Minor refactoring on the keepAliveListener and keepAliveConn

[ahrtr]

Two issues:

1. When starting etcd using ./etcd --socket-reuse-address --socket-reuse-port, actually etcd setting the SetKeepAlive or SetKeepAlivePeriod fails, but the current code ignores the errors. This issue can happen on both Linux and Mac.
2. When running the functional test on Mac, the etcd always runs into issue below. This issue only happens on Mac,

panic: interface conversion: transport.timeoutConn is not transport.keepAliveConn: missing method SetKeepAlive

goroutine 295 [running]:
go.etcd.io/etcd/client/pkg/v3/transport.(*keepaliveListener).Accept(0x1f)
        /Users/wachao/go/src/go.etcd.io/etcd/client/pkg/transport/keepalive_listener.go:53 +0x51
github.com/soheilhy/cmux.(*cMux).Serve(0xc00057c3c0)
        /Users/wachao/go/gopath/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:170 +0xa9
go.etcd.io/etcd/server/v3/embed.(*serveCtx).serve(0xc0001b51f0, 0xc00037c300, 0xc0001a6868, {0x1d789c0, 0xc0003a2400}, 0xc0002f9250, {0xc0002a4280, 0x2, 0x2})
        /Users/wachao/go/src/go.etcd.io/etcd/server/embed/serve.go:220 +0x191d
go.etcd.io/etcd/server/v3/embed.(*Etcd).serveClients.func1(0xc000280ba0)
        /Users/wachao/go/src/go.etcd.io/etcd/server/embed/etcd.go:735 +0xa8
created by go.etcd.io/etcd/server/v3/embed.(*Etcd).serveClients
        /Users/wachao/go/src/go.etcd.io/etcd/server/embed/etcd.go:734 +0x5f6
,"--socket-reuse-address=true","--socket-reuse-port=true"]}

Analysis and solution

Currently etcd defines an keepAliveConn interface, and the
limitListenerConn implements this interface.

The high level diagram/relationship is something like below (excluding the unix(s) and TLS for simplicity),

There are three issues:

1. Only net.TCPConn supports SetKeepAlive and SetKeepAlivePeriod by default. So the customized conns (previously the limitListenConn, now the keepAliveConn) use golang's type assertion to convert the net.Conn to net.TCPConn, and it just leverage the net.TCPConn to set the keepalive and keepalivePeriod. If there are multiple layers of net.Listener encapsulations, then it must be the one which is closest to the original net.Listener implementation, namely TCPListener. Otherwise, the type assertion fails. It's exactly the reason for the first issue above.
2. Mac (Darwin) doesn't somehow support FDLimit , so the net.Listener chain doesn't include the limitListener, which is the only one which implements the keepAliveConn interface. It's exactly the reason for the failure on Mac.
3. It doesn't make sense to let limitListenConn to support SetKeepAlive and SetKeepAlivePeriod, because it should focus on the functionality (rate limit) it's designed to.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[ahrtr]

cc @spzala

[spzala]

@ahrtr great, thank you! I have few nits inline.

[ahrtr]

Thanks @spzala for the quick review, and I resolved all the comments. PTAL, thx

[codecov-commenter]

Codecov Report

Merging #14356 (7450673) into main (ff6b85d) will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #14356      +/-   ##
==========================================
- Coverage   75.44%   75.26%   -0.19%     
==========================================
  Files         457      457              
  Lines       37122    37139      +17     
==========================================
- Hits        28007    27952      -55     
- Misses       7361     7418      +57     
- Partials     1754     1769      +15     

Flag	Coverage Δ
all	75.26% <ø> (-0.19%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/embed/etcd.go	75.58% <ø> (+0.24%)	⬆️
client/pkg/v3/transport/keepalive_listener.go	69.38% <0.00%> (-22.92%)	⬇️
client/pkg/v3/transport/limit_listen.go	57.14% <0.00%> (-21.43%)	⬇️
client/v3/experimental/recipes/priority_queue.go	56.66% <0.00%> (-10.00%)	⬇️
server/proxy/grpcproxy/register.go	69.76% <0.00%> (-9.31%)	⬇️
client/pkg/v3/fileutil/lock_linux.go	72.22% <0.00%> (-8.34%)	⬇️
raft/rafttest/node.go	95.00% <0.00%> (-5.00%)	⬇️
server/storage/wal/file_pipeline.go	90.69% <0.00%> (-4.66%)	⬇️
client/v3/concurrency/session.go	88.63% <0.00%> (-4.55%)	⬇️
server/etcdserver/api/rafthttp/msgappv2_codec.go	69.56% <0.00%> (-3.48%)	⬇️
... and 21 more

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[spzala]

Thanks @spzala for the quick review, and I resolved all the comments. PTAL, thx

Thank you @ahrtr for quickly addressing the comments.