Fix Several Race Conditions

[dammarco]

When testing the flakyness of our OCPP unit tests we found a few races happening when reconnection of OCPP clients happens really fast.

Bugs

Websocket: Panic Send on closed channel

Websockets error channel should only be used when the channel is not closed already (1f0c95e)

When shutting down Central System, errors are sent on a closed channel

When shutting down Central System, the error channel might be closed right before we try to send something on it. This is checked before now and protected with a lock. (09198de)

Dispatcher Disconnect Handling

When a client disconnects and connectrs really fast, the dispatcher is locked in a state where he is waiting for a request to be completed. The request got removed from the queue though and thus, the dispatcher is not able to find a matching request when the real client answers to the message. A cleaner way to handle the disconnection of clients was proposed. (5bb5806)

Dispatcher nil reference when peeking element from queue

Fix nil reference when the queue peeks a nil element (ddc83d7)

[coveralls]

Pull Request Test Coverage Report for Build 4041955883

• 43 of 65 (66.15%) changed or added relevant lines in 3 files are covered.
• 12 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.3%) to 87.383%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ws/websocket.go	3	5	60.0%
ocppj/dispatcher.go	36	42	85.71%
ocpp1.6/central_system.go	4	18	22.22%

Files with Coverage Reduction	New Missed Lines	%
ocpp2.0.1/v2.go	2	66.67%
ws/websocket.go	2	90.0%
ocppj/dispatcher.go	8	89.46%

Totals
Change from base Build 3627954280:	-0.3%
Covered Lines:	6323
Relevant Lines:	7236

---

💛 - Coveralls

[guelfey]

Generally it's problematic if you have a Goroutine closing a channel that another Goroutine can write to. Not too familiar with the rest of the code anymore, but is there any reason we need to close errC at all? It's mostly used for logging, right? Then I don't see a reason why we can't just keep it open. Writers could just do a select and throw the error away if the reader Goroutine already stopped.

[dammarco]

good idea 🤔 I'm also not familiar with it, I just fixed the issues :D
how would the writer know if the readers goroutine has stopped?

[guelfey]

The write to the channel would block, but you can just wrap it in a select with a default case to prevent it from causing an issue