irc: fix RELAYMSG nick missing required separator

[johnnyxmas]

Summary

doSend() sanitizes the outgoing RELAYMSG nick but never appends a separator/protocol suffix (e.g. alice/discord). Servers that enforce draft/relaymsg's separator requirement (e.g. Ergo, when relaymsg.separators is configured) reject every such RELAYMSG with ERR_INVALID_NICK, so no message from any bridged platform reaches IRC when the target network enforces this — reproducible 100% of the time, regardless of which user is speaking.

Also fixes a related bug: modifyUsername() in gateway/gateway.go unconditionally applies the destination bridge's RemoteNickFormat template (e.g. "{NICK}: ") to Username before it reaches the IRC bridge. For RELAYMSG destinations this bakes stray punctuation into the nick before sanitizeNick runs (e.g. "alice: " → "alice--" after sanitizing), producing a mangled/truncated nick even once the separator is fixed. RELAYMSG doesn't need a text-prefix format at all — the nick itself is the sender identity — so this skips RemoteNickFormat templating specifically for IRC destinations with UseRelayMsg enabled, while still honoring StripNick/ReplaceNicks.

Reproduction

Reproduced against a live Ergo instance with relaymsg.separators: ["/"] configured. Every Discord/Slack → IRC message failed identically:

> RELAYMSG #channel alice-- hello
< FAIL RELAYMSG INVALID_NICK :Relayed nicknames MUST contain a relaymsg separator from this set: /

Confirmed via debug logging (DebugLevel=2) that this is what matterbridge actually sends — not a config issue on the Ergo side. IRC → other platforms, and Discord ↔ Slack, were unaffected; only the RELAYMSG-based * → IRC path was broken.

Fix

• New relayMsgNick(nick, protocol, maxNickLen string/int) helper: sanitizes the nick, then appends /<protocol>. Truncates the base name (not the suffix) against the server's advertised NICKLEN when known, on a UTF-8 rune boundary, so long names can't silently truncate the required suffix back into the same failure.
• gateway.modifyUsername(): skip RemoteNickFormat templating for IRC+UseRelayMsg destinations, since RELAYMSG doesn't use a text-prefix format.
• Added unit tests for sanitizeNick/relayMsgNick covering the basic case, truncation + suffix preservation, UTF-8 rune-boundary safety, and the degenerate case where the suffix alone exceeds maxNickLen.

Test plan

• go build ./...
• go test ./... (full suite, no failures)
• go vet ./bridge/irc/... ./gateway/...
• gofmt -l clean on touched files
• New unit tests in bridge/irc/irc_test.go pass
• Manually verified against a live Ergo server: before the fix, 100% of Discord/Slack → IRC RELAYMSGs were rejected with INVALID_NICK; the fix produces valid nicks like alice/discord that Ergo accepts

🤖 Generated with Claude Code

[johnnyxmas]

This fix already landed on master via 3f7533c / 02daefd (irc: fix RELAYMSG nick missing required separator + lint follow-up). Closing as redundant.

[johnnyxmas]

Correction: my previous close comment was based on comparing against my own fork's master, not this repo's master — the fix has NOT actually landed upstream. Reopening; this PR is still valid and needed.

[johnnyxmas]

Maintainer has been inactive on this repo since August 2024 (over two years), so I'm maintaining these fixes on my own fork (johnnyxmas/matterbridge) instead of waiting on upstream review. Closing this PR.