[tor-bugs] #29180 [Applications/Tor Browser]: MAR download stalls when about dialog is opened

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#29180: MAR download stalls when about dialog is opened
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     Reporter:  mcs                       |      Owner:  tbb-team
         Type:  defect                    |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 In #28885, Georg encountered a bug that causes the browser update to
 stall. Here are the steps that Kathy and I used to reproduce it:
 1. Start a clean copy of Tor Browser 8.5a5 on a 64-bit Linux system.
 2. Set `app.update.auto` to `false` and `app.update.log` to `true`.
 3. Open the about dialog and click "Update to 8.5a6" to begin downloading
 the MAR file.
 4. Quickly close and reopen the about dialog. The download will stall and
 not make any further progress.

 The following was logged to the browser console:
  AUS:SVC UpdateService:downloadUpdate - no support for downloading more
 than one update at a time

 This problem is fallout from the fact that when the about dialog is
 opened, the browser cancels the download and starts a new one. Here is the
 code from the `startDownload()` function inside `browser/base/content
 /aboutDialog-appUpdater.js`:
  ...
  this.aus.pauseDownload();
  let state = this.aus.downloadUpdate(this.update, false);
  ...

 Looking at the code in `toolkit/mozapps/update/nsUpdateService.js`, we see
 that `pauseDownload()` cancels the first download via
 `nsIRequest.cancel()`, which does not take effect immediately. Cleanup of
 the `Downloader` occurs inside an `onStopRequest()` listener and that
 entire process is asynchronous.

 One possible fix is to not pause and restart the download when the about
 dialog is opened. Kathy and I do not yet know if that would introduce
 other problems.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29180>
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