Struct SessionRegistry 

pub struct SessionRegistry { /* private fields */ }

Manages active sessions.

Locking design (lock C)

Uses tokio::sync::Mutex because feed_response holds the lock while calling Session::feed_one() (which itself acquires the per-session std::sync::Mutex<SessionStatus>, lock A). The lock ordering invariant is always C → A — no code path acquires A then C, so deadlock is structurally impossible.

tokio::sync::Mutex is chosen here (rather than std::sync::Mutex) because feed_response must take the session out of the map for the async wait_event() call. The two-phase pattern (lock → remove → unlock → await → lock → reinsert) requires an async-aware mutex to avoid holding the lock across the wait_event().await.

Contention

list_snapshots() (from alc_status) holds lock C while iterating all sessions. During this time, feed_response for any session is blocked. Given that snapshot iteration is O(n) with n = active sessions (typically 1–3) and each snapshot takes microseconds, this is acceptable. If session count grows significantly, consider switching to a concurrent map or per-session locks.

Interaction with lock A

Session::snapshot() (called under lock C in list_snapshots) acquires lock A via ExecutionMetrics::snapshot(). This is safe:

• Lock order: C → A (consistent with feed_response)
• Lock A hold time: microseconds (JSON field reads)
• Lock A is per-session (no cross-session contention)

Implementations

impl SessionRegistry

pub fn new() -> Self

pub async fn start_execution( &self, session: Session, ) -> Result<(String, FeedResult), SessionError>

Start execution and wait for first event (pause or completion).

pub async fn feed_response( &self, session_id: &str, query_id: &QueryId, response: String, ) -> Result<FeedResult, SessionError>

Feed one response to a paused session by query_id.

If this completes all pending queries, the session resumes and returns the next event (Paused or Finished). If queries remain, returns Accepted { remaining }.

pub async fn list_snapshots(&self) -> HashMap<String, Value>

Snapshot all active sessions for external observation (alc_status).

Returns a map of session_id → snapshot JSON. Only includes sessions currently held in the registry (i.e. paused, awaiting responses). Sessions that have completed are already removed from the registry.

Trait Implementations

impl Default for SessionRegistry

fn default() -> Self

Returns the “default value” for a type.