kversion: switchup Versions API, finalize 2.7.0

As usual, a new release added new behavior: 2.7.0 has a gap in API keys
in the ApiVersions response.

This commit switches up the Versions type to a struct, which is a
breaking change but makes future changes unlikely to be breaking. The
gap is indicated with negative values.

This required small changes in the client guts, but I think the result
is easier to read.

Also removes an erroneous kversions bump for ApiVersions in tip.

pkg/kgo/broker.go

@@ -232,8 +232,7 @@ func (b *broker) handleReqs() {
 		}
 
 		if int(req.Key()) > len(cxn.versions[:]) ||
-			b.cl.cfg.maxVersions != nil &&
-				int(req.Key()) >= len(b.cl.cfg.maxVersions) {
+			b.cl.cfg.maxVersions != nil && !b.cl.cfg.maxVersions.HasKey(req.Key()) {
 			pr.promise(nil, ErrUnknownRequestKey)
 			continue
 		}
@@ -248,7 +247,7 @@ func (b *broker) handleReqs() {
 
 		ourMax := req.MaxVersion()
 		if b.cl.cfg.maxVersions != nil {
-			userMax := b.cl.cfg.maxVersions[req.Key()]
+			userMax, _ := b.cl.cfg.maxVersions.LookupVersion(req.Key()) // we validated HasKey above
 			if userMax < ourMax {
 				ourMax = userMax
 			}
@@ -265,11 +264,12 @@ func (b *broker) handleReqs() {
 		// If the version now (after potential broker downgrading) is
 		// lower than we desire, we fail the request for the broker is
 		// too old.
-		if b.cl.cfg.minVersions != nil &&
-			int(req.Key()) < len(b.cl.cfg.minVersions) &&
-			version < b.cl.cfg.minVersions[req.Key()] {
-			pr.promise(nil, ErrBrokerTooOld)
-			continue
+		if b.cl.cfg.minVersions != nil {
+			minVersion, minVersionExists := b.cl.cfg.minVersions.LookupVersion(req.Key())
+			if minVersionExists && version < minVersion {
+				pr.promise(nil, ErrBrokerTooOld)
+				continue
+			}
 		}
 
 		req.SetVersion(version) // always go for highest version
@@ -422,7 +422,7 @@ func (cxn *brokerCxn) init() error {
 		cxn.versions[i] = -1
 	}
 
-	if cxn.b.cl.cfg.maxVersions == nil || len(cxn.b.cl.cfg.maxVersions) >= 19 {
+	if cxn.b.cl.cfg.maxVersions == nil || cxn.b.cl.cfg.maxVersions.HasKey(18) {
 		if err := cxn.requestAPIVersions(); err != nil {
 			cxn.cl.cfg.logger.Log(LogLevelError, "unable to request api versions", "err", err)
 			return err

pkg/kgo/config.go

@@ -55,8 +55,8 @@ type cfg struct {
 	logger Logger
 
 	seedBrokers []string
-	maxVersions kversion.Versions
-	minVersions kversion.Versions
+	maxVersions *kversion.Versions
+	minVersions *kversion.Versions
 
 	retryBackoff          func(int) time.Duration
 	retries               int
@@ -142,7 +142,7 @@ func defaultCfg() cfg {
 		logger: new(nopLogger),
 
 		seedBrokers: []string{"127.0.0.1"},
-		maxVersions: kversion.Stable(),
+		maxVersions: func() *kversion.Versions { s := kversion.Stable(); return &s }(),
 
 		retryBackoff: func() func(int) time.Duration {
 			var rngMu sync.Mutex
@@ -300,7 +300,7 @@ func SeedBrokers(seeds ...string) Opt {
 // requests, it is recommended to pin versions so that new fields on requests
 // do not get invalid default zero values before you update your usage.
 func MaxVersions(versions kversion.Versions) Opt {
-	return clientOpt{func(cfg *cfg) { cfg.maxVersions = versions }}
+	return clientOpt{func(cfg *cfg) { cfg.maxVersions = &versions }}
 }
 
 // MinVersions sets the minimum Kafka version a request can be downgraded to,
@@ -316,7 +316,7 @@ func MaxVersions(versions kversion.Versions) Opt {
 // versions, the request is allowed. It is assumed that there is no lower bound
 // for that request.
 func MinVersions(versions kversion.Versions) Opt {
-	return clientOpt{func(cfg *cfg) { cfg.minVersions = versions }}
+	return clientOpt{func(cfg *cfg) { cfg.minVersions = &versions }}
 }
 
 // RetryBackoff sets the backoff strategy for how long to backoff for a given