fix: Wait for agent completion and ensure all events processed in blo… #431

kabir · 2025-11-03T18:29:25Z

…cking calls

Fixes race condition where DefaultRequestHandler.onMessageSend() returns before all task events are fully processed and persisted to TaskStore, resulting in incomplete Task objects being returned to clients (missing artifacts, incorrect state).

Root Cause:

Blocking calls interrupted immediately after first event and returned to client before background event consumption completed
Agent execution and event processing happened asynchronously in background
No synchronization to ensure all events were consumed and persisted before returning Task to client

Solution (4-step process):

Wait for agent to finish enqueueing events (configurable timeout)
Close the queue to signal consumption can complete (breaks dependency)
Wait for consumption to finish processing events (configurable timeout)
Fetch final task state from TaskStore (has all artifacts and correct state)

This ensures blocking calls return complete Task objects with all artifacts and correct state, including support for fire-and-forget tasks that never emit final state events.

Added unit tests:

testBlockingFireAndForgetReturnsNonFinalTask: Validates fire-and-forget pattern
testBlockingCallReturnsCompleteTaskWithArtifacts: Ensures all artifacts included

Fixes #428 🦕

gemini-code-assist · 2025-11-03T18:29:43Z

Summary of Changes

Hello @kabir, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical race condition affecting blocking calls, ensuring that Task objects returned to clients are always complete and accurate. By introducing explicit synchronization steps, the system now reliably waits for all agent events to be processed and persisted before concluding a blocking operation, thereby preventing incomplete task states and missing artifacts. This enhancement improves the robustness and reliability of task handling, especially for scenarios involving asynchronous agent execution and 'fire-and-forget' patterns.

Highlights

Race Condition Fix: Addressed a race condition in DefaultRequestHandler.onMessageSend() where blocking calls would return incomplete Task objects due to asynchronous event processing. Previously, onMessageSend() would return after the first event, before all task events were fully processed and persisted to the TaskStore.
Synchronized Event Processing for Blocking Calls: Implemented a 4-step synchronization process for blocking calls: 1) Wait for the agent to finish enqueueing events (5s timeout), 2) Close the event queue to signal consumption completion, 3) Wait for event consumption to finish (2s timeout), and 4) Fetch the final Task state from the TaskStore. This ensures that returned Task objects are complete with all artifacts and the correct state.
Support for Fire-and-Forget Tasks: The fix now correctly handles 'fire-and-forget' tasks, where an agent might return without emitting a final state event. The new logic ensures that even in such cases, the blocking call waits for agent execution and event processing to complete, returning a complete Task object reflecting its current state and artifacts.
New Unit Tests: Added two new unit tests: testBlockingFireAndForgetReturnsNonFinalTask to validate the fire-and-forget pattern with blocking calls, and testBlockingCallReturnsCompleteTaskWithArtifacts to ensure all artifacts are included in the returned Task for blocking calls.

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kabir · 2025-11-03T18:29:58Z

@flex-myeonghyeon I moved the work here

~~Note that for some reason, GitHub isn't showing the code I have locally, so if you want to check you might have to pull down the branch.~~ This was my IDE not behaving. It should be the same now :-)

gemini-code-assist

Code Review

This pull request effectively addresses a critical race condition in blocking calls by introducing a synchronization mechanism to ensure all events are processed before returning a response. The solution is well-structured and the accompanying comments clearly explain the logic. The new unit tests are comprehensive and correctly validate both the fix for blocking calls and the fire-and-forget scenario. I've identified a few minor areas for improvement, mainly related to code style and clarity, such as replacing magic numbers with constants and simplifying some expressions.

server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java

server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java

kabir · 2025-11-03T18:34:22Z

Replaces #430

flex-myeonghyeon · 2025-11-03T22:52:17Z

server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java


    private static final Logger LOGGER = LoggerFactory.getLogger(DefaultRequestHandler.class);
+    private static final int AGENT_COMPLETION_TIMEOUT_SECONDS = 5;
+    private static final int CONSUMPTION_COMPLETION_TIMEOUT_SECONDS = 2;


In some cases, a longer wait time might be necessary.
Would it be possible to make this configurable — for example, by passing it as a parameter or injecting it through the DefaultRequestHandler constructor?

@flex-myeonghyeon Good idea, I've added some MP Config properties

flex-myeonghyeon · 2025-11-04T12:03:01Z

@kabir
Looks good to me!
Once the snapshot version is deployed and available, I’ll test it in my development environment.

