[PATCH stable v2] worker: avoid potential partial write of pickled data

[Manuel Jacob]

On 22/05/2022 10.09, Manuel Jacob wrote:
> For some reason, CI fails: 
> https://foss.heptapod.net/mercurial/mercurial-devel/-/jobs/553261
> 
> The test is flaky on my machine as well.
> 
> Obviously this is related to my change, so please do not merge.

cPickle.load() does not properly handle EINTR. This is fixed in another 
patch "worker: prevent cPickle from directly interacting with C FILEs" 
that I sent to the mailing list. The reading side of the worker logic 
suppressed the error if cPickle raised IOError with EINTR (fixed by 
patch "worker: do not suppress EINTR"). In the failing test, this caused 
the stream to be in the middle of some pickled data when reading from 
the stream later, resulting in nonsense.

Writing to the stream in smaller pieces made it more likely that reading 
from the other end of the pipe errored with EINTR. If this patch is put 
on top of "worker: prevent cPickle from directly interacting with C 
FILEs", it works.

> On 22/05/2022 07.28, Manuel Jacob wrote:
>> # HG changeset patch
>> # User Manuel Jacob <me at manueljacob.de>
>> # Date 1653184234 -7200
>> #      Sun May 22 03:50:34 2022 +0200
>> # Branch stable
>> # Node ID b475b0ea695438f6b2994eba0ddb3189d8b4fd05
>> # Parent  477b5145e1a02715f846ce017b460858a58e03b1
>> # EXP-Topic worker-pickle-fix_partial_write
>> worker: avoid potential partial write of pickled data
>>
>> Previously, the code wrote the pickled data using os.write(). However,
>> os.write() can write less bytes than passed to it. To trigger the 
>> problem, the
>> pickled data had to be larger than 2147479552 bytes on my system.
>>
>> Instead, open a file object and pass it to pickle.dump(). This also 
>> has the
>> advantage that it doesn’t buffer the whole pickled data in memory.
>>
>> Note that the opened file must be buffered because pickle doesn’t support
>> unbuffered streams because unbuffered streams’ write() method might 
>> write less
>> bytes than passed to it (like os.write()) but pickle.dump() relies on 
>> that all
>> bytes are written (see https://github.com/python/cpython/issues/93050).
>>
>> The side effect of using a file object and a with statement is that 
>> wfd is
>> explicitly closed now while it seems like before it was implicitly 
>> closed by
>> process exit.
>>
>> diff --git a/mercurial/worker.py b/mercurial/worker.py
>> --- a/mercurial/worker.py
>> +++ b/mercurial/worker.py
>> @@ -255,8 +255,10 @@
>>                           os.close(r)
>>                           os.close(w)
>>                       os.close(rfd)
>> -                    for result in func(*(staticargs + (pargs,))):
>> -                        os.write(wfd, util.pickle.dumps(result))
>> +                    with os.fdopen(wfd, 'wb') as wf:
>> +                        for result in func(*(staticargs + (pargs,))):
>> +                            util.pickle.dump(result, wf)
>> +                            wf.flush()
>>                       return 0
>>                   ret = scmutil.callcatch(ui, workerfunc)
>> _______________________________________________
>> Mercurial-devel mailing list
>> Mercurial-devel at mercurial-scm.org
>> https://www.mercurial-scm.org/mailman/listinfo/mercurial-devel
> 
> _______________________________________________
> Mercurial-devel mailing list
> Mercurial-devel at mercurial-scm.org
> https://www.mercurial-scm.org/mailman/listinfo/mercurial-devel