fix: synchronize ClientTaskManager to prevent race condition in concu…

[kabir]

…rrent SSE event processing

ClientTaskManager was not thread-safe, causing intermittent test failures when
SSE events were delivered concurrently by the HTTP client. The race condition
occurred when multiple events (startWork, addArtifact, complete) were emitted
rapidly in streaming mode.

Without synchronization, concurrent threads could interleave read-modify-write
operations on currentTask, resulting in lost updates. Specifically, the artifact
update could be overwritten by a status update, leaving the final COMPLETED task
with no artifacts. The fast time to failure of the test 0.017s, indicates that the
COMPLETED status update was received, and that the latch did not time out.

This manifested as intermittent failures in agent-to-agent tests where the test
received a COMPLETED task but extractTextFromTask() returned an empty string
because the artifact list was empty.

Fix: Added synchronized keyword to all ClientTaskManager methods (getCurrentTask,
saveTaskEvent variants, updateWithMessage) to ensure atomic processing of events
regardless of HTTP client threading behavior.

Impact: All client transports (JSON-RPC, gRPC, REST) in streaming mode
Validated: 100 consecutive test iterations passed on CI

Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com

[gemini-code-assist]

Summary of Changes

Hello, 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 concurrency issue within the ClientTaskManager by implementing thread-safe access to its state. The change ensures that concurrent streaming events, particularly from HTTP clients, are processed atomically, thereby eliminating race conditions that led to lost task updates and intermittent test failures.

Highlights

• Concurrency Fix: Implemented synchronization for key methods within ClientTaskManager to prevent race conditions during concurrent SSE event processing.
• Thread Safety: Ensured thread-safe operations for getCurrentTask, saveTaskEvent variants, and updateWithMessage methods by adding the synchronized keyword.
• Bug Resolution: Addressed intermittent test failures where task artifacts were lost due to interleaved read-modify-write operations on the currentTask.

Changelog

• client/base/src/main/java/io/a2a/client/ClientTaskManager.java
  • The getCurrentTask() method was made synchronized.
  • The saveTaskEvent(Task) method was made synchronized.
  • The saveTaskEvent(TaskStatusUpdateEvent) method was made synchronized.
  • The saveTaskEvent(TaskArtifactUpdateEvent) method was made synchronized.
  • The updateWithMessage(Message, Task) method was made synchronized.

Ignored Files

• Ignored by pattern: .github/workflows/** (1)
  • .github/workflows/test-race-condition.yml

Activity

• No specific activity (comments, reviews, progress updates) has been provided in the context for this pull request.

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[gemini-code-assist]

Code Review

The pull request modifies ClientTaskManager.java by adding the synchronized keyword to several public methods, including getCurrentTask, saveTaskEvent (multiple overloads), and updateWithMessage. This change ensures thread safety for task management operations, preventing potential race conditions when accessing or modifying shared task-related state from multiple threads.

[jmesnil]

@kabir do you plan to keep it in the CI or remove it before merging?

[kabir]

I thought I had removed it. Done now