[Tech-debt] Tests for enable_dns_mode script don't exercise production code (#156 F1)

[flyhigher139]

Context

PR #156 added 3 tests to defend against #155's silent failure (proxy binary missing → DNS mode looks enabled but port 53 has no listener):

• test_pid_file_content_format — asserts the script contains expected echo "$PROXY_PID ..." line
• test_enable_script_contains_safety_layers — asserts the script has [ ! -x ] / kill -0 / log dump / PROXY_PID / networksetup ordering
• test_enable_script_loudly_fails_when_proxy_missing — runs a 6-line [ ! -x ] snippet through /bin/sh

Problem: the first two rebuild the script with their own format! macro call instead of invoking the production enable_dns_mode script generator. The shell test executes only a synthetic snippet, not the real PROXY_PID / kill -0 / log-dump path.

Net effect: reverting enable_dns_mode to the old set -e + cmd & echo "\$! ..." buggy form would keep all 3 tests passing. The tests guard against script-shape regression, not against silent failure on production paths.

Repro

git checkout fix/dns-mode-silent-failure-155 -- src-tauri/crates/mhost-dns/src/platform.rs
# revert just the script body inside enable_dns_mode to the old `echo "$! {proxy}"` form
cargo test --package mhost-dns --lib test_enable_script_contains_safety_layers test_pid_file_content_format test_enable_script_loudly_fails_when_proxy_missing
# All 3 pass against the buggy version

Fix

Extract production builders and have all tests consume them:

// in platform.rs
pub(crate) fn build_enable_script(
    proxy_path: &Path, dns_port: u16, pid_file: &Path, log_path: &Path, interface: &str,
) -> String { /* the actual script body */ }

pub(crate) fn validate_proxy_binary(path: &Path) -> Result<(), PlatformError> { /* pre-check */ }

Test refactor:

1. Structural tests consume build_enable_script(...) directly — assert against the production string.
2. Execution test writes the production script to a temp file, swaps in fake mhost-dns-proxy (long-running script that just sleeps) and fake networksetup (writes args to a log file, exits 0 or non-zero per flag), runs /bin/sh on it with MHOST_RUNTIME_DIR=/tmp/path with space and interface="Thunderbolt Ethernet", asserts:
   • Old script: exits 0 with no PID file (regression for [Bug] DNS mode 在 pnpm tauri dev 下静默失效：mhost-dns-proxy binary 缺失导致所有查询卡死 #155 root cause)
   • Fixed script: writes PID file, kills proxy on networksetup failure, etc.
3. Cover:
   • Path containing spaces (PID file / log / runtime_dir)
   • Interface containing spaces
   • Delayed proxy startup (kill -0 retry loop with timeout)
   • Proxy bind failure (early exit before bind)
   • networksetup failure post-proxy-startup (transactional cleanup)

Linked

• PR fix(dns): surface silent failures when mhost-dns-proxy binary is missing (#155) #156 (primary fix; this issue deferred as follow-up): fix(dns): surface silent failures when mhost-dns-proxy binary is missing (#155) #156
• Issue [Bug] DNS mode 在 pnpm tauri dev 下静默失效：mhost-dns-proxy binary 缺失导致所有查询卡死 #155 (original silent-failure report): [Bug] DNS mode 在 pnpm tauri dev 下静默失效：mhost-dns-proxy binary 缺失导致所有查询卡死 #155

[flyhigher139]

Closed via commit 9e80a7e on branch fix/dns-mode-test-coverage-158.

What changed

The F1/F2 structural-test fix from PR #156 review (commit 5e0b109) was already in place — build_enable_script / validate_proxy_binary were already extracted as pub(crate) and the existing 3 tests already consumed the production builder. The remaining gap from this issue was end-to-end execution coverage: existing tests only exercised the Layer-1 fail path (binary missing → exit 127).

This commit adds a fake-binary test harness + 5 execution tests that run the production builder's output through /bin/sh:

Test	Scenario
test_enable_script_happy_path_with_fakes	long-running proxy + succeeding networksetup → exit 0, PID file "PID FULL_PROXY_PATH", EXIT trap disarmed
test_enable_script_kill_zero_catches_immediate_exit	proxy exit 1 immediately → exit non-zero with "exited within 1s" + log dump
test_enable_script_transactional_cleanup_on_networksetup_failure	long proxy + failing networksetup → EXIT trap kills proxy + removes PID file + propagates exit code
test_enable_script_end_to_end_with_spaces_in_paths	full happy path with spaces in every path component
test_old_buggy_script_silently_succeeds_with_dead_pid	hand-builds the pre-#155 buggy script and demonstrates its silent failure (exit 0 + PID file pointing at a dead process). Regression proof for the production fix.

Test harness

• setup_fake_bin_env(proxy_contents, networksetup_contents) — writes fake binaries to a tempdir and prepends it to PATH so production scripts can be exercised without root.
• exec_production_enable_script(inputs) — writes builder output to a 0o700 temp file and shells out.
• kill_proxy_from_pid_file(path) — cleans up the long-running fake proxy left by happy-path tests.
• EnvRestore RAII guard — restores MHOST_RUNTIME_DIR + PATH on test exit, so a panic mid-test doesn't pollute siblings.

Verification

CI matrix locally:

• cargo fmt --all -- --check ✅
• cargo clippy --all-targets --all-features -- -D warnings ✅
• cargo test --all-features --workspace ✅ (109 mhost-dns tests, up from 104)

Ready to merge.