exist. Indeed, Firefox has supported Profiles forever, since way before Firefox 1.0. I generally have three instances of Firefox opened at the same time (four when I’m doing web development), and it works nicely.

With a few add-ons, you can get almost everything, although not entirely connected together:

• Profilist helps a lot with switching between profiles, and the dev version adds distinct icons;
• Firefox Themes implement distinct appearances;
• there are add-ons implementing whitelist browsing;
• there are add-ons implementing password-protected Firefox.

A few features are missing, but as you can see, the list is actually quite short:

• Pushing/pulling passwords and bookmarks between Agents (although that’s a subset of what Firefox Accounts can do).
• Attaching specific websites to specific Agents (although this doesn’t seem too difficult to implement).
• Connecting this all together.

What now?

I would like to browse with this Firefox. Would you?

Detecting slow add-ons

When it is at its best, Firefox is fast. Really, really fast. When things start slowing down, though, using Firefox is much less fun. So, one of main objectives of the developers of Firefox is making sure that Firefox is and remains as smooth and responsive as humanly possible. There is, however, one thing that can slow down Firefox, and that remains out of the control of the developers: add-ons. Good add-ons are extraordinary, but small coding errors – or sometimes necessary hacks – can quickly drive the performance of Firefox into the ground.

So, how can an add-on developer (or add-on reviewer) find out whether her add-on is fast? Sadly, not much. Testing certainly helps, and the Profiler is invaluable to help pinpoint a slowdown once it has been noticed, but what about the performance of add-ons in everyday use? What about the experience of users?

To solve this issue, we decided to work on a set of tools to help add-on developers and reviewers find out the performance of their add-ons. Oh, and also to let users find out quickly if an add-on is slowing down their everyday experience.

about:performance

On recent Nightly builds of Firefox, you may now open about:performance to get an overview of the performance cost of add-ons and webpages :

The main resources we monitor are :

• jank, which measures how much the add-on impacts the responsiveness of Firefox. For 60fps performance, jank should always remain ≤ 4. If an add-on regularly causes jank to increase past 6, you should be worried.
• CPOW aka blocking cross-process communications, which measures how much the add-on is causing Firefox to freeze waiting for a process to respond. Anything above 0 is bad.

Note that the design of this page is far from stable. I realise it’s not very user-friendly at the moment, so don’t hesitate to file bugs to help us improve it. Also note that, when running with e10s, the page doesn’t display all the useful information. We are working on it.

add-on Telemetry

Add-on developers and reviewers can now find information on the performance of their add-ons on a dedicated dashboard.

These are real-world performance data, as extracted from user’s computers. The two histograms available for the time being are:

• MISBEHAVING_ADDONS_JANK_LEVEL, which measures the jank, as detailed above;
• MISBEHAVING_ADDONS_CPOW_TIME_MS, which measure the amount of time spent in CPOW, as detailed above.

If you are an add-on developer, you should monitor regularly the performance of your add-on on this page. If you notice suspicious values, you should try and find out what causes these performance issues. Don’t hesitate and reach out to us, we will try and help you.

Slow add-on Notification

Add-on developers and reviewers, as well as end-users, are now informed when an add-on causes either jank or CPOW performance issues:

Note that this feature is not ready to ride the trains, and we do not have a specific idea of when it will be made available for users of Aurora/DeveloperEdition. This is partly because the UX is not good enough yet, partly because the thresholds will certainly change, and partly because we want to give add-on developers time to fix any issue before the users see a dialog that suggest that an add-on should be uninstalled.

Performance Stats API

By the way, we have an API for accessing performance stats. Very imaginatively, it’s called PerformanceStats.jsm [link]. While this API will still change during the coming weeks you can start playing with it if you are interested. Some add-ons may be able to throttle their performance use based on this data. Also, I hope that, in time, someone will be able to write a version of about:performance much nicer than mine 🙂

Challenges and work ahead

For the moment, we are in the process of stabilizing the API, its implementation and its performance. In parallel, we are working on making the UX of about:performance more useful. Once both are done, we are going to proceed with adding more measurements, making the code more e10s-friendly and measuring the performance of webpages.

