Implement PR #157 review feedback: simplify error detection and document security tradeoffs

[Copilot]

Addresses review comments on PR #157 which implements username conflict recovery for Discord headset linking.

Changes

Error Detection Simplification

• Removed redundant string check in isAlreadyExistsError(): "code = AlreadyExists" || "AlreadyExists" → "AlreadyExists"
• Second check is substring of first, making it redundant

Security Documentation

Added 21-line SECURITY NOTE documenting automatic custom_id linking tradeoffs:

• Concern: Attacker could pre-create account with victim's Discord username
• Mitigations: Comprehensive audit logging, unlinked-only recovery, unique usernames per server, duplicate rejection
• Race condition: LinkCustom fails with AlreadyExists if concurrent link occurs
• Justification: Discord auth is trusted, conflicts are legacy migrations, logging enables abuse detection

Test Fix

• Fixed evr_runtime_event_remotelogset_test.go calling non-existent operatorFromTag() → operatorFromStatField()
• Bug from PR Fix "username already exists" errors when linking #157 preventing test compilation

Example

// Simplified error detection
func (d *DiscordAppBot) isAlreadyExistsError(err error) bool {
    if err == nil {
        return false
    }
    st, ok := status.FromError(err)
    if ok && st.Code() == codes.AlreadyExists {
        return true
    }
    // Single check covers both "code = AlreadyExists" and "AlreadyExists"
    return strings.Contains(err.Error(), "AlreadyExists")
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Handle linking errors for "username already exists"</issue_title>
<issue_description>### Problem
When linking a Discord account, if Nakama returns a rpc error: code = AlreadyExists desc = Username is already in use. error (as in:

{"level":"error","ts":"2025-11-15T20:35:59.335Z","caller":"server/evr_discord_appbot_linking.go:79","msg":"Failed to link headset","runtime":"go","system":"discordAppBot","discord_id":"1380019639354130534","username":"someuser","guild_id":"779349159852769310","channel_id":"1227795372244729926","user_id":"","group_id":"147afc9d-2819-4197-926d-5b3f92790edc","discord_id":"1234123412341234","link_code":"RTTD","error":"failed to authenticate (or create) user 1234123412341234: rpc error: code = AlreadyExists desc = Username is already in use.","source":"github.com/heroiclabs/nakama/v3/server/evr_discord_appbot_linking.go:79"}

the process fails without further recovery options.

Proposed Solution

Implement a new handler for this linking case in Go using the Nakama server runtime APIs:

• On catching this AlreadyExists error, look up the Nakama account with the conflicting username.
  • If the discord user exists:
    • Update the Nakama username to the correct value and re-attempt linking.
  • If the discord user no longer exists:
    • Change the Nakama custom_id to the Discord ID of the user currently trying to link the headset.
    • Retry linking.
• If the link still fails, return a user error and guide them to file a support ticket.

Reference Nakama Go runtime API docs for authentication and account update calls.

Acceptance Criteria

• Graceful handling of username conflicts for Discord linking using a recovery handler
• Logic matches the error context given above, references server/evr_discord_appbot_linking.go
• Properly documents the control flow with code comments
• Returns actionable info to user on repeated failure
• Code samples/references from Nakama docs and error log are included for maintainers
  </issue_description>

<agent_instructions>review #157 carefully, implement the review suggestions.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Handle linking errors for "username already exists" #146

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.