fix deadloop bug

[johnlanni]

What this PR does:

Fixed an deadloop error that would occur when a zk connection was closed.

deadloop profile:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[wongoo]

@johnlanni how deadloop happens if z.Session is close?

[johnlanni]

@johnlanni how deadloop happens if z.Session is close?

gost/database/kv/zk/client.go

Line 223 in 927aad2

case event = <-z.Session:

This channel will be closed here:
https://github.com/dubbogo/go-zookeeper/blob/f9d2183d89d5f736496b45dfaae0cb7cb4f687cc/zk/conn.go#L233

and then this select will be a deadloop since the channel will always return with ok=false

[codecov-commenter]

Codecov Report

Merging #102 (b142e34) into master (927aad2) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   74.17%   74.22%   +0.05%     
==========================================
  Files          51       51              
  Lines        4929     4932       +3     
==========================================
+ Hits         3656     3661       +5     
+ Misses       1040     1039       -1     
+ Partials      233      232       -1     

Impacted Files	Coverage Δ
database/kv/zk/client.go	48.95% <100.00%> (+0.53%)	⬆️
time/timer.go	78.52% <0.00%> (+0.67%)	⬆️

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[wongoo]

@johnlanni For the code case event = <-z.Session:, if z.Session is closed, event is nil, switch event.State seems will panic.

[johnlanni]

@johnlanni For the code case event = <-z.Session:, if z.Session is closed, event is nil, switch event.State seems will panic.

event is not nil, it will be an empty struct of zk.Event, and the event.State is zero by default which means StateDisconnected

[AlexStocks]

@wongoo wait for your approve.

[wongoo]

@johnlanni For the code case event = <-z.Session:, if z.Session is closed, event is nil, switch event.State seems will panic.

event is not nil, it will be an empty struct of zk.Event, and the event.State is zero by default which means StateDisconnected

@johnlanni I have approved this PR. But I wonder whether should the Session <-chan zk.Event be defined as a reference channel, like <-chan *zk.Event, to avoid memory copy. We can discuss it in another issue.

[AlexStocks]

@johnlanni For the code case event = <-z.Session:, if z.Session is closed, event is nil, switch event.State seems will panic.

event is not nil, it will be an empty struct of zk.Event, and the event.State is zero by default which means StateDisconnected

@johnlanni I have approved this PR. But I wonder whether should the Session <-chan zk.Event be defined as a reference channel, like <-chan *zk.Event, to avoid memory copy. We can discuss it in another issue.

@johnlanni I have merge your pr. pls open another issue and answer @wongoo 's question.