1317542 - Rework history batch downloading

Status: NEW

(Firefox for iOS :: Sync, enhancement)

Description

[Richard Newman [:rnewman]]

Follow-on from Bug 1309571.

We kinda don't want to download 50,000 records.

We'd also like to get newest-first.

With Bug 1314171, we could reverse our downloading direction on first sync, and work new to old.

We didn't do this, because _in general_ you can't high-watermark and resume: if an old item is modified it'll disappear and hop 'behind' you, with a newer timestamp.

We can probably do this _once_, though: once we're done walking backwards as far as we want (ten thousand items?), we turn around and walk forwards.


Another approach is to simply fetch one recent batch to start with, then fill in the blanks oldest-to-newest with older=. That's a smaller piece of work, but it makes it very difficult to limit the amount of data downloaded.