kgo: add log for preferred replicas

pkg/kgo/source.go

@@ -5,7 +5,9 @@ import (
 	"encoding/binary"
 	"fmt"
 	"hash/crc32"
+	"slices"
 	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -262,6 +264,60 @@ func (p *cursorOffsetPreferred) move() {
 
 type cursorPreferreds []cursorOffsetPreferred
 
+func (cs cursorPreferreds) String() string {
+	type pnext struct {
+		p    int32
+		next int32
+	}
+	ts := make(map[string][]pnext)
+	for _, c := range cs {
+		t := c.from.topic
+		p := c.from.partition
+		ts[t] = append(ts[t], pnext{p, c.preferredReplica})
+	}
+	tsorted := make([]string, 0, len(ts))
+	for t, ps := range ts {
+		tsorted = append(tsorted, t)
+		slices.SortFunc(ps, func(l, r pnext) int {
+			if l.p < r.p {
+				return -1
+			}
+			if l.p > r.p {
+				return 1
+			}
+			if l.next < r.next {
+				return -1
+			}
+			if l.next > r.next {
+				return 1
+			}
+			return 0
+		})
+	}
+	slices.Sort(tsorted)
+
+	sb := new(strings.Builder)
+	for i, t := range tsorted {
+		ps := ts[t]
+		fmt.Fprintf(sb, "%s{", t)
+
+		for j, p := range ps {
+			if j < len(ps)-1 {
+				fmt.Fprintf(sb, "%d=>%d, ", p.p, p.next)
+			} else {
+				fmt.Fprintf(sb, "%d=>%d", p.p, p.next)
+			}
+		}
+
+		if i < len(tsorted)-1 {
+			fmt.Fprint(sb, "}, ")
+		} else {
+			fmt.Fprint(sb, "}")
+		}
+	}
+	return sb.String()
+}
+
 func (cs cursorPreferreds) eachPreferred(fn func(cursorOffsetPreferred)) {
 	for _, c := range cs {
 		fn(c)
@@ -832,6 +888,12 @@ func (s *source) fetch(consumerSession *consumerSession, doneFetch chan<- struct
 	// These two removals transition responsibility for finishing using the
 	// cursor from the request's used offsets to the new source or the
 	// reloading.
+	if len(preferreds) > 0 {
+		s.cl.cfg.logger.Log(LogLevelInfo, "fetch partitions returned preferred replicas",
+			"from_broker", s.nodeID,
+			"moves", preferreds.String(),
+		)
+	}
 	preferreds.eachPreferred(func(c cursorOffsetPreferred) {
 		c.move()
 		deleteReqUsedOffset(c.from.topic, c.from.partition)