max / alloy

Stop a test from changing HOME under the other tests `a_mount_expands_a_leading_tilde` set HOME process-wide and restored it, under a comment reading "SAFETY: single-threaded test". A Rust test binary is not that: the harness runs tests on a thread pool in one process, so for the length of that test every other test saw /home/tester. `a_host_export_calls_distrobox_export_once_per_binary` is the one that noticed. It builds an argv from export_dir(), which reads HOME, then compares it against a second call to the same function, so it failed whenever the fixture's HOME was live for exactly one of the two. Measured at roughly one run in five, and it passes alone every time, which is the shape that gets written off as noise. settings.rs had already solved this and said why: expand() delegates to expand_in(target, config, home) so the rules are testable without touching process environment, and its comment calls a HOME-changing test "a flake waiting for a slow machine". mount_path now splits the same way and the test passes a home instead of setting one. Twelve consecutive runs clean, against 4-of-5 before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Max Johnson <me@maxj.phd> · 2026-07-30 12:33 UTC

Signed with PGP, not checked

Commit: fb636cb24d40d3f2e727a2e4eafe521adbda00cb

Parent: 1d2d549

1 file changed, +36 insertions, -10 deletions

M crates/alloy/src/pkg.rs +36 -10

			@@ -706,11 +706,29 @@
706	706		/// against the same list, and it has to be the same expansion or the two
707	707		/// disagree about what `~/code` means.
708	708		fn mount_path(mount: &str) -> Result<String> {
	709	+	mount_path_in(mount, std::env::var("HOME").ok().as_deref())
	710	+	}
	711	+
	712	+	/// [`mount_path`] against an explicit home.
	713	+	///
	714	+	/// Split out for the same reason [`crate::settings::expand`] is, and its comment
	715	+	/// there already said why: `set_var` is unsafe in a threaded test binary, and a
	716	+	/// test that changes `HOME` under the other tests is a flake waiting for a slow
	717	+	/// machine. This module did not follow that and got the flake.
	718	+	///
	719	+	/// It was `a_mount_expands_a_leading_tilde` setting `HOME=/home/tester`
	720	+	/// process-wide under a comment reading "single-threaded test", which is not what
	721	+	/// a Rust test binary is: the harness runs tests on a thread pool in one process.
	722	+	/// Any test reading `HOME` concurrently saw the fixture's value.
	723	+	/// `a_host_export_calls_distrobox_export_once_per_binary` is the one that
	724	+	/// noticed, because it compares an argv built from [`export_dir`] against a
	725	+	/// second call to it, and failed roughly one run in five.
	726	+	fn mount_path_in(mount: &str, home: Option<&str>) -> Result<String> {
709	727		let path = mount.strip_suffix(":ro").unwrap_or(mount);
710	728
711	729		let path = match path.strip_prefix("~/") {
712	730		Some(rest) => {
713		-	let home = std::env::var("HOME").context("`~` in a mount needs HOME set")?;
	731	+	let home = home.context("`~` in a mount needs HOME set")?;
714	732		format!("{home}/{rest}")
715	733		}
716	734		None => path.to_string(),
			@@ -2499,17 +2517,25 @@
2499	2517
2500	2518		// Podman does no tilde expansion, so an unexpanded `~` would create a
2501	2519		// directory named `~` rather than mounting the home path meant.
	2520	+	//
	2521	+	// Against an explicit home rather than by setting one. The previous version
	2522	+	// of this test set `HOME` process-wide and restored it, under a comment
	2523	+	// claiming it was single-threaded; the harness runs tests on a thread pool in
	2524	+	// one process, so for the length of this test every other test saw
	2525	+	// `/home/tester`. `a_host_export_calls_distrobox_export_once_per_binary`
	2526	+	// reads `HOME` through `export_dir` and failed about one run in five.
2502	2527		#[test]
2503	2528		fn a_mount_expands_a_leading_tilde() {
2504		-	// SAFETY: single-threaded test, and the value is restored below.
2505		-	let previous = std::env::var_os("HOME");
2506		-	unsafe { std::env::set_var("HOME", "/home/tester") };
2507		-	let bound = bind("~/code/thing").unwrap();
2508		-	match previous {
2509		-	Some(home) => unsafe { std::env::set_var("HOME", home) },
2510		-	None => unsafe { std::env::remove_var("HOME") },
2511		-	}
2512		-	assert_eq!(bound, "/home/tester/code/thing:/home/tester/code/thing");
	2529	+	let path = mount_path_in("~/code/thing", Some("/home/tester")).unwrap();
	2530	+	assert_eq!(path, "/home/tester/code/thing");
	2531	+	}
	2532	+
	2533	+	// The error names the variable rather than the mount, because a `~` with no
	2534	+	// HOME is an environment problem and nothing about the spec would fix it.
	2535	+	#[test]
	2536	+	fn a_tilde_with_no_home_says_which_is_missing() {
	2537	+	let err = mount_path_in("~/code/thing", None).unwrap_err().to_string();
	2538	+	assert!(err.contains("HOME"), "got: {err}");
2513	2539		}
2514	2540
2515	2541		// Podman rejects a relative source too, but only once the command runs, and