H2 pool invariants and stale-slot cleanup

[Copilot]

Reviewed HTTP/2 pool for quality/performance in line with TigerBeetle style; ensure stale slots are reclaimed and invariants are explicit.

• Pool invariants

  • Assert backend list does not exceed fixed pool capacity.
  • Document synchronization expectations around slot availability and ref_count.

• Stale slot reclamation

  • While holding the backend mutex, destroy stale “available” connections and clear slots to prevent permanent exhaustion.

Example snippet:

if (self.isConnectionStale(conn)) {
    std.debug.assert(conn.ref_count.load(.monotonic) == 0);
    slot.* = null;
    state.* = .empty;
    self.destroyConnection(conn, io);
}

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)
• ziglang.org
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)
  • Triggering command: /usr/bin/curl curl -I REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

review this code for quality, performance and tiger beetle style

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