fix: MaxListenersExceededWarning for parallel tasks (shared AbortSignal)

[Arpan Das (adasarpan404)]

Affected issues:

• Possible EventTarget memory leak detected #1494 (Orchestrator-Worker pattern with many Send() calls)
• Possible EventTarget memory leak detected #1420 (Parallel tool calls in React agents)

Root Cause

In PregelRunner, all tasks scheduled in the same superstep (parallel execution tick) receive the exact same composedAbortSignal via config.signal.

Each LLM invocation (e.g. ChatOpenAI.invoke()) and HTTP client adds an abort listener to this shared signal. When fan-out exceeds Node.js default limit of 10 listeners, the warning is emitted.

This is not a real memory leak — it's Node.js protecting against potential leaks — but it creates noise in production and tutorial runs.

Solution

Fork the parent AbortSignal for each parallel task so every task gets its own clean AbortSignal instance while still fully propagating cancellation from the parent.

Changes

• Updated signal handling in task scheduling to use per-task child signals via AbortSignal.any() (modern & clean).
• Added fallback for older environments.
• Ensured cancellation semantics remain identical.
• Added test case for high fan-out (>10 parallel tasks) to prevent regression.

Testing

• Verified no MaxListenersExceededWarning with 15+ parallel LLM calls.
• Confirmed graceful abortion still works across all tasks.
• Existing single-task and low-parallelism behavior unchanged.

Ready for review! 🚀

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