How Can We Guarantee Context Cancellation Priority in Go's `select` Statement?

Prioritizing Go select Statement

Problem:

In a Go select statement, the order of evaluation for case blocks is non-deterministic. This can lead to situations where a specific case block is not entered promptly, even when its corresponding channel is ready to receive.

Scenario:

Consider a sendRegularHeartbeats() function that sends heartbeat messages periodically and terminates when its context is canceled. If the context is canceled before the first heartbeat message is sent, we expect no heartbeat messages to be transmitted.

Problem with Non-Deterministic Evaluation:

In some cases, a heartbeat message is sent despite the context being canceled before the function is started. This is because the order of evaluation is unpredictable, and the default or the heartbeat case may be entered before the context case.

Incorrect Suggestion:

One proposed solution is to add a "isContextclosed"-check in the heartbeat case. However, this is not a reliable solution as both the context and heartbeat channels can become ready for reading concurrently.

Proposed Solution: Primordial Select

To prioritize the context case, we can introduce a primordial select statement that tests for the context channel being ready. Only if the context channel is not ready will the second select statement be executed.

func sendRegularHeartbeats(ctx context.Context) {
    for {
        // Primordial select: Check for context channel being ready only.
        select {
        case <-ctx.Done():
            return
        default:
        }

        // Secondary select: Handle heartbeat logic.
        select {
        case <-ctx.Done():
            return
        case <-time.After(1 * time.Second):
            sendHeartbeat()
        }
    }
}

Imperfection:

While this approach effectively prioritizes the context case, it may still allow for "close enough" events. For example, if a heartbeat event arrives shortly after the context channel becomes ready, the heartbeat may be sent before the context case is entered. There is no perfect solution to this issue with the current Go language implementation.

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