Description
All search engines are missing in Staging using Firefox latest Nightly 79.0a1 (shows a blank list) when using Dump enabled on bucket "main-preview")

Environments
Windows 10, Ubuntu 18.04 and MacOS 10.15

create user.js containing:
user_pref("services.settings.server", "https://settings.stage.mozaws.net/v1");
user_pref("security.content.signature.root_hash", "DB:74:CE:58:E4:F9:D0:9E:E0:42:36:BE:6C:C5:C4:F6:6A:E7:74:7D:C0:21:42:7A:03:BC:2F:57:0C:8B:9B:90");
user_pref("services.settings.load_dump", true);
user_pref("services.settings.default_bucket", "main-preview");
user_pref("browser.search.region", "US");
user_pref("browser.search.log", true);
user_pref("services.settings.loglevel", "debug");

tested with locales and all had the same problem:
en-US/US
RO/RO

Steps
Download latest USA localized Firefox nightly build.
Create a new profile and add the specified user.js in the profile folder before start-up.
Start-up Firefox.
Check list of the search engines that are installed by going into hamburger menu > Options > Search.
Check Browser console for errors.

Actual Results
No engines are shown. the list is blank

Expected Result
List should appear, like it does on The latest Firefox Beta 78.0b4

see screenshots attached.

Mathieu, could this be a regression from bug 1640136?

I could reproduce this on production with main-preview as well. From the logs:

services.settings: main-preview/anti-tracking-url-decoration restore dump
services.settings: main-preview/anti-tracking-url-decoration 0 records before filtering.
services.settings: main-preview/anti-tracking-url-decoration 0 records after filtering.
services.settings: main-preview/search-config restore dump
services.settings: main-preview/search-config 0 records before filtering.
services.settings: main-preview/search-config 0 records after filtering.

mozregression

build_date: 2020-06-04 10:40:58.421000
build_url: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/UX-redRlTDuTjOnkj3u48w/runs/0/artifacts/public%2Fbuild%2Ftarget.zip
changeset: 3acd33c72d73eae69bf50536c87286342418daa1
pushlog_url: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=3acd33c72d73eae69bf50536c87286342418daa1&tochange=dd65196892ffa181e0a68b78d64cf672c0d2dd44

Mathieu, could this be a regression from bug 1640136?

Indeed, it was a regression. Good catch! Basically the dump was loaded from main/{cid}.json but also stored in local DB as main/{cid}, instead of main-preview/{cid}.

https://hg.mozilla.org/mozilla-central/rev/6060e2e8a0b0

I verified on windows10 64bit, mac 10.15 and also on ubuntu 18.04
using Firefox beta 79b1 and Firefox nightly 80.0a1
and the issue is not visible anymore, when i launch firefox i see all engines.

Depends on D84352

Comment on attachment 9165058 [details]
Bug 1644153 - Fix loading of JSON dump from main-preview r=standard8

ESR Uplift Approval Request

• If this is not a sec:{high,crit} bug, please state case for ESR consideration: This patch is the follow-up of 1640136, and has to be uplifted along
• User impact if declined: We would have to backport 1649700 with a piece of this patch.

It means that authors of Remote Settings collections won't be able to preview their changes on top of the default packaged data.

• Fix Landed on Version: 79
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): This patch landed a long time ago, and tests cover the changed part. Plus, users are not impacted by the changes, since it only affects previewing Remote Settings changes.
• String or UUID changes made by this patch:

Comment on attachment 9165058 [details]
Bug 1644153 - Fix loading of JSON dump from main-preview r=standard8

Approved for 78.1esr.

https://hg.mozilla.org/releases/mozilla-esr78/rev/0ba669e42081