Sync: Add fetch delay to mitigate S3 inconsistency

[ayumi]

Related to brave/sync#139

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

I'll have to dig into this some more... but in browser => browser testing, I'm seeing bookmarks not sync when both are open (some records are sent, but it appears to stop. No errors). However, after I quit/relaunch, it syncs up just fine.

My steps:

1. (pyramid1) npm start with a session that has a decent # bookmarks
2. (pyramid2) npm run start2 with a different session (has leftover bits from last sync, before I reset it)
3. enable sync on pyramid1
4. use keywords and join pyramid 1's group on pyramid2
5. Add some folders + bookmarks on pyramid1
6. it doesn't get synced
7. exit both browser instances
8. relaunch (step 1 + 2), it syncs up

[bsclifton]

is this correct (the subtraction)? should it be + config.offset?

[ayumi]

this is correct– this is "Find records whose timestamps are after 30s ago", looking in the past because records created 0–30s ago could be inconsistent.

[bridiver]

shouldn't you get the new startAt time (in a tmp var) before sending the FETCH_SYNC_RECORDS request? It's extremely unlikely now that you've added the offset, but in a resource starved machine there is still a potential for a gap between the sync end/next start times

[SergeyZhukovsky]

it is a good point, we do it before on Android

[ayumi]

@bsclifton that is weird– when the sync is failing, do you get anything in the sync extension console log (go to chrome-extension://cjnmeadmgmiihncdidmfiabhenbggfjm/_generated_background_page.html and open dev tools)?

(I tried your test plan twice and it seemed ok for me.)

[LaurenWags]

Post-Sync import Laptop-Laptop scenario (complex case) - this scenario brave/sync#146 (comment) with different pyramids:
Pyramid 0 was Linux/Ubuntu VM (without the PR)
Pyramid 1 was MacOS laptop (with the PR)

No bookmarks/folders were lost and both pyramids were open during sync. Approx 6k bookmarks were synced.