fix(background-agent): cancel all nested descendant tasks recursively (#107)

Previously, background_cancel(all=true) only cancelled direct child tasks, leaving grandchildren and deeper nested tasks uncancelled. This caused background agents to continue running even when their parent session was cancelled.

Changes:
- Added getAllDescendantTasks() method to BackgroundTaskManager for recursive task collection
- Updated background_cancel to use getAllDescendantTasks instead of getTasksByParentSession
- Added comprehensive test coverage for nested task cancellation scenarios

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/background-agent/manager.test.ts

@@ -0,0 +1,232 @@
+import { describe, test, expect, beforeEach } from "bun:test"
+import type { BackgroundTask } from "./types"
+
+class MockBackgroundManager {
+  private tasks: Map<string, BackgroundTask> = new Map()
+
+  addTask(task: BackgroundTask): void {
+    this.tasks.set(task.id, task)
+  }
+
+  getTask(id: string): BackgroundTask | undefined {
+    return this.tasks.get(id)
+  }
+
+  getTasksByParentSession(sessionID: string): BackgroundTask[] {
+    const result: BackgroundTask[] = []
+    for (const task of this.tasks.values()) {
+      if (task.parentSessionID === sessionID) {
+        result.push(task)
+      }
+    }
+    return result
+  }
+
+  getAllDescendantTasks(sessionID: string): BackgroundTask[] {
+    const result: BackgroundTask[] = []
+    const directChildren = this.getTasksByParentSession(sessionID)
+
+    for (const child of directChildren) {
+      result.push(child)
+      const descendants = this.getAllDescendantTasks(child.sessionID)
+      result.push(...descendants)
+    }
+
+    return result
+  }
+}
+
+function createMockTask(overrides: Partial<BackgroundTask> & { id: string; sessionID: string; parentSessionID: string }): BackgroundTask {
+  return {
+    parentMessageID: "mock-message-id",
+    description: "test task",
+    prompt: "test prompt",
+    agent: "test-agent",
+    status: "running",
+    startedAt: new Date(),
+    ...overrides,
+  }
+}
+
+describe("BackgroundManager.getAllDescendantTasks", () => {
+  let manager: MockBackgroundManager
+
+  beforeEach(() => {
+    // #given
+    manager = new MockBackgroundManager()
+  })
+
+  test("should return empty array when no tasks exist", () => {
+    // #given - empty manager
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toEqual([])
+  })
+
+  test("should return direct children only when no nested tasks", () => {
+    // #given
+    const taskB = createMockTask({
+      id: "task-b",
+      sessionID: "session-b",
+      parentSessionID: "session-a",
+    })
+    manager.addTask(taskB)
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe("task-b")
+  })
+
+  test("should return all nested descendants (2 levels deep)", () => {
+    // #given
+    // Session A -> Task B -> Task C
+    const taskB = createMockTask({
+      id: "task-b",
+      sessionID: "session-b",
+      parentSessionID: "session-a",
+    })
+    const taskC = createMockTask({
+      id: "task-c",
+      sessionID: "session-c",
+      parentSessionID: "session-b",
+    })
+    manager.addTask(taskB)
+    manager.addTask(taskC)
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toHaveLength(2)
+    expect(result.map(t => t.id)).toContain("task-b")
+    expect(result.map(t => t.id)).toContain("task-c")
+  })
+
+  test("should return all nested descendants (3 levels deep)", () => {
+    // #given
+    // Session A -> Task B -> Task C -> Task D
+    const taskB = createMockTask({
+      id: "task-b",
+      sessionID: "session-b",
+      parentSessionID: "session-a",
+    })
+    const taskC = createMockTask({
+      id: "task-c",
+      sessionID: "session-c",
+      parentSessionID: "session-b",
+    })
+    const taskD = createMockTask({
+      id: "task-d",
+      sessionID: "session-d",
+      parentSessionID: "session-c",
