How does Go's select handle multiple ready cases, and how do you implement true priority?

When multiple cases in a select are simultaneously ready, Go picks one uniformly at random. This prevents deterministic starvation but does not guarantee priority. Implementing true priority requires nested selects or a dedicated check before the main select.

// Standard select: random among ready cases
// Even if ctx is cancelled AND a job is ready, ~50% of the time the job wins
for {
    select {
    case <-ctx.Done(): return
    case job := <-jobs: process(job)
    }
}

// Priority select: always check cancellation first
// Step 1: fast non-blocking check of the high-priority case
for {
    select {
    case <-ctx.Done(): return   // check first: if done, exit immediately
    default:                    // not done: fall through to main select
    }

    // Step 2: main blocking select
    select {
    case <-ctx.Done(): return
    case job := <-jobs: process(job)
    }
}

// True channel priority (hi > lo): nested select pattern
func drainWithPriority(hi, lo <-chan int) {
    for {
        // Always drain hi before touching lo
        select {
        case v := <-hi: handle(v); continue
        default:
        }
        // hi empty: process either
        select {
        case v := <-hi: handle(v)
        case v := <-lo: handle(v)
        }
    }
}

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