test(runtime): isolate session and git metadata checks

rust/crates/runtime/src/session_control.rs

@@ -828,10 +828,40 @@ mod tests {
     use std::fs;
     use std::path::{Path, PathBuf};
     use std::sync::atomic::{AtomicU64, Ordering};
+    use std::sync::{Mutex, OnceLock};
     use std::time::{SystemTime, UNIX_EPOCH};
 
     static TEMP_COUNTER: AtomicU64 = AtomicU64::new(0);
 
+    fn env_lock() -> std::sync::MutexGuard<'static, ()> {
+        static LOCK: OnceLock<Mutex<()>> = OnceLock::new();
+        LOCK.get_or_init(|| Mutex::new(()))
+            .lock()
+            .expect("env lock")
+    }
+
+    struct EnvVarGuard {
+        key: &'static str,
+        previous: Option<std::ffi::OsString>,
+    }
+
+    impl EnvVarGuard {
+        fn set(key: &'static str, value: &Path) -> Self {
+            let previous = std::env::var_os(key);
+            std::env::set_var(key, value);
+            Self { key, previous }
+        }
+    }
+
+    impl Drop for EnvVarGuard {
+        fn drop(&mut self) {
+            match &self.previous {
+                Some(value) => std::env::set_var(self.key, value),
+                None => std::env::remove_var(self.key),
+            }
+        }
+    }
+
     fn temp_dir() -> PathBuf {
         let nanos = SystemTime::now()
             .duration_since(UNIX_EPOCH)
@@ -1290,8 +1320,11 @@ mod tests {
     #[test]
     fn latest_session_returns_all_empty_error_when_sessions_exist_but_have_no_messages() {
         // given — create sessions with 0 messages (empty)
+        let _env_guard = env_lock();
         let base = temp_dir();
         fs::create_dir_all(&base).expect("base dir should exist");
+        let isolated_config_home = base.join("config-home");
+        let _claw_config_home = EnvVarGuard::set("CLAW_CONFIG_HOME", &isolated_config_home);
         let store = SessionStore::from_cwd(&base).expect("store should build");
 
         let empty_handle = store.create_handle("empty-session");

rust/crates/runtime/src/worker_boot.rs

@@ -1644,16 +1644,13 @@ mod tests {
 
         let tmp = tempfile::tempdir().expect("tempdir");
         let worktree = tmp.path().join("worktree");
-        let git_dir = tmp.path().join("external-gitdir");
         fs::create_dir_all(&worktree).expect("worktree dir");
-        fs::create_dir_all(git_dir.join("objects")).expect("objects dir");
+        Command::new("git")
-        fs::create_dir_all(git_dir.join("refs/heads")).expect("refs dir");
+            .arg("init")
-        fs::write(git_dir.join("HEAD"), "ref: refs/heads/main\n").expect("HEAD");
+            .current_dir(&worktree)
-        fs::write(
+            .output()
-            worktree.join(".git"),
+            .expect("git init should run");
-            format!("gitdir: {}\n", git_dir.display()),
+        let git_dir = worktree.join(".git");
-        )
-        .expect(".git file");
 
         let original_permissions = fs::metadata(&git_dir)
             .expect("gitdir metadata")