[tor-bugs] #21766 [Applications/Tor Browser]: Tor Browser based on ESR52 with e10s enabled crashed while trying to download a file

[Tor Bug Tracker & Wiki]

#21766: Tor Browser based on ESR52 with e10s enabled crashed while trying to
download a file
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 Reporter:  gk                                   |          Owner:  mcs
     Type:  defect                               |         Status:
                                                 |  assigned
 Priority:  Very High                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Critical                             |     Resolution:
 Keywords:  ff52-esr, tbb-7.0-must-alpha, tbb-   |  Actual Points:
  e10s, tbb-crash, TorBrowserTeam201705          |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor4
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Comment (by mcs):

 Replying to [comment:24 gk]:
 > While pondering #21886 I was wondering whether it would help to just get
 rid of the observer idea and leave the extension out of the picture: could
 we just show a dialog at places where we are currently using the observer
 mechanism?

 We could try that, but I think the main problem is that a modal dialog is
 displayed (as a cypherpunk mentioned somewhere). With e10s enabled, it
 seems that while a modal dialog is open events related to the IPC
 mechanism are not handled correctly. This causes the chrome process to
 kill the content process (to the user, this appears as a tab crash).
 Therefore I think we will still have a problem if we move the dialog into
 the browser/C++ code.

 Mozilla does show dialogs during a download, but they are not modal and
 they do not block the network activity (so, for example, the download
 proceeds while the user is answering the "What would you like to do with
 this file? open/save" prompt. I assume we want to stop the network
 activity as in Tor Browser 6.5.2.

 The most promising approach seems to be to suspend and then resume the
 request, but when e10s is enabled that does not work reliably when we try
 it from any of the obvious locations, e.g., not from `DoContent()` inside
 `uriloader/exthandler/nsExternalHelperAppService.cpp`, not from
 `OnStartRequest()`, not inside the first call to `OnDataAvailable()`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21766#comment:25>
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