[Libguestfs] [libnbd PATCH v2 1/5] python: tests: Fix error handling

[Laszlo Ersek]

On 02/03/22 23:49, Nir Soffer wrote:
> On Thu, Feb 3, 2022 at 10:26 PM Eric Blake <eblake at redhat.com> wrote:
>>
>> Like a lot of the C examples, the aio copy test ignores read and write
>> errors in the completion callback, which can cause silent data
>> corruption. The failure in the test is not critical, but this is a bad
>> example that may be copied by developers to a real application.
>>
>> The test dies with an assertion failure now if completion callback
>> fails.  Tested with the temporary patch of:
>>
>> | diff --git i/python/t/590-aio-copy.py w/python/t/590-aio-copy.py
>> | index 861fa6c8..4cd64d83 100644
>> | --- i/python/t/590-aio-copy.py
>> | +++ w/python/t/590-aio-copy.py
>> | @@ -117,7 +117,8 @@ src.set_handle_name("src")
>> |  dst = nbd.NBD()
>> |  dst.set_handle_name("dst")
>> |  src.connect_command(["nbdkit", "-s", "--exit-with-parent", "-r",
>> | -                     "pattern", "size=%d" % disk_size])
>> | +                     "--filter=error", "pattern", "error-pread-rate=1",
>> | +                     "size=%d" % disk_size])
>> |  dst.connect_command(["nbdkit", "-s", "--exit-with-parent",
>> |                       "memory", "size=%d" % disk_size])
>> |  asynch_copy(src, dst)
>> ---
>>  python/t/590-aio-copy.py | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/python/t/590-aio-copy.py b/python/t/590-aio-copy.py
>> index 6cde5934..861fa6c8 100644
>> --- a/python/t/590-aio-copy.py
>> +++ b/python/t/590-aio-copy.py
>> @@ -1,5 +1,5 @@
>>  # libnbd Python bindings
>> -# Copyright (C) 2010-2019 Red Hat Inc.
>> +# Copyright (C) 2010-2022 Red Hat Inc.
>>  #
>>  # This program is free software; you can redistribute it and/or modify
>>  # it under the terms of the GNU General Public License as published by
>> @@ -36,6 +36,7 @@ def asynch_copy(src, dst):
>>      # This callback is called when any pread from the source
>>      # has completed.
>>      def read_completed(buf, offset, error):
>> +        assert not error
> 
> This works for the test, since the test is not compiled
> to pyc file, which removes the asserts (like C -DNODBUG)
> by default when building rpms.
> 
> If someone will copy this to a real application they will have no
> error checking.

I consider this a catastrophic bug in python, to be honest. Eliminating
assertions should never be done without an explicit request from whoever
builds the package.

Laszlo

> 
> I would use:
> 
>     if error != 0:
>         raise RuntimeError(f"read: {os.strerror(error)}")
> 
> Which also teaches the reader that error is an errno value.
> 
>>          global bytes_read
>>          bytes_read += buf.size()
>>          wr = (buf, offset)
>> @@ -46,6 +47,7 @@ def asynch_copy(src, dst):
>>      # This callback is called when any pwrite to the destination
>>      # has completed.
>>      def write_completed(buf, error):
>> +        assert not error
>>          global bytes_written
>>          bytes_written += buf.size()
>>          # By returning 1 here we auto-retire the pwrite command.
>> --
>> 2.34.1
> 
> Nir
> 
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