+    })
+    manager.addTask(taskB)
+    manager.addTask(taskC)
+    manager.addTask(taskD)
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toHaveLength(3)
+    expect(result.map(t => t.id)).toContain("task-b")
+    expect(result.map(t => t.id)).toContain("task-c")
+    expect(result.map(t => t.id)).toContain("task-d")
+  })
+
+  test("should handle multiple branches (tree structure)", () => {
+    // #given
+    // Session A -> Task B1 -> Task C1
+    //           -> Task B2 -> Task C2
+    const taskB1 = createMockTask({
+      id: "task-b1",
+      sessionID: "session-b1",
+      parentSessionID: "session-a",
+    })
+    const taskB2 = createMockTask({
+      id: "task-b2",
+      sessionID: "session-b2",
+      parentSessionID: "session-a",
+    })
+    const taskC1 = createMockTask({
+      id: "task-c1",
+      sessionID: "session-c1",
+      parentSessionID: "session-b1",
+    })
+    const taskC2 = createMockTask({
+      id: "task-c2",
+      sessionID: "session-c2",
+      parentSessionID: "session-b2",
+    })
+    manager.addTask(taskB1)
+    manager.addTask(taskB2)
+    manager.addTask(taskC1)
+    manager.addTask(taskC2)
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toHaveLength(4)
+    expect(result.map(t => t.id)).toContain("task-b1")
+    expect(result.map(t => t.id)).toContain("task-b2")
+    expect(result.map(t => t.id)).toContain("task-c1")
+    expect(result.map(t => t.id)).toContain("task-c2")
+  })
+
+  test("should not include tasks from unrelated sessions", () => {
+    // #given
+    // Session A -> Task B
+    // Session X -> Task Y (unrelated)
+    const taskB = createMockTask({
+      id: "task-b",
+      sessionID: "session-b",
+      parentSessionID: "session-a",
+    })
+    const taskY = createMockTask({
+      id: "task-y",
+      sessionID: "session-y",
+      parentSessionID: "session-x",
+    })
+    manager.addTask(taskB)
+    manager.addTask(taskY)
+
+    // #when
+    const result = manager.getAllDescendantTasks("session-a")
+
+    // #then
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe("task-b")
+    expect(result.map(t => t.id)).not.toContain("task-y")
+  })
+
+  test("getTasksByParentSession should only return direct children (not recursive)", () => {
+    // #given
+    // Session A -> Task B -> Task C
+    const taskB = createMockTask({
+      id: "task-b",
+      sessionID: "session-b",
+      parentSessionID: "session-a",
+    })
+    const taskC = createMockTask({
+      id: "task-c",
+      sessionID: "session-c",
+      parentSessionID: "session-b",
+    })
+    manager.addTask(taskB)
+    manager.addTask(taskC)
+
+    // #when
+    const result = manager.getTasksByParentSession("session-a")
+
+    // #then
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe("task-b")
+  })
+})

src/features/background-agent/manager.ts

@@ -150,6 +150,19 @@ export class BackgroundManager {
     return result
   }
 
+  getAllDescendantTasks(sessionID: string): BackgroundTask[] {
+    const result: BackgroundTask[] = []
+    const directChildren = this.getTasksByParentSession(sessionID)
+
+    for (const child of directChildren) {
+      result.push(child)
+      const descendants = this.getAllDescendantTasks(child.sessionID)
+      result.push(...descendants)
+    }
+
+    return result
+  }
+
   findBySession(sessionID: string): BackgroundTask | undefined {
     for (const task of this.tasks.values()) {
       if (task.sessionID === sessionID) {

src/tools/background-task/tools.ts

@@ -275,7 +275,7 @@ export function createBackgroundCancel(manager: BackgroundManager, client: Openc
         }
 
         if (cancelAll) {
-          const tasks = manager.getTasksByParentSession(toolContext.sessionID)
+          const tasks = manager.getAllDescendantTasks(toolContext.sessionID)
           const runningTasks = tasks.filter(t => t.status === "running")
 
           if (runningTasks.length === 0) {