headed mode (#607)

* headed mode

* fixes

* fixes

* docs

* fixes

* fixes

* fixes

cli/src/test_utils.rs

@@ -0,0 +1,49 @@
+use std::sync::{Mutex, MutexGuard};
+
+/// Global mutex shared across all test modules to prevent parallel tests from
+/// interfering with each other when mutating environment variables.
+pub static ENV_MUTEX: Mutex<()> = Mutex::new(());
+
+/// RAII guard that locks [`ENV_MUTEX`] and restores environment variables on drop.
+pub struct EnvGuard<'a> {
+    _lock: MutexGuard<'a, ()>,
+    vars: Vec<(String, Option<String>)>,
+}
+
+impl<'a> EnvGuard<'a> {
+    pub fn new(var_names: &[&str]) -> Self {
+        let lock = ENV_MUTEX.lock().unwrap();
+        let vars = var_names
+            .iter()
+            .map(|&name| (name.to_string(), std::env::var(name).ok()))
+            .collect();
+        Self { _lock: lock, vars }
+    }
+
+    pub fn set(&self, name: &str, value: &str) {
+        debug_assert!(
+            self.vars.iter().any(|(n, _)| n == name),
+            "EnvGuard::set called with unregistered var: {name}"
+        );
+        std::env::set_var(name, value);
+    }
+
+    pub fn remove(&self, name: &str) {
+        debug_assert!(
+            self.vars.iter().any(|(n, _)| n == name),
+            "EnvGuard::remove called with unregistered var: {name}"
+        );
+        std::env::remove_var(name);
+    }
+}
+
+impl Drop for EnvGuard<'_> {
+    fn drop(&mut self) {
+        for (name, value) in &self.vars {
+            match value {
+                Some(v) => std::env::set_var(name, v),
+                None => std::env::remove_var(name),
+            }
+        }
+    }
+}