If you are an add-on developer and if you feel that your add-on is tagged as slow by error, or if you have great ideas on how to make this data useful, feel free to ping me, preferably on IRC. You can find me on irc.mozilla.org, channel #developers, where I am Yoric.

Je Suis Charlie

The Future of Promise

If you are writing JavaScript in mozilla-central or in an add-on, or if you are writing WebIDL code, by now, you have probably made use of Promise. You may even have noticed that we now have several implementations of Promise in mozilla-central, and that things are moving fast, and sometimes breaking.

At the moment, we have two active implementations of Promise:

• Promise.jsm, as provided by the toolkit;

• DOM Promise, as provided by the DOM.

(as well as a little code using an older, long deprecated, implementation of Promise)

This is somewhat confusing, but the good news is that we are working hard at making it simpler and moving everything to DOM Promise.

General Overview

Many components of mozilla-central have been using Promise for several years, way before a standard was adopted, or even discussed. So we had to come up with our implementation(s) of Promise. These implementations were progressively folded into Promise.jsm, which is now used pervasively in mozilla-central and add-ons.

In parallel, Promise were specified, submitted for standardisation, implemented in Firefox, and finally standardised. This is the second implementation we call DOM Promise. This implementation is starting to be used in many places on the web.

Having two implementations of Promise with the same feature set doesn’t make sense. Fortunately, Promise.jsm was designed to match the API of Promise that we believed would be standardised, and was progressively refactored and extended to follow these developments, so both APIs are almost identical.

Our objective is to move entirely to DOM Promise. There are still a few things that need to happen before this is possible, but we are getting close. I hope that we can get there by the end of 2014.

Missing pieces

Debugging and testing

At the moment, Promise.jsm is much better than DOM Promise in two aspects:

• it is easier to inspect a promise from Promise.jsm for debugging purposes (not anymore, things have been moving fast while I was writing this blog entry);
  

• Promise.jsm integrates nicely in the test suite, to make sure that uncaught errors are reported and cause test failures.

In both topics, we are hard at work bringing DOM Promise to feature parity with Promise.jsm and then some (bug 989960, bug 1083361). Most of the patches are in the pipeline already.

API differences

• Promise.jsm offers an additional function Promise.defer, which didn’t make it to standardization.

This function may easily be written on top of DOM Promise, so this is not a hard blocker. We are going to add this function to a module `PromiseUtils.jsm`.

• Also, there is a slight bug in DOM Promise that gives it a slightly unexpected behavior in a few edge cases. This should not hit developers who use DOM Promise as expected, but this might surprise people who know the exact scheduling algorithm and expect it to be consistent between Promise.jsm and DOM Promise.
  

Oh, wait, that’s fixed already.

Wrapping it up

Once we have done all of this, we will be able to replace Promise.jsm with an empty shell that defers all implementations to DOM Promise. Eventually, we will deprecate and remove this module.

As a developer, what should I do?

For the moment, you should keep using Promise.jsm, because of the better testing/debugging support. However, please do not use Promise.defer. Rather, use PromiseUtils.defer, which is strictly equivalent but is not going away.

We will inform everyone once DOM Promise becomes the right choice for everything.

If your code doesn’t use Promise.defer, migrating to DOM Promise should be as simple as removing the line that imports Promise.jsm in your module.

Firefox, the Browser that has your Back[up]

One of the most important features of Firefox, in my opinion, is Session Restore. This component is responsible for ensuring that, even in case of crash, or if you upgrade your browser or an add-on that requires restart, your browser can reopen immediately and in the state in which you left it. As far as I am concerned, this feature is a life-safer.

Unfortunately, there are a few situations in which the Session Restore file may be corrupted – typically, if the computer is rebooted before the write is complete, or if it loses power, or if the operating system crashes or the disk is disconnected, we may end up losing our precious Session Restore. While any of these circumstances happens quite seldom, it needs to be applied as part of the following formula:

seldom · .5 billion users = a lot

I am excited to announce that we have just landed a new and improved Session Restore component in Firefox 33 that protects your precious data better than ever.

How it works

Firefox needs Session Restore to handle the following situations:

• restarting Firefox without data loss after a crash of either Firefox, the Operating System, a driver or the hardware, or after Firefox has been killed by the Operating System during shutdown;
• restarting Firefox without data loss after Firefox has been restarted due to an add-on or an upgrade;
• quitting Firefox and, later, restarting without data loss.

