broker: batch reminder delivery in single db transaction

hive-c0re/src/broker.rs

@@ -717,27 +717,74 @@ impl Broker {
     /// Emits a `Sent` event on the broadcast channel after the transaction
     /// commits (so subscribers see the inbox message but never see a
     /// "phantom" send for a transaction that rolled back).
-    pub fn deliver_reminder(&self, id: i64, agent: &str, message: &str) -> Result<()> {
+    /// Deliver a batch of reminders in a single transaction, reducing
+    /// lock contention on the shared sqlite connection under high
+    /// reminder volume. Returns per-item results so the scheduler can
+    /// record individual failures without aborting successful ones.
+    ///
+    /// Items where the INSERT+UPDATE succeeds get a `MessageEvent::Sent`
+    /// emitted after the transaction commits. Items that fail are
+    /// returned as `Err` in the output vec (index-aligned with input).
+    pub fn deliver_reminders_batch(
+        &self,
+        items: &[(i64, String, String)], // (reminder_id, agent, body)
+    ) -> Vec<Result<()>> {
+        if items.is_empty() {
+            return Vec::new();
+        }
         let now = now_unix();
         let mut conn = self.conn.lock().unwrap();
-        let tx = conn.transaction()?;
-        tx.execute(
-            "INSERT INTO messages (sender, recipient, body, sent_at) VALUES (?1, ?2, ?3, ?4)",
-            params!["reminder", agent, message, now],
-        )?;
-        tx.execute(
-            "UPDATE reminders SET sent_at = ?1 WHERE id = ?2",
-            params![now, id],
-        )?;
-        tx.commit()?;
+        // Build one transaction for all deliveries so we hold the lock
+        // once rather than N times. On a batch-level error (e.g. DB
+        // corruption), fall back to returning per-item errors so the
+        // scheduler records the failure cleanly.
+        let tx = match conn.transaction() {
+            Ok(t) => t,
+            Err(e) => {
+                let err_str = format!("{e:#}");
+                return items
+                    .iter()
+                    .map(|_| Err(anyhow::anyhow!("{}", err_str.clone())))
+                    .collect();
+            }
+        };
+        let mut results: Vec<Result<()>> = Vec::with_capacity(items.len());
+        for (id, agent, body) in items {
+            let r = (|| -> Result<()> {
+                tx.execute(
+                    "INSERT INTO messages (sender, recipient, body, sent_at) \
+                     VALUES (?1, ?2, ?3, ?4)",
+                    params!["reminder", agent, body, now],
+                )?;
+                tx.execute(
+                    "UPDATE reminders SET sent_at = ?1 WHERE id = ?2",
+                    params![now, id],
+                )?;
+                Ok(())
+            })();
+            results.push(r);
+        }
+        if let Err(e) = tx.commit() {
+            let err_str = format!("{e:#}");
+            return items
+                .iter()
+                .map(|_| Err(anyhow::anyhow!("{}", err_str.clone())))
+                .collect();
+        }
         drop(conn);
-        let _ = self.events.send(MessageEvent::Sent {
-            from: "reminder".to_owned(),
-            to: agent.to_owned(),
-            body: message.to_owned(),
-            at: now,
-        });
-        Ok(())
+        // Emit per-row Sent events (only for rows that succeeded).
+        for ((id, agent, body), result) in items.iter().zip(results.iter()) {
+            if result.is_ok() {
+                let _ = self.events.send(MessageEvent::Sent {
+                    from: "reminder".to_owned(),
+                    to: agent.clone(),
+                    body: body.clone(),
+                    at: now,
+                });
+                tracing::debug!(reminder_id = id, %agent, "reminder delivered");
+            }
+        }
+        results
     }
 }
 

hive-c0re/src/reminder_scheduler.rs

@@ -27,8 +27,8 @@
 //!   small and the bulky payload can be read out of band.
 //!
 //! Atomicity of the inbox INSERT + `reminders.sent_at` UPDATE is handled
-//! inside `Broker::deliver_reminder`; this module only computes the
-//! body string before calling it.
+//! inside `Broker::deliver_reminders_batch`; this module only computes the
+//! body strings before calling it.
 
 use std::io::Write;
 use std::os::unix::fs::OpenOptionsExt;
@@ -75,9 +75,23 @@ fn tick(coord: &Arc<Coordinator>) {
             return;
         }
     };
-    for (agent, id, message, file_path) in due {
-        let body = prepare_body(&agent, &message, file_path.as_deref());
-        if let Err(e) = coord.broker.deliver_reminder(id, &agent, &body) {
+    if due.is_empty() {
+        return;
+    }
+    // Resolve body strings (file-path writes / inline) before entering
+    // the batch transaction so the DB lock is held as briefly as possible.
+    let items: Vec<(i64, String, String)> = due
+        .iter()
+        .map(|(agent, id, message, file_path)| {
+            let body = prepare_body(agent, message, file_path.as_deref());
+            (*id, agent.clone(), body)
+        })
+        .collect();
+    // Single-transaction batch: one DB lock acquisition for N reminders
+    // instead of N sequential lock/unlock cycles.
+    let results = coord.broker.deliver_reminders_batch(&items);
+    for ((id, agent, _body), result) in items.iter().zip(results.iter()) {
+        if let Err(e) = result {
             let reason = format!("{e:#}");
             tracing::warn!(
                 reminder_id = id,
@@ -85,11 +99,8 @@ fn tick(coord: &Arc<Coordinator>) {
                 error = %reason,
                 "failed to deliver reminder"
             );
-            // Persist the failure so the dashboard can surface it +
-            // bump attempt_count. After MAX_REMINDER_ATTEMPTS the
-            // row drops out of `get_due_reminders` and waits for
-            // operator retry / cancel.
-            if let Err(persist_err) = coord.broker.record_reminder_failure(id, &reason) {
+            // Persist the failure so the dashboard can surface it.
+            if let Err(persist_err) = coord.broker.record_reminder_failure(*id, &reason) {
                 tracing::warn!(
                     reminder_id = id,
                     error = ?persist_err,