The update orphaning dashboard[1] shows that over 58% of users who are more than two version out-of-date are reporting an update check code of CHK_IS_DOWNLOADING. We should investigate if this is due to people getting stuck downloading an update or if other factors are contributing to this large percentage. [1] https://telemetry.mozilla.org/update-orphaning/

I'd be curious if users stuck in a 'download update, fail to apply it, goto start' loop end up downloading most of the time, reproducing this kind of result.

rstrong, mhowell and I met in Portland 5/2 - 5/6 and investigated this by establishing a set of hypotheses and trying to prove/disprove them by diving deeper into our telemetry data. The main takeaway was that approximately 50% of users in the CHK_IS_DOWNLOADING state encounter an error of NS_ERROR_DOCUMENT_NOT_CACHED. Through code inspection, we determined that this error code may occur when users are offline, or switch from being online to offline (details still to be investigated). If users encounter this error, we currently delete the partially downloaded update and start fresh. We filed bug 1272585 where we will try to continue an interrupted download instead of removing it. This matches our code that should already have been dealing with interrupted downloads due to going offline. Another notable takeaway was that we could not determine any significant correlation between users in the CHK_IS_DOWNLOADING state and specific addons. Since bug 1272585 is the actionable consequence of the investigation here, I'm closing this bug for now. We will reopen and continue our investigation should CHK_IS_DOWNLOADING pings continue to be represented by a large percentage on our update orphaning dashboard.