In order to handle all of this, Firefox needs to take a snapshot of the state of the browser whenever anything happens, whether the user browses, fills a form, scrolls, or an application sets a Session Cookie, Session Storage, etc. (this is actually capped to one save every 15 seconds, to avoid overloading the computer). In addition, Firefox performs a clean save during shutdown.

While at the level of the application, the write mechanism itself is simple and robust, a number of things beyond the control of the developer can prevent either the Operating System or the hard drive itself from completing this write consistently – a typical example being tripping on the power plug of a desktop computer during the write.

The new mechanism involves two parts:

• keeping smart backups to maximize the chances that at least one copy will be readable;
• making use of the available backups to transparently avoid or minimize data loss.

The implementation actually takes very few lines of code, the key being to know the risks against which we defend.

Keeping backups

During runtime, Firefox remembers which files are known to be valid backups and which files should be discarded. Whenever a user interaction or a script requires it, Firefox writes the contents of Session Restore to a file called sessionstore-backups/recovery.js. If it is known to be good, the previous version of sessionstore-backups/recovery.js is first moved to sessionstore-backups/recovery.bak. In most cases, both files are valid and recovery.js contains a state less than 15 seconds old, while recovery.bak contains a state less than 30 seconds old. Additionally, the writes on both files are separated by at least 15 seconds. In most circumstances, this is sufficient to ensure that, even of hard drive crash during a write to recover.js, at least recovery.bak has been entirely written to disk.

During shutdown, Firefox writes a clean startup file to sessionstore.js. In most cases, this file is valid and contains the exact state of Firefox at the time of shutdown (minus some privacy filters). During startup, if sessionstore.js is valid, Firefox moves it to sessiontore-backup/previous.js. Whenever this file exists, it is valid and contains the exact state of Firefox at the time of the latest clean shutdown/startup. Note that, in case of crash, the latest clean shutdown/startup might be older than the latest actual startup, but this backup is useful nevertheless.

Finally, on the first startup after an update, Firefox copies sessionstore.js, if it is available and valid, to sessionstore-backups/upgrade.js-[build id]. This mechanism is designed primarily for testers of Firefox Nightly, who keep on the very edge, upgrading Firefox every day to check for bugs. Testers, if we introduce a bug that affects Session Restore, this can save your life.

As a side-note, we never use the operating system’s flush call, as 1/ it does not provide the guarantees that most developers expect; 2/ on most operating systems, it causes catastrophic slowdowns.

Recovering

All in all, Session Restore may contain the following files:

• sessionstore.js (contains the state of Firefox during the latest shutdown – this file is absent in case of crash);
• sessionstore-backups/recovery.js (contains the state of Firefox ≤ 15 seconds before the latest shutdown or crash – the file is absent in case of clean shutdown, if privacy settings instruct us to wipe it during shutdown, and after the write to sessionstore.js has returned);
• sessionstore-backups/recovery.bak (contains the state of Firefox ≤ 30 seconds before the latest shutdown or crash – the file is absent in case of clean shutdown, if privacy settings instruct us to wipe it during shutdown, and after the removal of sessionstore-backups/recovery.js has returned);
• sessionstore-backups/previous.js (contains the state of Firefox during the previous successful shutdown);
• sessionstore-backups/upgrade.js-[build id] (contains the state of Firefox after your latest upgrade).

All these files use the JSON format. While this format has drawbacks, it has two huge advantages in this setting:

• it is quite human-readable, which makes it easy to recover manually in case of an extreme crash;
• its syntax is quite rigid, which makes it easy to find out whether it was written incompletely.

As our main threat is a crash that prevents us from writing the file entirely, we take advantage of the latter quality to determine whether a file is valid. Based on this, we test each file in the order indicated above, until we find one that is valid. We then proceed to restore it.

