[Bug 2142228] [NEW] `stdbuf -oL cut` doesn't work in a pipeline

[Chow Loong Jin]

Public bug reported:

The following shell pipeline works with gnucut, but not with cut from
rust:

% while sleep 1; do echo hello,world; done | stdbuf -oL cut -f1 -d, |
cat

What's supposed to happen: outputs "hello" every second
What actually happens: no output at all

This breaks scripts that rely on `stdbuf -oL cut` to work in a line-
buffered manner, e.g. this network throughput monitor script for waybar:

    bwm-ng -o csv -t 5000 -C, -u bytes |
        grep -e wlp46s0 -e wlp3s0 -e wlp0s20f3 --line-buffered |
        stdbuf -oL gnucut -f3-4 -d, |
        stdbuf -oL numfmt -d, --to=iec --field=1,2 --suffix=B/s --format=%10.1f |
        stdbuf -oL sed -re 's/K/k/g; s#(.*),(.*)#\2 \1#g'

% lsb_release -rd
Description:    Ubuntu 25.10
Release:        25.10

% apt policy coreutils-from-uutils
coreutils-from-uutils:
  Installed: 0.0.0~ubuntu24
  Candidate: 0.0.0~ubuntu24
  Version table:
 *** 0.0.0~ubuntu24 500
        500 http://sg.archive.ubuntu.com/ubuntu questing/main amd64 Packages
        500 http://sg.archive.ubuntu.com/ubuntu questing/main i386 Packages
        100 /var/lib/dpkg/status

% apt policy rust-coreutils
rust-coreutils:
  Installed: 0.2.2-0ubuntu2.1
  Candidate: 0.2.2-0ubuntu2.1
  Version table:
 *** 0.2.2-0ubuntu2.1 500
        500 http://sg.archive.ubuntu.com/ubuntu questing-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu questing-security/main amd64 Packages
        100 /var/lib/dpkg/status
     0.2.2-0ubuntu2 500
        500 http://sg.archive.ubuntu.com/ubuntu questing/main amd64 Packages

** Affects: rust-coreutils (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2142228

Title:
  `stdbuf -oL cut` doesn't work in a pipeline

Status in rust-coreutils package in Ubuntu:
  New

Bug description:
  The following shell pipeline works with gnucut, but not with cut from
  rust:

  % while sleep 1; do echo hello,world; done | stdbuf -oL cut -f1 -d, |
  cat

  What's supposed to happen: outputs "hello" every second
  What actually happens: no output at all

  This breaks scripts that rely on `stdbuf -oL cut` to work in a line-
  buffered manner, e.g. this network throughput monitor script for
  waybar:

      bwm-ng -o csv -t 5000 -C, -u bytes |
          grep -e wlp46s0 -e wlp3s0 -e wlp0s20f3 --line-buffered |
          stdbuf -oL gnucut -f3-4 -d, |
          stdbuf -oL numfmt -d, --to=iec --field=1,2 --suffix=B/s --format=%10.1f |
          stdbuf -oL sed -re 's/K/k/g; s#(.*),(.*)#\2 \1#g'

  % lsb_release -rd
  Description:    Ubuntu 25.10
  Release:        25.10

  % apt policy coreutils-from-uutils
  coreutils-from-uutils:
    Installed: 0.0.0~ubuntu24
    Candidate: 0.0.0~ubuntu24
    Version table:
   *** 0.0.0~ubuntu24 500
          500 http://sg.archive.ubuntu.com/ubuntu questing/main amd64 Packages
          500 http://sg.archive.ubuntu.com/ubuntu questing/main i386 Packages
          100 /var/lib/dpkg/status

  % apt policy rust-coreutils
  rust-coreutils:
    Installed: 0.2.2-0ubuntu2.1
    Candidate: 0.2.2-0ubuntu2.1
    Version table:
   *** 0.2.2-0ubuntu2.1 500
          500 http://sg.archive.ubuntu.com/ubuntu questing-updates/main amd64 Packages
          500 http://security.ubuntu.com/ubuntu questing-security/main amd64 Packages
          100 /var/lib/dpkg/status
       0.2.2-0ubuntu2 500
          500 http://sg.archive.ubuntu.com/ubuntu questing/main amd64 Packages

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