Re: [Qemu-devel] [PATCH v6 1/6] iotests: allow Valgrind checking all QEMU processes

[Andrey Shinkevich]

On 29/08/2019 03:30, Eric Blake wrote:
> On 8/28/19 5:58 PM, John Snow wrote:
> 
>>> +++ b/tests/qemu-iotests/common.rc
>>> @@ -60,61 +60,132 @@ if ! . ./common.config
>>>       exit 1
>>>   fi
>>>   
>>> +# Unset the variables to turn Valgrind off for specific processes, e.g.
> 
> That's not unsetting, that's setting to the empty string.
> 

Thanks Eric, I will make the correction of the comment. Any string other 
than "y", including the empty one, fits.

>>> +# $ VALGRIND_QEMU_IO= ./check -qcow2 -valgrind 015
>>> +
>>> +: ${VALGRIND_QEMU_VM='y'}
>>> +: ${VALGRIND_QEMU_IMG='y'}
>>> +: ${VALGRIND_QEMU_IO='y'}
>>> +: ${VALGRIND_QEMU_NBD='y'}
>>> +: ${VALGRIND_QEMU_VXHS='y'}
>>> +
>>

I am going to make the change:

: ${VALGRIND_QEMU_VM=$VALGRIND_QEMU}
: ${VALGRIND_QEMU_IMG=$VALGRIND_QEMU}
: ${VALGRIND_QEMU_IO=$VALGRIND_QEMU}
: ${VALGRIND_QEMU_NBD=$VALGRIND_QEMU}
: ${VALGRIND_QEMU_VXHS=$VALGRIND_QEMU}

and get rid of the local VALGRIND_ON="${VALGRIND_QEMU}"

so that the code will be optimized.

>> I have to admit to you that I'm not familiar with this trick. I'm
>> looking it up and I see := documented, but not = alone.
> 
> It's been a repeated complaint to the bash developer that the manual is
> doing a disservice to its users by not documenting ${var=val} in an
> easily searchable form.  It IS documented, but only by virtue of
> ${var:=val} occurring under a section header that states:
> 
>         When not performing substring expansion,  using  the  forms
> documented
>         below  (e.g.,  :-),  bash  tests for a parameter that is unset or
> null.
>         Omitting the colon results in a test  only  for  a  parameter
> that  is
>         unset.
> 
> So the choice is whether you want to special case a variable set to an
> empty string the same as an unset variable, or the same as a variable
> with a non-empty value.
> 

Thank you all for your reviews and comments. The purpose why I omitted 
the colon is to allow a user writing the shorter command syntax like
$ VALGRIND_QEMU_IO= ./check -valgrind <test#>
rather than
$ VALGRIND_QEMU_IO=" 'no' or 'off' or else anything other than 'y' " 
./check -valgrind <test#>
so, no need to strike the Shift key twice and guess at what else is 
acceptable to type )))

The variable default value 'y' looks good to me to implement the new 
functionality that is compatible with the existing one when we just set 
the '-valgrind' switch. The general idea behind using the Valgrind is to 
make a careful search for memory issues. Once found, a user can tune the 
particular test with extra variables to save their development/testing 
time as John suggested. Also, no need to specify all the five long name 
variables each time a user writes the command if default values aren't set.

I am flexible to make a change that is good for all. So, what solution 
will we come to?

Andrey

>>
>> It doesn't seem documented here at all:
>> https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
>>
>> I see it here, though:
>> https://www.tldp.org/LDP/abs/html/parameter-substitution.html
>>
>> And it seems to work, but I'm not sure if this works with BSD or OSX's
>> sh. I see Eric comment on that compatibility a lot, so maybe I'll let
>> him chime in.
> 
> It's quite portable; POSIX requires it, and autoconf relies on it.
> 

-- 
With the best regards,
Andrey Shinkevich