kgo: add EnsureProduceConnectionIsOpen

This can help reduce latency if you produce infrequently, but know
you'll be producing shortly.

Closes #807.

.golangci.yml

@@ -24,7 +24,6 @@ linters:
     - asciicheck
     - bidichk
     - bodyclose
-    - copyloopvar
     - durationcheck
     - exhaustive
     - gocritic

pkg/kgo/broker.go

@@ -41,6 +41,8 @@ func (p *pinReq) SetVersion(v int16) {
 	p.Request.SetVersion(v)
 }
 
+type forceOpenReq struct{ kmsg.Request }
+
 type promisedReq struct {
 	ctx     context.Context
 	req     kmsg.Request
@@ -408,6 +410,12 @@ start:
 	default:
 	}
 
+	if _, isForceOpen := req.(*forceOpenReq); isForceOpen {
+		cxn.lastWrite.Store(time.Now().UnixNano())
+		pr.promise(nil, nil)
+		return
+	}
+
 	// Produce requests (and only produce requests) can be written
 	// without receiving a reply. If we see required acks is 0,
 	// then we immediately call the promise with no response.

pkg/kgo/producer.go

@@ -101,6 +101,81 @@ func (cl *Client) BufferedProduceBytes() int64 {
 	return cl.producer.bufferedBytes + cl.producer.blockedBytes
 }
 
+// EnsureProduceConnectionIsOpen attempts to open a produce connection to all
+// specified brokers, or all brokers if `brokers` is empty or contains -1.
+//
+// This can be used in an attempt to reduce the latency when producing if your
+// application produces infrequently: you can force open a produce connection a
+// bit earlier than you intend to produce, rather than at the moment you
+// produce. In rare circumstances, it is possible that a connection that was
+// ensured to be open may close before you produce.
+//
+// This returns an errors.Join'd error that merges a message for all brokers
+// that failed to be opened, as well as why.
+//
+// Note that if opening connections to all brokers, this only opens connections
+// to all discovered brokers. If no internal metadata request has ever been
+// issued and returned, this will not open a connection to any broker.
+func (cl *Client) EnsureProduceConnectionIsOpen(ctx context.Context, brokers ...int32) error {
+	var (
+		keep = brokers[:0]
+		all  bool
+		wg   sync.WaitGroup
+		mu   sync.Mutex
+		errs []error
+	)
+	for _, b := range brokers {
+		switch {
+		case b < -1:
+		case b == -1:
+			all = true
+		case b > -1:
+			keep = append(keep, b)
+		}
+	}
+	var toOpen []*broker
+	if all || len(brokers) == 0 {
+		cl.brokersMu.RLock()
+		toOpen = cl.brokers
+		cl.brokersMu.RUnlock()
+	} else {
+		for _, b := range brokers {
+			b := b
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+
+				br, err := cl.brokerOrErr(ctx, b, errUnknownBroker)
+
+				mu.Lock()
+				defer mu.Unlock()
+				if err != nil {
+					errs = append(errs, fmt.Errorf("%d: %w", b, err))
+					return
+				}
+				toOpen = append(toOpen, br)
+			}()
+		}
+		wg.Wait()
+	}
+
+	for _, br := range toOpen {
+		br := br
+		wg.Add(1)
+		br.do(ctx, &forceOpenReq{new(kmsg.ProduceRequest)}, func(_ kmsg.Response, err error) {
+			defer wg.Done()
+			if err != nil {
+				mu.Lock()
+				errs = append(errs, fmt.Errorf("%d: %w", br.meta.NodeID, err))
+				mu.Unlock()
+			}
+		})
+	}
+	wg.Wait()
+
+	return errors.Join(errs...)
+}
+
 type unknownTopicProduces struct {
 	buffered []promisedRec
 	wait     chan error // retryable errors