Flexible allocation hangs when the output of `scontrol show` is large

[vkarak]

$ ./bin/reframe -C config/cscs.py -c ../misc/reframe-tests/flexible/hostname_check.py -r
Command line: ./bin/reframe -C config/cscs.py -c ../misc/reframe-tests/flexible/hostname_check.py -r
Reframe version: 2.15-dev2
Launched by user: karakasv
Launched on host: daint102
Reframe paths
=============
    Check prefix      :
    Check search path : '../misc/reframe-tests/flexible/hostname_check.py'
    Stage dir prefix     : /users/karakasv/Devel/reframe/stage/
    Output dir prefix    : /users/karakasv/Devel/reframe/output/
    Perf. logging prefix : /users/karakasv/Devel/reframe/perflogs
[==========] Running 1 check(s)
[==========] Started on Mon Nov  5 23:16:20 2018

[----------] started processing HostnameCheck (HostnameCheck)
[ RUN      ] HostnameCheck on daint:gpu using PrgEnv-cray

scontrol seems to be hanging for some reason:

$ ps -elf | grep scontrol
0 S karakasv  7892  7880  0  80   0 - 28803 pipe_w 23:16 ?        00:00:00 scontrol -a show -o nodes
0 S karakasv 25636 18079  0  80   0 -  2023 pipe_w 23:29 pts/9    00:00:00 grep --color=always scontrol

$ pstree -u $USER
sshd───bash───python3───scontrol

[vkarak]

The problem is caused the use of the wait() and the fact that the output is large enough.

[teojgo]

The problem happens due to the large number of lines produced when trying to show all the nodes in a large system such as daint.