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!