[merge][#330248] don't crash on incorrect progress bar updates

[John Arbash Meinel]

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Martin Pool wrote:
> The UI model is that there is one correctly-nested progress bar stack
> at any time; this should correspond to a nested set of try/finally
> blocks that close them off as tasks complete.  There is code that
> tries to check it and issue warnings, but if you try to finish a
> progress bar when none are active at all it crashes bzr, as seen in
> <https://bugs.edge.launchpad.net/bzr/+bug/330248>.
> 
> In that bug the root cause is probably
> <https://bugs.edge.launchpad.net/bzr/+bug/303568> but 330248 is for
> the knock-on mess in the progress bar.
> 
> Arguably we should allow an arbitrary forest of progress tasks, and
> arguably we should not show any warnings if the program gets it wrong,
> though there have been a couple of cases where it found bugs.  I
> intend to send a patch that at least suppresses them for release
> builds.  But I think this change is still worthwhile as it is.
> 
> 
BB:approve
John
=:->

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