restoring Safari preferences from backup files in OS X Mavericks

Recently I had the misfortune of having to restore some Safari settings from backup, on OS X 10.9 Mavericks. I have done this many times before on older OS X versions, without incident — simply pull the various preference files such as com.apple.Safari.plist from backup and replace the damaged/unwanted ones. Takes all of 2 minutes, and years ago, I had already wrote a shell script to do exactly that.

It turns out that after Mavericks, Safari is incredibly resistant to conventional methods of preference file backup and restoration. Considering that preferences in OS X have always been stored in XML-based preference lists, you would think (as in previous operating systems from 10.0 to 10.7) taking the relevant preference files and replacing the unwanted new ones in ~/Library/Preferences would be enough. But no, an incredible amount of effort is now required for a simple task:

  1. unhiding the Library directory, because clearly we’re all Windows babies who cannot be trusted to see where application preferences are stored
  2. replacing the actual preference files, scattered around the system in ~/Library/Safari, ~/Library/Preferences, ~/Library/Caches, and hopefully remember to have turned off iCloud, or otherwise see changes being clobbered by iCloud sync (or, worse yet, having experimental changes reflected all across a network of Mac and iDevices)
  3. resetting the preference cache daemon, cfprefsd so that preference changes can be reflected in a running system. This where a lot of people get stuck in general, judging by Google results; when they replace preference files and find that their changes aren’t being reflected, it leads them into a wild goose chase for “hidden preference files” for Safari, when the answer lies in a simply yet utterly non-obvious background daemon.
  4. restoring the list of installed extensions — which, incredibly, is NOT stored in the deceptively named com.apple.Safari.Extensions.plist, but in the login.keychain.

Context

Had some issues where certain websites were behaving differently under private browsing mode than normal browsing mode. I deduced there was some kind of corrupted stored state, whether it was a cookie or localstorage issue. Had the brilliant idea of setting Safari preferences aside, thus resetting Safari to factory state, and then divide-and-conquer by restoring parts of the settings until the problem recurs. I’ve done this many times before.

First I turned off iCloud sync, having been bitten by sync propagation of experimental changes in the past. This is pretty important if you don’t want to blow up Safari bookmarks (at the very least) across all Apple-manufactured, iCloud-compatible devices. I then removed ~/Library/com.apple.Safari.plist, ~/Library/com.apple.Safari.Extensions.plist, ~/Library/Safari, and ~/Library/Caches/Safari, ~/Library/Cookies. After resetting to confirm some issues have disappeared, I moved some files from backup to original locations. Imagine my surprise when nothing became restored, and all my Safari extensions (installed from the extension store or custom-built by me) disappeared.

Process

Increasingly desperate, I started to trace filesystem accesses using fs_usage. It showed nothing out of the ordinary. 30 mins of reviewing useless forum posts later, I pieced together a multi-stage solution. It turns out there were two separate obstacles.

Preference caching

Presumably to save energy, OS X Mavericks caches application preferences (in RAM?) using a daemon called cfprefsd. Instead of applications pulling their preferences from XML files on disk at launch, it requests this from the daemon instead. The defaults command has been modified to operate with this daemon, so if you had been working with preferences from the command line (as I have been), the changes have been transparent.

However if the preference files are changed or edited directly, this change is not propagated to the preference cache daemon. When the app is opened again, the cached version takes precedence, and is re-written out to disk, clobbering the restored versions.

This does not mean there are hidden Safari preferences somewhere that you haven’t found, though you might think this at first. When Safari is reset manually from the filesystem, or if the plist files are edited, cfprefsd must be reset as well.

There exists a cfprefsd daemon for every logged in user, running under that user’s privileges, as well as a root-owned one. Safari preferences are stored under the user domain, so the user-specific daemon is the one that needs a reset when files change. Can also quit the process from Activity Viewer, or killall cfprefsd. A login-logout cycle would also reset the user-specific daemon.

Extension list caching

Having done this reset with the backup files in place, most preferences will be restored on next launch, *except* the list of extensions you had installed previously. That will remain empty. Even though all the extensions and their settings have been restored to ~/Library/Safari.

For a long time I traced ~/Library/com.apple.Safari.Extensions.plist, and wondered why it wasn’t being read.

An Apple discussion forum post (shockingly enough) gave a vital clue. There exists an “application password” in the login keychain titled “Safari Extensions List”. Whether it is merely a cryptographic key, or the actual list of extensions, is unknown, but that is the critical preference to restore extensions. Having reset extensions by moving them away, this preference is apparently emptied out. The entire login keychain, being an encrypted document, has to be restored to a corresponding previous version to restore access to previous extensions. Without this, all extensions would have to be reinstalled manually (and get a new copy of the extension file stored into ~/Library/Safari/Extensions, instead of the previous version being reused).

Discussion

Given recent focus on energy consumption, I can understand preference caching. However, it’s not that hard to track filesystem changes (the Time Machine/Spotlight APIs explicitly do this!) and reload appropriate preferences when they are changed on disk. It would show respect for power users and developers who might need to interact with the preferences system in a more convenient way.

But stuffing extension lists in an obscure corner of a password keychain? What sense does that make? Are my list of extensions (not actual extension data or settings, mind you — those are in plaintext on the disk for anyone to copy and look at) such privileged information that it has to reside alongside my login password? Why can’t you just read the list of extensions, oh I don’t know, from the list of extension files installed into the Extensions directory? Wouldn’t that be a lot more reasonable?

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