[erlang-questions] os:cmd hang on OTP 18

[Dániel Szoboszlay]

Honestly, I didn't try with other command variations. There are many
commands that do not hang when run from os:cmd, regardless of the OTP
version. But this particular command does hang with one OTP version, and
not with the other OTP version. So the difference is in OTP, and I want to
find out what has changed.

Daniel

On Wed, 26 Jul 2017 at 21:34 Dmytro Lytovchenko <
dmytro.lytovchenko@REDACTED> wrote:

> Is it something lbzip2 related?
> Did you try normal single-thread bzip2? (-j flag or --bzip2)
> What is you use gzip? (-z or --gzip)
>
> 2017-07-26 21:27 GMT+02:00 Dániel Szoboszlay <dszoboszlay@REDACTED>:
>
>> Hi,
>>
>> I've encountered a strange problem with os:cmd when running tar and
>> lbzip2. Steps to reproduce:
>>
>> # create some lbzip2 compressed data
>>
>> dd if=/dev/urandom of=/tmp/testfile count=10
>> tar -cf - -C /tmp testfile | lbzip2 -6 -n 4 | dd of=/tmp/tartest status=none
>>
>>
>> # try to extract the archive from Erlang with os:cmd
>>
>> erl -noinput -eval 'os:cmd("tar -C /tmp/ -xf /tmp/tartest --use-compress-program=lbzip2"), init:stop().'
>>
>>
>> This worked fine with OTP 17.5.6.7, but with OTP 18.3.4.5 the command
>> hangs: lbzip2 just sits in a rt_sigsuspend syscall waiting for a USR2, PIPE
>> or XFSZ signal. And its parent, the tar process waits in a wait4 syscall
>> for lbzip2 to terminate.
>>
>> I don't have at the moment any newer OTP version installed, I'm not sure
>> how OTP 19 or 20 would behave.
>>
>> I tried to strace the processes, but there's too much noise, I couldn't
>> yet figure out anything interesting there.
>>
>> I also tried to diff OTP 17 & 18, but os:cmd/1 and friends didn't change.
>> I'm not sure about the port code, but at least the release notes didn't
>> mention anything major. Or did I miss something? Does anyone have an idea
>> what may have changed between these OTP versions?
>>
>> Thanks,
>> Daniel
>>
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>>
>>
>
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