Subprocess(fix[encoding]): Enforce UTF-8 decoding for tmux output

[tony]

Summary

• Add regression test reproducing Enforce UTF-8 encoding #678: tmux_cmd uses subprocess.Popen(text=True) without encoding="utf-8", causing FORMAT_SEPARATOR (U+241E) corruption on non-UTF-8 locales
• Fix: add encoding="utf-8" to the subprocess.Popen call in tmux_cmd.__init__

Commit sequence (hermetic proof)

Commit	What	Test result
922a326a	xfail regression test — monkeypatches locale.getencoding → "latin-1", runs list-sessions through tmux_cmd, asserts FORMAT_SEPARATOR survives	XFAIL — separator corrupted to â\x90\x9e, assertion fails at the right place
e05c8ca0	Fix: encoding="utf-8" in subprocess.Popen — zero test changes	XPASS(strict) → CI fails, proving the fix resolves the reproduction
c8279acf	Remove xfail marker — zero code changes	1257 passed

Root cause

Commit 1a5e69a2 (2025-02-02) removed console_to_str() (which had an explicit UTF-8 fallback) and switched to bare text=True. Without encoding="utf-8", CPython's subprocess._text_encoding() falls back to locale.getencoding(). On non-UTF-8 locales, the 3-byte UTF-8 sequence for ␞ (U+241E) is decoded as three latin-1 code points (â\x90\x9e), making .split(FORMAT_SEPARATOR) produce a single unsplit element. This cascades to parse_output() where zip(..., strict=True) raises ValueError, and list accessors like server.sessions return empty results.

tmux has mandated UTF-8 since 2015 — hardcoding encoding="utf-8" matches tmux's output contract.

Fixes: #678
Related: tmux-python/tmuxp#1044

Test plan

• uv run ruff check . --fix --show-fixes — passes
• uv run ruff format . — no changes
• uv run mypy — no issues
• uv run pytest --reruns 0 -vvv — 1257 passed, 2 skipped
• just build-docs — builds successfully

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.29%. Comparing base (5089f31) to head (fc5e6c4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #679   +/-   ##
=======================================
  Coverage   51.29%   51.29%           
=======================================
  Files          25       25           
  Lines        3488     3488           
  Branches      686      686           
=======================================
  Hits         1789     1789           
  Misses       1404     1404           
  Partials      295      295           

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[tony]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

🤖 Generated with Claude Code