What is the 'done channel' pattern and why has context.Context largely superseded it?

Before context.Context was standardised in Go 1.7, a chan struct{} was the primary way to broadcast a stop signal. Closing the channel (rather than sending) is used because it unblocks all waiting receivers simultaneously — a broadcast, not a point-to-point signal.

// Done channel pattern (pre-1.7 idiom)
done := make(chan struct{})

func startWorker(done <-chan struct{}, jobs <-chan Job) {
    for {
        select {
        case <-done: return          // all goroutines unblock at once
        case job := <-jobs: process(job)
        }
    }
}

for i := 0; i < 5; i++ { go startWorker(done, jobs) }
close(done) // broadcasts stop to all 5 workers in O(1)

// Why close() instead of sending N values?
// close() is O(1) and unblocks ALL receivers simultaneously.
// Sending N values requires knowing N and sending N times.

// Why context.Context is preferred today:
// 1. Propagates deadlines/timeouts automatically
// 2. Carries the reason: ctx.Err() returns Canceled or DeadlineExceeded
// 3. Standard API — all stdlib network/IO functions accept context
// 4. No manual channel plumbing through every function signature

// Modern equivalent:
ctx, cancel := context.WithCancel(context.Background())
for i := 0; i < 5; i++ {
    go func() {
        for {
            select {
            case <-ctx.Done(): return
            case job := <-jobs: process(job)
            }
        }
    }()
}
cancel() // equivalent to close(done)

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