Can Chained Channel Operations in Go\'s `select` Case Lead to Data Loss?

Chained Channel Operations in a Single select Case and its Impact on Data Loss

In Go, the select statement provides a convenient mechanism for multiplexing multiple channel operations. This capability enables the concurrent processing of events from various sources. However, certain chained channel operations can lead to unintended consequences when used within a select case.

Let's consider a scenario where we have two channels, A and B, each sending messages with different delays. We use a fan-in channel to collect messages from both channels and send them to the main function for printing. Here's the simplified code snippet:

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
}

This code correctly multiplexes the messages from both channels. However, if we modify the select case to use chained channel operations as follows:

select {
    case ch <- <-input1:
    case ch <- <-input2:
}

We encounter a perplexing issue. While the first few messages are received correctly, subsequent messages are dropped, and the program eventually deadlocks.

This behavior arises because only one channel operation within a select case is non-blocking. In our modified code, both channel operations are non-blocking, resulting in the dropped messages.

To understand the mechanics behind this unexpected behavior, let's examine the sequence of events that occur:

1. The for loop in the fan-in goroutine initiates a non-blocking read operation (Send) on input1.
2. If the main function loop has not yet consumed the value from the combined channel (ch), it is possible for the input1 channel to block while waiting to write to ch.
3. This blocking operation prevents the for loop from evaluating the second select case (the one involving input2).
4. If the main function loop eventually consumes the value from ch, the for loop will be able to progress to the next iteration and evaluate the second select case.
5. However, by this time, the value sent by input2 in the previous iteration might have been lost, as the main function loop had not yet consumed it.

This repeated loss of messages eventually leads to a deadlock situation where no messages are left on either channel, and the main function is waiting to read from the combined channel indefinitely.

Therefore, when using chained channel operations in a single select case, it is crucial to ensure that only one channel operation is non-blocking. This prevents the blocking of other channel operations and the subsequent loss of messages.

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