create UDP connection only when needed

[mitnk]

A further step to try to fix #802

[coveralls]

Coverage remained the same at 0.0% when pulling 4016a0d on mitnk:udp-idle into e9316f7 on xtaci:master.

[mitnk]

Changed the logging items to let them more clear:

2020/12/19 14:51:01 main.go:447: accepted an TCP conn: 127.0.0.1:63581
2020/12/19 14:51:01 main.go:393: smux version: 1 on connection: 0.0.0.0:58146 -> x.x.x.x:12345
2020/12/19 14:51:04 main.go:447: accepted an TCP conn: 127.0.0.1:63583
2020/12/19 14:51:04 main.go:393: smux version: 1 on connection: 0.0.0.0:51598 -> x.x.x.x:12345
2020/12/19 14:51:06 main.go:447: accepted an TCP conn: 127.0.0.1:63585
2020/12/19 14:51:06 main.go:393: smux version: 1 on connection: 0.0.0.0:50933 -> x.x.x.x:12345
2020/12/19 14:51:21 main.go:488: scavenger: session closed due to ttl: 0.0.0.0:58146
2020/12/19 14:51:24 main.go:488: scavenger: session closed due to ttl: 0.0.0.0:51598
2020/12/19 14:51:26 main.go:488: scavenger: session closed due to ttl: 0.0.0.0:50933

[mitnk]

@xtaci Now that only when AutoExpire > that scavengettl will have meanings or routine scavengettl will have work to do:

kcptun/client/main.go

Line 459 in e9316f7

if config.AutoExpire > 0 { // only when autoexpire set

So a question is that should we update option --scavengettl's description like `this option take effects only if AutoExpire is enabled? Or make logic change that scavengettl take effects no matter whether AutoExpire enabled or not?

[xtaci]

scavengettl make sense ONLY when autoexpire(actively switching to new connection) has set.

[mitnk]

@xtaci Ok, make sense to me. This PR is ready for review, please let me know if any adjust needed

[xtaci]

why setting nil for line 440

[mitnk]

@xtaci Set sessions to nil when make the pool is because 1) there may no soon traffic coming to kcptun-client when it starts up and 2) make session's creation only happens in a single place (create only when a TCP conn accepted) which makes a better readability.

[mitnk]

Here in scavenger(), we better not depend on config.AutoExpire, instead to fetch the expire info from the session item. So that a change to the timedSession.expiryDate built logic won't need to touch the code in scavenger too.

[mitnk]

I don't see any place of using -1, so I guess the description need to be updated?

[mitnk]

@xtaci I just updated a bit more. It's ready for another look. Please let me know if you'd like to do any adjustments.

[xtaci]

https://github.com/xtaci/kcptun/pull/825/files#diff-39e4af22263d62c01d979fa1f6ec1770d58fc1b03b0aeec2f17ba84e7136c0dbR439-R442

Line 439-442 can be delete for lazy initialization of session

[mitnk]

@xtaci hmm, good point. Updated.

[xtaci]

looks good to me .