How Can I Prioritize Context Cancellation in Go's `select` Statement?

Priority Control in Go's Select Statement

In Go, the select statement allows for asynchronous communication between goroutines. However, the evaluation order of case blocks can sometimes lead to unexpected results. This issue arises when attempting to prioritize the handling of a specific case, such as immediately exiting a routine when a context is canceled.

Consider the following code:

func sendRegularHeartbeats(ctx context.Context) {
    for {
        select {
        case <-ctx.Done():
            return
        case <-time.After(1 * time.Second):
            sendHeartbeat()
        }
    }
}

The Problem:
In this code, the intent is to prioritize the handling of the ctx.Done() case over the time.After() case. However, due to Go's non-deterministic evaluation order, the time.After() case may sometimes be evaluated first, leading to the transmission of a heartbeat even when the context is canceled.

Solution:
The proposed solution in the accepted answer suggests adding a nested select statement with a non-blocking check for ctx.Done(). However, this does not fully address the issue, as the probability of the heartbeat being sent remains high.

A more effective approach is to prioritize the handling of the ctx.Done() case by adding a default case to the outer select statement:

func sendRegularHeartbeats(ctx context.Context) {
    ticker := time.NewTicker(time.Second)
    defer ticker.Stop()

    for {
        select {
        case <-ctx.Done():
            return
        default:
        }

        select {
        case <-ctx.Done():
            return
        case <-ticker.C:
            sendHeartbeat()
        }
    }
}

This ensures that the ctx.Done() case is evaluated first in the outer select statement, giving it higher priority. If the context is canceled, the routine will immediately return and no heartbeats will be sent. If the time.After() case arrives first, it will be ignored due to the default case in the outer select statement.

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