Hi,
I've found a regression in ncurses 6.6 (patchdate 20251230) where
setupterm() fails on valid Linux pseudo-terminals, causing tmux's
tty_init() to fail and "open terminal failed: not a terminal" on attach.
The issue does NOT occur with ncurses 6.5 (patchdate 20240427) or
the Ubuntu system ncurses 6.4.
ENVIRONMENT
OS: Ubuntu 24.04.4 LTS
Kernel: 6.8.0-107-generic x86_64
tmux: 3.6b (Homebrew)
ncurses: 6.6.20251230 (Homebrew, wide-char build)
6.5.20240427 (built from source, same configure flags)
6.4+20240113 (Ubuntu system /lib/x86_64-linux-gnu)
REPRODUCTION
# Using tmux 3.6b linked against ncurses 6.6:
$ tmux new -d -s test
$ tmux attach -t test
open terminal failed: not a terminal
This happens in:
- Interactive SSH sessions (ssh user@host, then tmux a)
- VS Code Remote integrated terminal
- PTYs created by script(1)
The ONLY working case is ssh -t host "tmux a -t test" (forced PTY
with terminal size forwarding from the SSH client).
ISOLATION
Same tmux 3.6b binary, different ncurses library at runtime:
| ncurses | method | result |
|-------------------|-------------------------------------|---------|
| 6.6.20251230 | default brew linkage | FAIL |
| 6.5.20240427 | LD_LIBRARY_PATH=.../ncurses-6.5/lib | OK |
| 6.4 (system) | LD_LIBRARY_PATH=/lib/x86_64-linux-gnu | OK |
Commands to reproduce the isolation test:
# FAIL with 6.6
script -qc 'tmux -S /tmp/t66 new -d -s x && \
tmux -S /tmp/t66 a -t x' /dev/null
# OK with 6.5
script -qc 'LD_LIBRARY_PATH=/path/to/ncurses-6.5/lib \
tmux -S /tmp/t65 new -d -s x && \
LD_LIBRARY_PATH=/path/to/ncurses-6.5/lib \
tmux -S /tmp/t65 a -t x' /dev/null
MECHANISM
tmux's server receives the client's PTY fd via SCM_RIGHTS, then
calls tty_init() -> setupterm(). With ncurses 6.6, setupterm()
apparently rejects the terminal, causing tty_init() to return -1.
This leaves the CLIENT_TERMINAL flag unset, and server_client_open()
returns "not a terminal".
strace shows the PTY is valid — TCGETS succeeds, the fd resolves
to /dev/pts/N, and permissions are correct (crw--w---- owner:tty).
The tmux source code for this path is identical between 3.4 and
3.6b, confirming the behavioral change is in ncurses, not tmux.
BUILD FLAGS (both 6.5 and 6.6 built with):
./configure --prefix=... --with-shared --with-widec \
--enable-pc-files --enable-sigwinch --enable-ext-colors \
--without-debug --without-ada
I haven't bisected the exact ncurses patch between 6.5 and 6.6 yet.
Happy to test patches or provide additional traces if helpful.
Thanks,
Liam
