Python 2.7/3.6: issue with output line buffering

[thiell]

I discovered today that the fix for #528 might have affected line output buffering in Python 2.7 and possibly Python 3.6. This can be a problem when clush commands are piped to other tools like grep.

Reproducer below. I'm using lctl mark to display a debug marker in syslog while clush | grep is running.

In Python 2.7.5 (EL7), it only works if PYTHONUNBUFFERED is set to 1 (ugly).

# clush -w oak-h05v[06,16-17] journalctl -n0 -fk | grep MARKER
<nothing... this is the bug>

# PYTHONUNBUFFERED=1 clush -w oak-h05v[06,16-17] journalctl -n0 -fk | grep MARKER
oak-h05v06: Dec 02 18:42:24 oak-h05v06.sunet kernel: Lustre: DEBUG MARKER: Tue Dec  2 18:42:24 2025
...

In Python 3.9.18 (EL9), we're good by default:

# clush -w elm-rcf-mr-h[01-04]s01 journalctl -n0 -fk  | grep MARKER
elm-rcf-mr-h01s01: Dec 02 18:46:38 elm-rcf-mr-h01s01 kernel: Lustre: DEBUG MARKER: Tue Dec  2 18:46:38 2025

Not sure how much effort we want to put into fixing this as nobody has reported it and Python 2.7/3.6 are both quite old.

The impacted code is likely here:

clustershell/lib/ClusterShell/CLI/Display.py

Line 119 in 0cc8cc2

# GH#528 enable line buffering

[degremont]

If what the code does right now is "the future" i'm fine keeping it and considering older Python releases are not fixed, if they are mostly deprecated.

• Python 2.7 is old enough and could be ignored here.
• But for Python 3.6 is the default for RHEL 8 and I'm afraid too many people still use it.