(*net.OpError).Error() concatenates Source/Addr fields, so the previous
disconnectMsg surfaced internal source IP/port and upstream server address
to clients via SSE error frames and UpstreamFailoverError.ResponseBody
(reported by @Wei-Shaw on PR #2066).
- Add sanitizeStreamError that maps known errors (io.ErrUnexpectedEOF,
context.Canceled, syscall.ECONNRESET/EPIPE/ETIMEDOUT/...) to fixed
descriptions and falls back to a generic placeholder, with an explicit
*net.OpError branch that drops Source/Addr fields entirely.
- Use sanitized message in client-facing disconnectMsg; full ev.err is
still preserved in the existing operator log line for diagnosis.
- Tests cover net.OpError redaction, the failover ResponseBody path, and
every known sanitized error mapping.
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