Why Do Chained Channel Operations in Go\'s `select` Statement Lead to Deadlock?

Chained Channel Operations in a Select Case: Understanding Deadlock

In Go, the fanIn function aggregates values from multiple input channels into a single output channel, providing a form of multiplexing. The select statement in the fanIn function uses non-blocking <- operations to read from multiple input channels concurrently.

However, if the select statement is modified to include chained channel operations, such as:

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

strange behavior can occur. Some values may be dropped, leading to a deadlock.

To understand why this happens, it's essential to remember that in a select statement, only one channel read or write operation is non-blocking. Others behave as if the block operator was used.

In the modified fanIn function, the first case reads a value from input1 and attempts to write it to ch in a non-blocking manner. This can succeed if the main function consumes the value promptly. However, there's a small chance that the main function loop may not be fast enough to do this.

In the subsequent iterations of the fanIn loop, the second case is likely to be selected, and by this time, the second goroutine may have written a value to ch. However, if the main function hasn't yet consumed the value from the previous iteration, it will be dropped.

This cycle of dropping values eventually leads to a situation where there are no more writers, but a reader is still waiting for values, resulting in a deadlock.

A modified version of the code demonstrates this issue more clearly: https://play.golang.org/p/lcM5OKx09Dj

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