Fixes an issue where users were no longer receiving messages after
restoration. During restoration, the restoring device copies the
transferred database files to a special "hotswap" directory since we
don't want to overwrite our currently in-use database. Once finished, it
re-opens the database pointing to our hotswap directory. It would then
move the hotswap database to its primary location on the next app
launch.
The problem here is our extensions don't know how to read the hotswap
directory. Even if we added that capability, it's going to be tricky to
coordinate which database they read and when as the main app shuffles
files around.
This fix adjusts our restoration flow. Instead of having a special
"primary" and "hotswap" directory that we need to fix up on the next
launch, we instead record a UserDefaults entry that points to our
current database directory.
Once transfer has completed, the main app only needs to update the
current database directory in UserDefaults. No post-launch swapping
needs to occur.
Extensions will listen for updates to this database location by
registering KVO on NSUserDefaults.
This change also adjusts the restoration flow to break it up into
discrete stages. The motivation here is we need to make sure that a
partial restoration doesn't put the extensions in an inconsistent state.
We've observed races between the NSE+app handoff that can cause a
message to be decrypted twice. This works around this by quitting the
NSE if we decide to wake the main app. This isn't a good fix, but it
should address these out-of-sync session issues for now.
The new hex stripper is a bit too aggressive and will strip out
timestamps as well. Adjusting the minimum number of bytes from 6 to 7
buys us a couple hundred years.
Initial: "timestamp: 1628539368550"
Before: "timestamp: [ REDACTED_HEX:...550 ]"
After: "timestamp: 1628539368550"
