From a80d61f8e1ae8bea931491af6433b5f3180a37c9 Mon Sep 17 00:00:00 2001
From: LinYushen <linyushen@proton.me>
Date: Thu, 2 Apr 2026 14:12:00 +0800
Subject: [PATCH] fix(task): enforce per-issue serial execution in task
 claiming (#330)

Add NOT EXISTS check to ClaimAgentTask SQL to prevent claiming a queued
task when the same issue already has a dispatched/running task. This
ensures serial execution within an issue while preserving parallel
execution across different issues (concurrency group pattern).

Also add defensive guard in the frontend task:dispatch handler to avoid
replacing an active task's LiveLog timeline mid-execution.

Closes MUL-183

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 apps/web/features/issues/components/agent-live-card.tsx | 7 +++++--
 server/pkg/db/generated/agent.sql.go                    | 9 +++++++++
 server/pkg/db/queries/agent.sql                         | 9 +++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/apps/web/features/issues/components/agent-live-card.tsx b/apps/web/features/issues/components/agent-live-card.tsx
index 6917fc0f..aff2a124 100644
--- a/apps/web/features/issues/components/agent-live-card.tsx
+++ b/apps/web/features/issues/components/agent-live-card.tsx
@@ -194,10 +194,13 @@ export function AgentLiveCard({ issueId, agentName }: AgentLiveCardProps) {
     }, [issueId]),
   );
 
-  // Pick up new tasks
+  // Pick up new tasks — skip if we're already showing an active task to avoid
+  // replacing its timeline mid-execution (per-issue serialization in the
+  // backend prevents this race, but this is a defensive safeguard).
   useWSEvent(
     "task:dispatch",
     useCallback(() => {
+      if (activeTask) return;
       api.getActiveTaskForIssue(issueId).then(({ task }) => {
         if (task) {
           setActiveTask(task);
@@ -205,7 +208,7 @@ export function AgentLiveCard({ issueId, agentName }: AgentLiveCardProps) {
           seenSeqs.current.clear();
         }
       }).catch(() => {});
-    }, [issueId]),
+    }, [issueId, activeTask]),
   );
 
   // Elapsed time
diff --git a/server/pkg/db/generated/agent.sql.go b/server/pkg/db/generated/agent.sql.go
index befca00e..06984472 100644
--- a/server/pkg/db/generated/agent.sql.go
+++ b/server/pkg/db/generated/agent.sql.go
@@ -59,6 +59,11 @@ SET status = 'dispatched', dispatched_at = now()
 WHERE id = (
     SELECT atq.id FROM agent_task_queue atq
     WHERE atq.agent_id = $1 AND atq.status = 'queued'
+      AND NOT EXISTS (
+          SELECT 1 FROM agent_task_queue active
+          WHERE active.issue_id = atq.issue_id
+            AND active.status IN ('dispatched', 'running')
+      )
     ORDER BY atq.priority DESC, atq.created_at ASC
     LIMIT 1
     FOR UPDATE SKIP LOCKED
@@ -66,6 +71,10 @@ WHERE id = (
 RETURNING id, agent_id, issue_id, status, priority, dispatched_at, started_at, completed_at, result, error, created_at, context, runtime_id, session_id, work_dir, trigger_comment_id
 `
 
+// Claims the next queued task for an agent, enforcing per-issue serialization:
+// a task is only claimable when no other task for the same issue is already
+// dispatched or running. This guarantees serial execution within an issue
+// while allowing parallel execution across different issues.
 func (q *Queries) ClaimAgentTask(ctx context.Context, agentID pgtype.UUID) (AgentTaskQueue, error) {
 	row := q.db.QueryRow(ctx, claimAgentTask, agentID)
 	var i AgentTaskQueue
diff --git a/server/pkg/db/queries/agent.sql b/server/pkg/db/queries/agent.sql
index 4511200b..b55476b8 100644
--- a/server/pkg/db/queries/agent.sql
+++ b/server/pkg/db/queries/agent.sql
@@ -60,11 +60,20 @@ SELECT * FROM agent_task_queue
 WHERE id = $1;
 
 -- name: ClaimAgentTask :one
+-- Claims the next queued task for an agent, enforcing per-issue serialization:
+-- a task is only claimable when no other task for the same issue is already
+-- dispatched or running. This guarantees serial execution within an issue
+-- while allowing parallel execution across different issues.
 UPDATE agent_task_queue
 SET status = 'dispatched', dispatched_at = now()
 WHERE id = (
     SELECT atq.id FROM agent_task_queue atq
     WHERE atq.agent_id = $1 AND atq.status = 'queued'
+      AND NOT EXISTS (
+          SELECT 1 FROM agent_task_queue active
+          WHERE active.issue_id = atq.issue_id
+            AND active.status IN ('dispatched', 'running')
+      )
     ORDER BY atq.priority DESC, atq.created_at ASC
     LIMIT 1
     FOR UPDATE SKIP LOCKED