Why Do Chained Channel Operations in Go\'s `select` Case Cause Deadlocks?

Chained Channel Operations in a Single select Case: Decoding the Behavior

In the pursuit of designing concurrent and asynchronous programs, Go's select construct provides a powerful tool for multiplexing channels. However, one often encounters unexpected outcomes when combining multiple operations within a single select case.

Consider the following scenario: two channels, A and B, send messages at different time intervals (10 milliseconds for A and 1 second for B). We employ select to listen to both channels and forward the received values to a fan-in channel.

func main() {
    ch := fanIn(talk("A", 10), talk("B", 1000))

    for i := 0; i < 10; i++ {
        fmt.Printf("%q\n", <-ch)
    }
    fmt.Printf("Done\n")
}

The expected result is:

"A 0"
"B 0"
"A 1"
"A 2"
"A 3"
"A 4"
"B 1"
"B 2"
"B 3"
"B 4"
Done

However, when we modify the select case to use chained channel operations:

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

we observe a peculiar behavior:

"B 0"
"A 1"
"B 2"
"A 3"
"A 4"
fatal error: all goroutines are asleep - deadlock!

Behind the Scenes

The key to understanding this behavior lies in the non-blocking nature of channel operations within a select case. In a typical select case, only one channel operation (either read or write) can be non-blocking.

When we use chained channel operations, we are effectively attempting multiple channel operations within a single case. The first operation is always blocking, while the subsequent operations are non-blocking.

In our modified code, the first operation blocks to receive a value from input1. After receiving the value, it attempts to write it to the ch channel non-blocking. However, if the receiver of the ch channel is not ready to accept the value, the write operation will fail.

The Chain Reaction

The failed write operation does not stop the select case. Instead, it moves on to the second case, which is now the only viable case. This results in a potential deadlock scenario.

Over time, multiple values from both channels are received but not forwarded to the fan-in channel due to the failed writes. Consequently, the fan-in channel eventually becomes empty, leading to a deadlock as no more values can be received.

Resolving the Issue

To avoid this issue, it is crucial to ensure that the channel operations within a select case are executed serially. This can be achieved by using a temporary variable to store the received value and then executing the write operation as a separate statement outside the select case.

var msg string
select {
    case msg = <-input1:
    case msg = <-input2:
}

ch <- msg

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