-- from code review 5574063: net: disable SO_REUSEADDR and SO_REUSEPORT on openbsd
(issue #5574063) on golang-dev

Sorry, I should have been more specific here. The problem is not due
to use of SO_REUSEADDR on the listening socket - the issue is that we
are setting it on all sockets. As a result, the client performs a
connect() which results in the allocation of a randomised port number
- since SO_REUSEADDR/SO_REUSEPORT are enabled the port number is
permitted to already be in use and the normal duplication checks are
not performed. We then attempt to establish a TCP connection using
this already in use port number, which fails:

 19425 test.out CALL  socket(PF_INET,SOCK_STREAM,IPPROTO_IP)
 19425 test.out RET   socket 10/0xa
 19425 test.out CALL  fcntl(0xa,F_SETFD,FD_CLOEXEC)
 19425 test.out RET   fcntl 0
 19425 test.out CALL  setsockopt(0xa,SOL_SOCKET,SO_REUSEADDR,0xf84019ea00,0x4)
 19425 test.out RET   setsockopt 0
 19425 test.out CALL  setsockopt(0xa,SOL_SOCKET,SO_REUSEPORT,0xf84019ea08,0x4)
 19425 test.out RET   setsockopt 0
 19425 test.out CALL  setsockopt(0xa,SOL_SOCKET,SO_BROADCAST,0xf84019ea10,0x4)
 19425 test.out RET   setsockopt 0
 19425 test.out CALL  fcntl(0xa,F_GETFL,0)
 19425 test.out RET   fcntl 2
 19425 test.out CALL  fcntl(0xa,F_SETFL,0x6<O_RDWR|O_NONBLOCK>)
 19425 test.out RET   fcntl 0
 19425 test.out CALL  connect(0xa,0xf840186768,0x10)
 19425 test.out STRU  struct sockaddr { AF_INET, 127.0.0.1:43658 }
 19425 test.out RET   connect -1 errno 48 Address already in use

The other (and probably better) solution is to only set
SO_REUSEADDR/SO_REUSEPORT on sockets that will be listening, probably
via the addition of a func setDefaultListenSockopts(fd *netFD) that
would be called from ListenTCP() et al.