Read SO_REUSEPORT on socket before setting

[stuartnelson3]

See if we're able to read the value from the
socket. If that fails, skip setting the value on
the port.

Note: I don't have any machines with an old enough kernel lying around to test that it fails correctly, but perhaps someone out there could jump in and give it a try.

Given this comment: #802 (comment)
I'm not sure where you want this to live, or if it's on the user to know they can use SO_REUSEPORT or not.

[miekg]

Might be simpler to always do this, instead of the conditional compilation? Would there be any downsides to that?

[codecov-io]

Codecov Report

Merging #803 into master will decrease coverage by 0.06%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage   57.62%   57.56%   -0.07%     
==========================================
  Files          42       42              
  Lines       10856    10859       +3     
==========================================
- Hits         6256     6251       -5     
- Misses       3511     3517       +6     
- Partials     1089     1091       +2

Impacted Files	Coverage Δ
listen_go111.go	66.66% <25%> (-11.12%)	⬇️
msg.go	77.7% <0%> (-0.65%)	⬇️
server.go	71.21% <0%> (-0.25%)	⬇️

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[stuartnelson3]

Based on this comment: #654 (comment)

It sounds like this behavior is only supported in go1.11+?

[chantra]

shouldn't this explicitly return opErr?

[miekg]

[ Quoting <[email protected]> in "Re: [miekg/dns] Read SO_REUSEPORT o..." ]

chantra commented on this pull request. > @@ -16,6 +17,11 @@ const supportsReusePort = true func reuseportControl(network, address string, c syscall.RawConn) error { var opErr error err := c.Control(func(fd uintptr) { + _, err := unix.GetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT) + if err != nil { + opErr = fmt.Errorf("could not get SO_REUSEPORT: %s", err.Error()) + return shouldn't this explicitly return opErr?

this code already has a toggle to allow you to use it, there is no direct reason to tweak this. In coredns we ignore the error here.

[stuartnelson3]

Closing, as users are allowed to opt-in to this code.