net/syncthing: failed to increase receive buffer size (quic-go)

Hi,

When you start Syncthing, one of the first log entries is this warning:

2026-01-24 12:33:01 INF failed to increase receive buffer size (wanted: 7168 kiB, got 0 kiB). See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.

The linked wiki page explains that quic-go tries to automatically increase the buffer size on some platforms (not BSDs), and it has instructions for manually doing it on others; but the section on BSDs doesn't apply to OpenBSD. There is an open issue [1] about it.

Someone there has figured out that the sysctl variable to change here is "net.inet.udp.recvspace", but it can only be increased to about 2 MB (2097152) — well below the target 7.5 MB (8441037). I have no idea whether quic-go is able to work properly with this. Either way, the warning persists.

So, my idea is to:

1. Modify our package README with instructions to increase the buffer size to maximum.
2. Silence the warning by setting the variable "QUIC_GO_DISABLE_RECEIVE_BUFFER_WARNING" [2] [3]

What do you think?


[1] https://github.com/quic-go/quic-go/issues/3476
[2] https://github.com/quic-go/quic-go/blob/4a8af22606c2d237da39212a734b80026a9a0bb0/sys_conn.go#L59
[3] https://github.com/quic-go/quic-go/issues/3801#issuecomment-1882365505