If Firefox was shutdown cleanly:

1. In most cases, sessionstore.js is valid;
2. In most cases in which sessionstore.js is invalid, sessionstore-backups/recovery.js is still present and valid (the likelihood of it being present is obviously higher if privacy settings do not instruct Firefox to remove it during shutdown);
3. In most cases in which sessionstore-backups/recovery.js is invalid, sessionstore-backups/recovery.bak is still present, with an even higher likelihood of being valid (the likelihood of it being present is obviously higher if privacy settings do not instruct Firefox to remove it during shutdown);
4. In most cases in which the previous files are absent or invalid, sessionstore-backups/previous.js is still present, in which case it is always valid;
5. In most cases in which the previous files are absent or invalid, sessionstore-backups/upgrade.js-[…] is still present, in which case it is always valid.

Similarly, if Firefox crashed or was killed:

1. In most cases, sessionstore-backups/recovery.js is present and valid;
2. In most cases in which sessionstore-backups/recovery.js is invalid, sessionstore-backups/recovery.bak is pressent, with an even higher likelihood of being valid;
3. In most cases in which the previous files are absent or invalid, sessionstore-backups/previous.js is still present, in which case it is always valid;
4. In most cases in which the previous files are absent or invalid, sessionstore-backups/upgrade.js-[…] is still present, in which case it is always valid.

Numbers crunching

Statistics collected on Firefox Nightly 32 suggest that, out of 11.95 millions of startups, 75,310 involved a corrupted sessionstore.js. That’s roughly a corrupted sessionstore.js every 158 startups, which is quite a lot. This may be influenced by the fact that users of Firefox Nightly live on pre-alpha, so are more likely to encounter crashes or Firefox bugs than regular users, and that some of them use add-ons that may modify sessionstore.js themselves.

With the new algorithm, assuming that the probability for each file to be corrupted is independent and is p = 1/158, the probability of losing more than 30 seconds of data after a crash goes down to p^3 ≅ 1 / 4,000,000. If we haven’t removed the recovery files, the probability of losing more than 30 seconds of data after a clean shutdown and restart goes down to p^4 ≅ 1 / 630,000,000. This still means that , statistically speaking, at every startup, there is one user of Firefox somewhere around the world who will lose more than 30 seconds of data, but this is much, better than the previous situation by several orders of magnitude.

It is my hope that this new mechanism will transparently make your life better. Have fun with Firefox!

Revisiting uncaught asynchronous errors in the Mozilla Platform

Consider the following feature and its xpcshell test:

// In a module Foo
function doSomething() {
  // ...
  OS.File.writeAtomic("/an invalid path", "foo");
  // ...
}

// In the corresponding unit test
add_task(function*() {
  // ...
  Foo.doSomething();
  // ...
});

Function doSomething is obviously wrong, as it performs a write operation that cannot succeed. Until we started our work on uncaught asynchronous errors, the test passed without any warning. A few months ago, we managed to rework Promise to ensure that the test at least produced a warning. Now, this test will actually fail with the following message:

A promise chain failed to handle a rejection – Error during operation ‘write’ at …

This is particularly useful for tracking subsystems that completely forget to handle errors or tasks that forget to call yield.

Who is affected?

This change does not affect the runtime behavior of application, only test suites.

• xpcshell: landed as part of bug 976205;
• mochitest / devtools tests: waiting for all existing offending tests to be fixed, code is ready as part of bug 1016387;
• add-on sdk: no started, bug 998277.

This change only affects the use of Promise.jsm. Support for DOM Promise is in bug 989960.

Details

We obtain a rejected Promise by:

• throwing from inside a Task; or
• throwing from a Promise handler; or
• calling Promise.reject.

A rejection can be handled by any client of the rejected promise by registering a rejection handler. To complicate things, the rejection handler can be registered either before the rejection or after it.

In this series of patches, we cause a test failure if we end up with a Promise that is rejected and has no rejection handler either:

• immediately after the Promise is garbage-collected;
• at the end of the add_task during which the rejection took place;
• at the end of the entire xpcshell test;

(whichever comes first).

Opting out

There are extremely few tests that should need to raise asynchronous errors and not catch them. So far, we have needed this two tests: one that tests the asynchronous error mechanism itself and another one that willingly crashes subprocesses to ensure that Firefox remains stable.

You should not need to opt out of this mechanism. However, if you absolutely need to, we have a mechanism for opting out. For more details, see object Promise.Debugging in Promise.jsm.

Any question?

Feel free to contact either me or Paolo Amadini.