Channel direction linter

Ideally whenever it's possible, I always prefer to declare the direction of channel operations used in function, e.g.:

func ReadsFromChan(chanName <-chan Type) { }

func WritesToChan(chanName chan<- Type) { }

Therefore, we end up having send- or receive-only channel and compiler is going to throw an error in case of unintended misuse.

Apparently I wasn't able to easily find a linter which would warn me about opportunities to declare channel direction, so I decided to write one as toy project and mess around with Go's AST.

This is definitely not a production-grade tool. At least not yet! :) I took some shortcuts here and there, plus I've strongly favoured having working PoC rather than "state of the art" Go code.

Example usage

Just to prove it's working nicely I've cloned Vitess repo and ran my linter against it:

vitess % ./channel-direction-linter ./...
/Users/anon/vitess/go/test/stress/stress.go:296:38: Function `startStressClient` uses channel `resultCh` as send-only, consider `func startStressClient(resultCh chan<- T`
/Users/anon/vitess/go/vt/vtgate/executor_framework_test.go:422:18: Function `getQueryLog` uses channel `logChan` as receive-only, consider `func getQueryLog(logChan <-chan T`

Aparrently the suggestions were accurate at that point of time :) See stress.go:296 and executor_framework_test.go:422

Known issues/limitations

1. Currently this linter ignores ellipsis (a.k.a. parameter lists):

   func FuncName(chanArr ...chan bool) { }

2. If the channel is passed as an argument to a different function, it won't be analysed - e.g:

    func aFunc(chan1, chan2 chan struct{}) struct{} {
        x := <-testChan2
    	anotherFunc(chan1) // this makes `chan1` usage analysis ignored
        return x
    }

Misc

• Go AST viewer I've used -> http://goast.yuroyoro.net, super helpful!
• A very neat blog post on writing a linter, which helped me with kicking off work on this one