perf: concurrent txpool fetcher with separate IPC connection

[vbuilder69420]

Summary

Fix the txpool subscription dying under high transaction load by:

1. Using a separate IPC connection for get_raw_transaction_by_hash RPC calls
2. Fetching transactions concurrently (64 parallel) instead of sequentially
3. Increasing the subscription broadcast channel from 16 to 16,384 items

Root Cause

The previous implementation had three cascading bottlenecks:

Sequential per-tx RPC calls block the subscription consumer — For each tx hash notification, rbuilder made a blocking get_raw_transaction_by_hash call over the same IPC connection before reading the next notification. At 500+ TPS, the subscription produced hashes faster than rbuilder could fetch full txs one-by-one.

Broadcast channel of 16 items — The alloy PubSubFrontend defaults the broadcast channel to 16 items. At 500+ TPS, this overflows instantly and notifications are silently dropped (SubscriptionStream ignores Lagged errors).

No backpressure — The unbounded channels between the IPC backend and PubSub service meant the kernel socket buffer could fill up, causing reth to reset the connection.

Before / After

Metric	Before	After
IPC survival at 500 TPS	~5 blocks (~30s)	Full run (46+ blocks)
IPC survival at 2000 TPS	~2 blocks (~10s)	5+ minutes (52 blocks)
Max orders in one block	~100	616

Design

Before:
  subscription stream → [hash] → get_raw_tx (BLOCKS) → [hash] → get_raw_tx (BLOCKS) → ...
                         ↑ same IPC connection for both ↑

After:
  IPC conn 1: subscription stream → [hash] → spawn task → [hash] → spawn task → ...
  IPC conn 2:                        ↓                      ↓
                                   get_raw_tx            get_raw_tx    (up to 64 concurrent)
                                     ↓                      ↓
                                   → orderpool           → orderpool

Test plan

• Builds cleanly
• IPC survives full run at 500 TPS (was dying after ~5 blocks)
• IPC survives 5+ minutes at 2000 TPS (was dying after ~2 blocks)
• Existing test test_fetcher_retrieves_transactions preserved
• Integration tests pass

🤖 Generated with Claude Code