Re: [PATCH v6 6/9] iotests: use python logging for iotests.log()

[John Snow <jsnow@redhat.com>]

On 2/27/20 9:21 AM, Max Reitz wrote:
> On 27.02.20 01:06, John Snow wrote:
>> We can turn logging on/off globally instead of per-function.
>>
>> Remove use_log from run_job, and use python logging to turn on
>> diffable output when we run through a script entry point.
>>
>> iotest 245 changes output order due to buffering reasons.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>  tests/qemu-iotests/030        |  4 +--
>>  tests/qemu-iotests/245        |  1 +
>>  tests/qemu-iotests/245.out    | 24 ++++++++---------
>>  tests/qemu-iotests/iotests.py | 50 +++++++++++++++++++++--------------
>>  4 files changed, 45 insertions(+), 34 deletions(-)
> 
> [...]
> 
>> diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
>> index b02d7932fa..60c4c7f736 100644
>> --- a/tests/qemu-iotests/iotests.py
>> +++ b/tests/qemu-iotests/iotests.py
>> @@ -35,6 +35,14 @@
>>  
>>  assert sys.version_info >= (3, 6)
>>  
>> +# Use this logger for logging messages directly from the iotests module
>> +logger = logging.getLogger('qemu.iotests')
>> +logger.addHandler(logging.NullHandler())
> 
> Hm, I never see another handler added to this, so how can these messages
> actually be printed?  Will enabling debug mode somehow make all loggers
> print everything?
> 

Implicit fallback to root logger which handles them when the root logger
is configured, which happens when logging.basicConfig is called.

>> +# Use this logger for messages that ought to be used for diff output.
>> +test_logger = logging.getLogger('qemu.iotests.diff_io')
> 
> Also, why does logger get a null handler and this by default does not?
> I’m asking because test_logger makes it look like you don’t necessarily
> need a handler for output to be silently discarded.
> 
> Max
> 

It's a library trick. By adding a null handler at `qemu.iotests` I add a
default handler to everything produced by iotests. When logging is not
configured, this stops messages from being printed -- there IS a default
"nonconfigured" logging behavior and this stops it.

What I learned since the last time I wrote this patchset is that you
only need a NullHandler at some particular root, so "qemu.iotests"
suffices. "qemu.iotests.diff_io" is a child of that other logger.

>>  # This will not work if arguments contain spaces but is necessary if we
>>  # want to support the override options that ./check supports.
>>  qemu_img_args = [os.environ.get('QEMU_IMG_PROG', 'qemu-img')]
> 

-- 
—js