swarm: bash task status keys expire after 72 hours

Nothing deletes the key of a finished task, so the bucket would grow with every
task an agent runs. The shared bucket helper takes an optional max age, applied
when the bucket is created; a live task's republishing keeps its own key.

swarm-wire/src/bash_task.rs

@@ -33,6 +33,11 @@ pub const HEARTBEAT_S: i64 = 60;
 /// A `running` status whose [`Status::ts`] is older than this is stale.
 pub const STALE_AFTER_S: i64 = 3 * HEARTBEAT_S;
 
+/// How long a task's key stays in the bucket after its last write. A live task
+/// is republished every [`HEARTBEAT_S`], which keeps its key; a finished one
+/// is only worth listing for a few days, and nothing else deletes the key.
+pub const STATUS_MAX_AGE: std::time::Duration = std::time::Duration::from_hours(72);
+
 /// The longest [`Status::cmd`], in characters; a longer command is cut.
 pub const CMD_MAX_CHARS: usize = 200;
 
@@ -218,6 +223,14 @@ mod tests {
         assert!(!status(State::Done, 10_000).is_running(10_000));
     }
 
+    /// A live task's key must outlive the gap between two heartbeats and the
+    /// stale window by a wide margin, or a running task could expire.
+    #[test]
+    fn a_key_outlives_the_stale_window_by_far() {
+        let stale = std::time::Duration::from_secs(STALE_AFTER_S.unsigned_abs());
+        assert!(STATUS_MAX_AGE >= stale * 100);
+    }
+
     #[test]
     fn a_long_command_is_cut_with_an_ellipsis() {
         assert_eq!(clip_cmd("ls"), "ls");