fix: use atomic.bool to ensure atomic operation of isWriting

[rfyiamcool]

summary

I think isWriting as bool type is not thread-safe operation under concurrency. 😁

For example, multiple Goroutines determine whether iswriting is false before writing data, and if isWriting is false, write data. but, the two operations of querying isWriting and setting isWriting are not atomic.

example code

the following scene

goroutine 1

func (w *messageWriter) flushFrame(final bool, extra []byte) error {
	...
	
	if c.isWriting {
		panic("concurrent write to websocket connection")
	}

 	// run here 😁

	c.isWriting = true

        c.write(....)

        ....
}

goroutine 2

func (w *messageWriter) flushFrame(final bool, extra []byte) error {
	...
	if c.isWriting {
		panic("concurrent write to websocket connection")
	}

 	// run here 😁

	c.isWriting = true

        c.write(....)

        ....
}

[FZambia]

Hello, as written in comment for isWriting - it's a best effort protection from concurrent writes. If you have a race here it's a signal you are writing concurrently - thus a signal you are using Gorilla WebSocket API in a wrong way. It does not support concurrent writes. So for me it seems to be working as intended.

[ghost]

I agree with FZambia's comment. Here's some more information:

The current code is similar to the check for concurrent writes in Go maps. The Go map check uses a bit flag, not an atomic bool (see uses of hashWriting in runtime/map.go).

The websocket package documentation says:

Connections support one concurrent reader and one concurrent writer.

There is not a race when the application uses the API per the documentation.

[rfyiamcool]

I agree with FZambia's comment. Here's some more information:

The current code is similar to the check for concurrent writes in Go maps. The Go map check uses a bit flag, not an atomic bool (see uses of hashWriting in runtime/map.go).

The websocket package documentation says:

Connections support one concurrent reader and one concurrent writer.

There is not a race when the application uses the API per the documentation.

You are right. 😁 @pennystevens

[BroadbentJim]

Might be worth closing / rejecting this PR.