[vim/vim] Unix set_child_environment: setenv/putenv return values ignored for TERM/ROWS/LINES/COLUMNS (and related vars) (Issue #20823)

[Shixin Tan]

tsx543 created an issue (vim/vim#20823)

Steps to reproduce

1. Build Unix/POSIX Vim with job/terminal/shell support (normal default).
2. Spawn an external command / job / :terminal so the child environment is set via set_child_environment() / set_default_child_environment() (e.g. :!ls, :terminal, or a job that inherits Vim’s tty size vars).
3. Inspect src/os_unix.c: every setenv() / putenv() call discards its return value. If any call fails (typically ENOMEM), the child is still launched with a missing/stale TERM / ROWS / LINES / COLUMNS / COLORS / VIM_TERMINAL / VIM_SERVERNAME.
   Relevant code in current master (src/os_unix.c):

#ifdef HAVE_SETENV
    setenv("TERM", term, 1);
    ...
    setenv("ROWS", buf, 1);
    setenv("LINES", buf, 1);
    setenv("COLUMNS", buf, 1);
    ...
    /* COLORS / VIM_TERMINAL / VIM_SERVERNAME similarly unchecked */
#else
    /* putenv() fallback also ignores return values */
#endif

Callers include the default shell path and job/terminal startup in the same file (e.g. around the mch_call_shell / job-init paths that invoke set_child_environment()).

Expected behaviour

If setenv() / putenv() fails, Vim should surface an error (or otherwise abort the spawn path) instead of silently starting the child with an incomplete or stale terminal environment.

Version of Vim

9.2.0838

Environment

Operating system: Linux
Terminal: any
Value of $TERM: any
Shell: any

Logs and stack traces

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