客户端向GraphQL订阅服务器发送关闭信号方式及ctx.Done()触发疑问

我正在开发一个GraphQL服务器，其中包含一个订阅API，以下是我在gqlgen文档中找到的示例代码：

// CurrentTime is the resolver for the currentTime field.
func (r *subscriptionResolver) CurrentTime(ctx context.Context) (<-chan *model.Time, error) {
    // First you'll need to `make()` your channel. Use your type here!
    ch := make(chan *model.Time)

    // You can (and probably should) handle your channels in a central place outside of `schema.resolvers.go`.
    // For this example we'll simply use a Goroutine with a simple loop.
    go func() {
        // Handle deregistration of the channel here. Note the `defer`
        defer close(ch)

        for {
            // In our example we'll send the current time every second.
            time.Sleep(1 * time.Second)
            fmt.Println("Tick")

            // Prepare your object.
            currentTime := time.Now()
            t := &model.Time{
                UnixTime:  int(currentTime.Unix()),
                TimeStamp: currentTime.Format(time.RFC3339),
            }

            // The subscription may have got closed due to the client disconnecting.
            // Hence we do send in a select block with a check for context cancellation.
            // This avoids goroutine getting blocked forever or panicking,
            select {
            case <-ctx.Done(): // This runs when context gets cancelled. Subscription closes.
                fmt.Println("Subscription Closed")
                // Handle deregistration of the channel here. `close(ch)`
                return // Remember to return to end the routine.
            
            case ch <- t: // This is the actual send.
                // Our message went through, do nothing 
            }
        }
    }()

    // We return the channel and no error.
    return ch, nil
}

我想了解接收到ctx.Done()信号时的具体情况：该信号是客户端取消订阅或关闭订阅时发送的？还是可能因闲置超时自动触发？另外，服务端设置的超时是否会触发ctx.Done()信号？

触发ctx.Done()的核心场景： #

• 客户端主动终止：当客户端主动取消订阅（调用客户端SDK的取消方法）、关闭连接（比如关闭浏览器标签页、断开WebSocket）时，gqlgen会自动取消当前订阅绑定的Context，触发ctx.Done()。这是代码注释中提到的最常见场景。
• 服务端闲置超时触发：如果你的GraphQL服务配置了订阅的闲置超时（比如通过gqlgen配置项、底层WebSocket服务器的超时设置），当客户端在指定时间内无任何交互（包括心跳包），服务端会主动终止订阅，此时也会触发ctx.Done()。
• 服务端主动配置的超时：如果你在Resolver或服务端中间件中，通过context.WithTimeout或context.WithDeadline给订阅Context设置了超时，当超时时间到达时，Context会被取消，进而触发ctx.Done()。默认情况下gqlgen不会给订阅Context添加超时，需要你主动配置。

补充说明：代码中defer close(ch)的写法很合理，当ctx.Done()触发后，goroutine退出前会关闭channel，能有效避免goroutine泄漏和资源浪费。

内容的提问来源于stack exchange，提问作者mrasoolmirza