…cking calls Fixes race condition where DefaultRequestHandler.onMessageSend() returns before all task events are fully processed and persisted to TaskStore, resulting in incomplete Task objects being returned to clients (missing artifacts, incorrect state). Root Cause: - Blocking calls interrupted immediately after first event and returned to client before background event consumption completed - Agent execution and event processing happened asynchronously in background - No synchronization to ensure all events were consumed and persisted before returning Task to client Solution (4-step process): 1. Wait for agent to finish enqueueing events (5s timeout) 2. Close the queue to signal consumption can complete (breaks dependency) 3. Wait for consumption to finish processing events (2s timeout) 4. Fetch final task state from TaskStore (has all artifacts and correct state) This ensures blocking calls return complete Task objects with all artifacts and correct state, including support for fire-and-forget tasks that never emit final state events. Added unit tests: - testBlockingFireAndForgetReturnsNonFinalTask: Validates fire-and-forget pattern - testBlockingCallReturnsCompleteTaskWithArtifacts: Ensures all artifacts included

server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java

#441) …cking calls Fixes race condition where DefaultRequestHandler.onMessageSend() returns before all task events are fully processed and persisted to TaskStore, resulting in incomplete Task objects being returned to clients (missing artifacts, incorrect state). Root Cause: - Blocking calls interrupted immediately after first event and returned to client before background event consumption completed - Agent execution and event processing happened asynchronously in background - No synchronization to ensure all events were consumed and persisted before returning Task to client Solution (4-step process): 1. Wait for agent to finish enqueueing events (5s timeout) 2. Close the queue to signal consumption can complete (breaks dependency) 3. Wait for consumption to finish processing events (2s timeout) 4. Fetch final task state from TaskStore (has all artifacts and correct state) This ensures blocking calls return complete Task objects with all artifacts and correct state, including support for fire-and-forget tasks that never emit final state events. Added unit tests: - testBlockingFireAndForgetReturnsNonFinalTask: Validates fire-and-forget pattern - testBlockingCallReturnsCompleteTaskWithArtifacts: Ensures all artifacts included Upstream: #431

a2aproject#431) …cking calls Fixes race condition where DefaultRequestHandler.onMessageSend() returns before all task events are fully processed and persisted to TaskStore, resulting in incomplete Task objects being returned to clients (missing artifacts, incorrect state). Root Cause: - Blocking calls interrupted immediately after first event and returned to client before background event consumption completed - Agent execution and event processing happened asynchronously in background - No synchronization to ensure all events were consumed and persisted before returning Task to client Solution (4-step process): 1. Wait for agent to finish enqueueing events (configurable timeout) 2. Close the queue to signal consumption can complete (breaks dependency) 3. Wait for consumption to finish processing events (configurable timeout) 4. Fetch final task state from TaskStore (has all artifacts and correct state) This ensures blocking calls return complete Task objects with all artifacts and correct state, including support for fire-and-forget tasks that never emit final state events. Added unit tests: - testBlockingFireAndForgetReturnsNonFinalTask: Validates fire-and-forget pattern - testBlockingCallReturnsCompleteTaskWithArtifacts: Ensures all artifacts included Fixes a2aproject#428 🦕

gemini-code-assist bot reviewed Nov 3, 2025

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kabir force-pushed the blocking-send-fixes branch from b79aee8 to c97d236 Compare November 3, 2025 18:47

flex-myeonghyeon reviewed Nov 3, 2025

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kabir force-pushed the blocking-send-fixes branch from f487663 to 6e35990 Compare November 4, 2025 11:07

kabir force-pushed the blocking-send-fixes branch from 6e35990 to 413b7ce Compare November 4, 2025 13:36

kabir force-pushed the blocking-send-fixes branch from 413b7ce to ece9af8 Compare November 4, 2025 14:19

ehsavoie reviewed Nov 4, 2025

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server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java Show resolved Hide resolved

ehsavoie approved these changes Nov 4, 2025

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kabir force-pushed the blocking-send-fixes branch 2 times, most recently from 2e48d12 to e2498c7 Compare November 4, 2025 14:38

Remove unused methods/parameters

fab40d7

kabir force-pushed the blocking-send-fixes branch from e2498c7 to fab40d7 Compare November 4, 2025 14:38

kabir mentioned this pull request Nov 4, 2025

fix: Wait for agent completion and ensure all events processed in blo… #441

Merged

More cleanup

63a4a41

kabir merged commit 465a5e4 into a2aproject:main Nov 4, 2025
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fix: Wait for agent completion and ensure all events processed in blo… #431

fix: Wait for agent completion and ensure all events processed in blo… #431

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