Batch autojoin channels into comma-separated JOIN commands

[kofany]

Problem

When connecting to a server with multiple autojoin channels configured, the client sends individual JOIN commands for each channel:

JOIN #channel1
JOIN #channel2
JOIN #channel3
...

For a typical configuration with 20 channels, this generates 20 separate messages. With any form of server-side or client-side flood protection, this introduces significant delay — each JOIN carries a penalty (~2s per command on most IRCd implementations following RFC 2813 §5.8), resulting in ~34 seconds to complete autojoin for 20 channels.

Solution

Batch autojoin channels into comma-separated JOIN commands per RFC 2812 §3.2.1:

JOIN #channel1,#channel2,#channel3,...,#channelN

Key design decisions

1. Keyed channels placed first — RFC 2812 specifies positional key matching (JOIN #a,#b,#c key1,key2 maps key1→#a, key2→#b, #c has no key). Channels with keys must precede keyless channels within each batch to maintain correct alignment.

2. Respects 512-byte IRC line limit — The batching algorithm splits into multiple JOIN commands when adding another channel would exceed 512 bytes (including JOIN , commas, the space-separated keylist, and \r\n). This is the only real constraint — RFC 2812 does not impose a limit on the number of channels per JOIN.

3. Preserves config order within groups — Keyed and keyless channels each maintain their original configuration order.

4. Re-joined channels also batched — Previously joined channels (tracked in chanlists) that aren't in the config are also batched when re-joining after reconnect.

Example

Config with channels [#public, #secret, #general] where #secret has key mypass:

Before: JOIN #public\r\n + JOIN #secret mypass\r\n + JOIN #general\r\n (3 messages)
After: JOIN #secret,#public,#general mypass\r\n (1 message)

Testing

• All 73 existing tests updated and passing
• 6 new unit tests for build_batched_joins:
  • All keyless channels → single batch
  • Keyed channels placed first with correct key ordering
  • Multiple keyed channels with positional key alignment
  • Empty channel list → no batches
  • Long channel names exceeding 512-byte limit → correct splitting
  • Keyed channels with keys exceeding limit → correct splitting with keys preserved
• cargo test --no-default-features ✓
• cargo clippy --all-targets --all-features — no new warnings
• cargo fmt --check ✓
• git diff --check — no whitespace issues