Closes #5696. (Two prior attempts at this — #5133 and #5882 — were closed when their contributors went inactive. Picking it back up.)

Background

Every fatal path in mitmproxy / mitmdump / mitmweb funnels through sys.exit(1). A parent process (CI runner, supervisor, watcher script) only sees the integer code, so it can't distinguish

• "you typed --invalid-opt" from
• "the file we're saving flows to became unwritable mid-stream" from
• "no TTY available, can't run the console UI"

…all three return 1. Real users have asked for this since #4669 in 2021.

Change

Add mitmproxy/utils/exit_codes.py with categorised constants. Categories are spaced by tens so future additions can slot into the appropriate group without renumbering existing ones (matches the design @Prinzhorn proposed in #5133):

GENERIC_ERROR        = 1     # backwards-compat catch-all
STARTUP_ERROR        = 10    # 10-19: startup-time problems
NO_UTF_CONSOLE       = 20    # 20-29: environment problems
NO_TTY               = 21
INVALID_ARGS         = 30    # 30-39: user-supplied input
INVALID_OPTIONS      = 31
CANNOT_PRINT         = 40    # 40-49: I/O problems
CANNOT_WRITE_TO_FILE = 41
DEBUG_EXIT           = 50    # 50-59: debugging hooks

Then thread the constants through every existing sys.exit(1) call site:

The commented-out experimental # sys.exit(1) for the non-UTF-8 console branch in console/master.py:221 is left untouched — that's an active "let's see if not exiting affects users" experiment from 2022, not a fatal path that needs categorisation.

Tests

• test/mitmproxy/utils/test_exit_codes.py (new) pins the codes for termlog (cannot print), save (cannot write), and errorcheck (startup error), plus a sanity check that all codes are distinct.
• test/mitmproxy/addons/test_termlog.py::test_cannot_print already asserted on exit_code == 1; updated to use the new exit_codes.CANNOT_PRINT constant so the existing regression coverage now pins the right code.

$ pytest test/mitmproxy/addons/test_termlog.py test/mitmproxy/addons/test_errorcheck.py test/mitmproxy/addons/test_save.py test/mitmproxy/utils/ test/mitmproxy/tools/
229 passed, 1 skipped, 724 warnings in 3.47s

Notes

• Diff is +185 / -9 across 10 files (one new module + one new test file + small edits at each call site).
• No public-API change beyond the new exit_codes module — existing callers that read SystemExit.code from a subprocess just see a different integer for branches they previously couldn't disambiguate.
• Numbering follows @Prinzhorn's original proposal exactly for backwards-compat with anyone who already wrote tooling against the prior PR's codes; DEBUG_EXIT = 50 is the only addition (the prior PRs didn't categorise the MITMPROXY_DEBUG_EXIT env-var path).
• Happy to drop DEBUG_EXIT and revert utils/debug.py to sys.exit(1) if you'd prefer keeping the debug-only path uncategorised — let me know.