Thursday 19 October 2017

Go Concurrency Patterns #1 - Signalling Multiple Readers - Closing Channels in Go

The traditional use of a channel close is to signal from the writer to the reader that there is no more data to be consumed. When a channel is closed and empty a reader will exit its 'for range' loop over such a channel. This is because of some handy shorthand in 'for range', note that the two expressions are functionally equivalent:

In the first snippet the check for the channel close is implicit. The two value receive format is how we normally check for a channel close, this is what the spec has to say on it:

 A receive expression used in an assignment or initialization of the special form

 x, ok = <-ch
 x, ok := <-ch
 var x, ok = <-ch
 var x, ok T = <-ch

 yields an additional untyped boolean result reporting whether the communication succeeded. The value of ok is true if the value received was delivered by a successful send operation to the channel, or false if it is a zero value generated because the channel is closed and empty.

Here is a functional example of a writer closing a data channel to signal a reader to stop reading, this is handled nicely in the language with the implicit close check in for range.

The close of a data channel should be performed by the writer as a write on a closed channel causes a panic, thus a close by the reader has the potential to induce a panic. Similarly a close with multiple writers could also induce a panic. The good news is that the excellent go race detector can detect such races.

However the reader can signal the writer to stop writing by utilising a secondary signalling channel. This could be done via a send to the secondary channel. However, as a signalling mechanism, closing a channel instead of a channel send has the benefit of working for multiple readers of the signalling channel. It is important this signalling channel has no data written to it as we only receive a close signal when the channel is both closed and empty.

The use of a channel of type empty struct has the benefit that it cannot be used to pass data and thus its usage as a signalling channel is more apparent. Dave Cheney has an interesting post on the curious empty struct type.

Here is an example that demonstrates the use of a secondary channel to shutdown multiple writers initiated by the reader:

Hopefully you find this useful for shutting down errant goroutines. Indeed, this is how the context package implements its own cancellation functionality. Context is the idiomatic way to perform cancellation that should also be honoured by http transports and libraries and the like.

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