[PATCH 1 of 8 py3] bundle2: raise a more helpful error if building a bundle part header fails

[Augie Fackler]

> On Sep 16, 2017, at 7:54 AM, Yuya Nishihara <yuya at tcha.org> wrote:
> 
> On Fri, 15 Sep 2017 19:14:04 -0400, Augie Fackler wrote:
>> # HG changeset patch
>> # User Augie Fackler <raf at durin42.com>
>> # Date 1505515049 14400
>> #      Fri Sep 15 18:37:29 2017 -0400
>> # Node ID 662bbd6d96952985eff807f424dd128663724672
>> # Parent  209120041d12b524648fa856732aa404dfedd91d
>> bundle2: raise a more helpful error if building a bundle part header fails
>> 
>> I've tripped on this several times now, and am tired of debugging. Now
>> the header parts are part of the error message when the ''.join()
>> fails, which makes debugging obvious.
>> 
>> diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
>> --- a/mercurial/bundle2.py
>> +++ b/mercurial/bundle2.py
>> @@ -1050,7 +1050,11 @@ class bundlepart(object):
>>             header.append(key)
>>             header.append(value)
>>         ## finalize header
>> -        headerchunk = ''.join(header)
>> +        try:
>> +            headerchunk = ''.join(header)
>> +        except TypeError:
>> +            raise TypeError(u'Found a non-bytes trying to '
>> +                            u'build bundle part header: %r' % header)
> 
> I was making it a r'' string, but probably ProgrammingError would be better.

I was conflicted on this one. Preserving the type of the exception seemed reasonable, since this can only happen with weird bugs, but it’s also something I can just sit on for a while as a debugging aid.
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