test(detach): ask the marker rule, not the machine the suite runs on

src/daemon/detach.rs

@@ -17,7 +17,18 @@ const ALREADY_DETACHED: &str = "NIGHTCROW_DETACHED";
 
 /// Whether this process is the backgrounded copy.
 pub fn is_detached_child() -> bool {
-    std::env::var_os(ALREADY_DETACHED).is_some()
+    marker_says_detached(std::env::var_os(ALREADY_DETACHED).as_deref())
+}
+
+/// The rule the marker carries, split from reading it.
+///
+/// Reading the environment inside the rule made the test answer for the
+/// machine it ran on: a suite started from inside a nightcrow pane inherits
+/// the marker from the daemon that spawned the pane, and the foreground case
+/// then failed while saying nothing about the rule. Presence is what counts —
+/// the child is spawned with `"1"`, but an empty value is still a marker.
+fn marker_says_detached(marker: Option<&std::ffi::OsStr>) -> bool {
+    marker.is_some()
 }
 
 /// Re-exec this binary in its own session and return.

src/daemon/detach_tests.rs

@@ -1,5 +1,5 @@
-use super::{background_command, child_args, is_detached_child};
-use std::ffi::OsString;
+use super::{background_command, child_args, marker_says_detached};
+use std::ffi::{OsStr, OsString};
 
 fn args(list: &[&str]) -> Vec<OsString> {
     list.iter().map(OsString::from).collect()
@@ -10,9 +10,25 @@ fn args(list: &[&str]) -> Vec<OsString> {
 /// hypothetical: it happened, and 46 tests failed while dozens of copies of the
 /// harness fought over the same machine. The spawn itself is covered by running
 /// the real thing, not from in here.
+/// The marker is passed in rather than read from the environment: a suite run
+/// from inside a nightcrow pane inherits it from the daemon that spawned the
+/// pane, and asking the environment here would fail on the machine rather than
+/// on the rule.
 #[test]
 fn a_process_with_no_marker_is_the_foreground_copy() {
-    assert!(!is_detached_child());
+    assert!(!marker_says_detached(None));
+}
+
+#[test]
+fn a_process_carrying_the_marker_is_the_background_copy() {
+    assert!(marker_says_detached(Some(OsStr::new("1"))));
+}
+
+#[test]
+fn the_marker_counts_by_being_set_rather_than_by_its_value() {
+    // The child is spawned with "1", but nothing downstream reads the value —
+    // so an empty one must not read as the foreground copy and detach again.
+    assert!(marker_says_detached(Some(OsStr::new(""))));
 }
 
 #[test]