Reduce duplicates places queries near startup

[Mardak]

Also see https://bugzilla.mozilla.org/show_bug.cgi?id=1401817
Right now TopSitesFeed and HighlightsFeed both call and expire places results at least 2 times on INIT, NEW_TAB_LOAD.

HighlightsFeed additionally does work on TOP_SITES_UPDATED.

[piatra]

Both TopSitesFeed and HighlightsFeed refresh twice from INIT and NEW_TAB_LOAD and it looks like NEW_TAB_LOAD is always second. The event is triggered by ActivityStreamMessageChannel that is created after the feeds are initialized.

• TopSites are not affected by NEW_TAB_LOAD because they update only once every 30 minutes.
• Highlights will update on every NEW_TAB_LOAD if they have less than 9 entries.
  • I think we could easily add an extra && Date.now() - this.lastUpdated > 30 seconds to counter this.
  • Highlights already clear the cache on changes to bookmarks and places so actual "stale data" could not occur as a result of a 30s delay.

The extra work done by the HighlightsFeed on TOP_SITES_UPDATED isn't that important. It will hit the cache. Additionally we could first check if there is any overlap between topsites and highlights before actually calling though to fetchHighlights.

Thoughts? @Mardak @k88hudson

[piatra]

I'm also looking into writing a mochitest that loads AS and tests how many times we call into NewTabUtils like getHighlights.

[Mardak]

I believe if you look at dispatch calls during startup, you should be able to see two from TopSites because the refresh call from INIT and NEW_TAB_LOAD happen concurrently (blocked on this._tippyTopProvider.init).

Also, even if Highlights hits the cache, it's most likely unnecessarily dispatching. Although that could be treated as its own issue as one was filed in terms of places queries.

I believe @k88hudson has mentioned just removing the "fewer than 9" Highlights check similar to how it was removed from TopSites.