How Can Chained Channel Operations in Go\'s `select` Statement Lead to Deadlocks and Data Loss?

Chaining Channel Operations in a Single select Case: Analyzing a Potential Pitfall

In Go, the select statement provides a convenient way to multiplex multiple channels. Consider the following scenario: we have two channels, A and B, transmitting messages on different intervals. We want to create a fan-in channel that receives messages from both A and B.

The code below demonstrates this:

func fanIn(input1, input2 <-chan string) <-chan string {
    ch := make(chan string)
    go func() {
        for {
            select {
                case t := <-input1:
                    ch <- t
                case t := <-input2:
                    ch <- t
            }
        }
    }()

    return ch
}

When we run this code, we expect to receive messages from both channels in an interleaved manner. However, if we modify the select case statement as follows, we encounter an unexpected behavior:

func fanIn(input1, input2 <-chan string) <-chan string {
    ch := make(chan string)
    go func() {
        for {
            select {
                case ch <- <-input1:
                case ch <- <-input2:
            }
        }
    }()

    return ch
}

In this case, we receive some messages correctly, but then we experience dropped values and eventually a deadlock. The reason for this behavior lies in the fundamental workings of select.

In select, only one channel read or write operation is non-blocking. All other operations behave normally. In the modified code, both cases contain channel writes, which are non-blocking. This leads to a situation where messages from input channels queue up, but the fan-in channel can only consume one at a time.

As a result, messages can be dropped and a deadlock occurs when the fan-in channel has no writers and the reader is waiting for more values.

To avoid this issue, it's crucial to understand that in a select statement, only one operation should be non-blocking. If you need to perform multiple channel operations in a single select case, consider using a non-blocking select helper function like this:

func nonBlockingSelect(cases []reflect.SelectCase) (chosen int, recv interface{}, ok bool) {
    for i, c := range cases {
        if c.Dir == reflect.SelectSend && c.Chan == nil {
            continue
        }
        v, ok := reflect.Select(cases)
        return v.Index, v.Elem().Interface(), ok
    }

    return -1, nil, false
}

Then, the modified fan-in function can be rewritten as:

func fanIn(input1, input2 <-chan string) <-chan string {
    ch := make(chan string)
    go func() {
        for {
            select {
                case c1 := <-input1:
                    nonBlockingSelect([]reflect.SelectCase{
                        {Dir: reflect.SelectSend, Chan: reflect.ValueOf(ch), Send: reflect.ValueOf(c1)},
                    })
                case c2 := <-input2:
                    nonBlockingSelect([]reflect.SelectCase{
                        {Dir: reflect.SelectSend, Chan: reflect.ValueOf(ch), Send: reflect.ValueOf(c2)},
                    })
            }
        }
    }()

    return ch
}

Using the non-blocking select helper ensures that only one channel operation is non-blocking, preventing the issue of dropped values and deadlocks.

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