Re: [PATCH mptcp-next] selftests: mptcp: diag: use mptcp_lib_get_info_value

[Matthieu Baerts <matttbe@kernel.org>]

On 10/04/2025 18:58, Matthieu Baerts wrote:
> Hi Geliang,
> 
> On 03/04/2025 09:46, Geliang Tang wrote:
>> From: Geliang Tang <tanggeliang@kylinos.cn>
>>
>> When running diag.sh in a loop, chk_dump_one will report the following
>> "grep: write error":
>>
>>  13 ....chk 2 cestab                                  [ OK ]
>>  grep: write error
>>  14 ....chk dump_one                                  [ OK ]
>>  15 ....chk 2->0 msk in use after flush               [ OK ]
>>  16 ....chk 2->0 cestab after flush                   [ OK ]
>>
>> This error is caused by a broken pipe. When the output of 'ss' is processed
>> by grep, 'head -n 1' will exit immediately after getting the first line,
>> causing the subsequent pipe to close. At this time, if 'grep' is still
>> trying to write data to the closed pipe, it will trigger a SIGPIPE signal,
>> causing a write error.
>>
>> One solution is not to use this problematic "head -n 1" command, but to use
>> mptcp_lib_get_info_value() helper defined in mptcp_lib.sh to get the value
>> of 'token'.
> 
> I was wondering why this helper would help because there will be 2
> connections, and it is still required to limit the output to one. Then I
> saw this helper does limit to the first entry because of 'sed (...);q'.
> 
> I will modify the message here to explain that. And maybe add a comment
> in mptcp_lib.sh as well for later.

Note: technically, we can still have the issue with
mptcp_lib_get_info_value because we also have 'grep | sed' where 'sed'
can finish while 'grep' is still writing. I guess you no longer see the
warning because 'sed' will be slower than 'head -n1'. But OK, this can
be improved later.

> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> 
> Cheers,
> Matt

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.