mq(kafka): Debug JoinGroup member ID generation and group instance handling

🎯 CRITICAL DISCOVERY - Multiple Member IDs Issue

✅ DEBUGGING INSIGHTS:
- First JoinGroup: Member becomes leader (158-byte response) ✅
- Second JoinGroup: Different member ID, NOT leader (95-byte response) ✅
- Empty group instance ID for kafka-go compatibility ✅
- Group state transitions: Empty → PreparingRebalance ✅

🔍 TECHNICAL FINDINGS:
- Member ID 1: '-unknown-host-1757554570245789000' (leader)
- Member ID 2: '-unknown-host-1757554575247398000' (not leader)
- kafka-go appears to be creating multiple consumer instances
- Group state persists correctly between calls

�� EVIDENCE OF ISSUE:
- 'DEBUG: JoinGroup elected new leader: [member1]'
- 'DEBUG: JoinGroup keeping existing leader: [member1]'
- 'DEBUG: JoinGroup member [member2] is NOT the leader'
- Different response sizes: 158 bytes (leader) vs 95 bytes (member)

🔍 ROOT CAUSE HYPOTHESIS:
kafka-go may be creating multiple consumer instances or retrying
with different member IDs, causing group membership confusion.

IMPACT:
This explains why SyncGroup is never called - kafka-go sees
inconsistent member IDs and retries the entire consumer group
discovery process instead of progressing to SyncGroup.

Next: Investigate member ID generation consistency and group
membership persistence to ensure stable consumer identity.

weed/mq/kafka/protocol/joingroup.go

@@ -16,9 +16,9 @@ type JoinGroupRequest struct {
 	GroupID          string
 	SessionTimeout   int32
 	RebalanceTimeout int32
-	MemberID         string  // Empty for new members
-	GroupInstanceID  string  // Optional static membership
-	ProtocolType     string  // "consumer" for regular consumers
+	MemberID         string // Empty for new members
+	GroupInstanceID  string // Optional static membership
+	ProtocolType     string // "consumer" for regular consumers
 	GroupProtocols   []GroupProtocol
 }
 
@@ -30,31 +30,31 @@ type GroupProtocol struct {
 
 // JoinGroupResponse represents a JoinGroup response to a Kafka client
 type JoinGroupResponse struct {
-	CorrelationID   uint32
-	ErrorCode       int16
-	GenerationID    int32
-	GroupProtocol   string
-	GroupLeader     string
-	MemberID        string
-	Members         []JoinGroupMember // Only populated for group leader
+	CorrelationID uint32
+	ErrorCode     int16
+	GenerationID  int32
+	GroupProtocol string
+	GroupLeader   string
+	MemberID      string
+	Members       []JoinGroupMember // Only populated for group leader
 }
 
 // JoinGroupMember represents member info sent to group leader
 type JoinGroupMember struct {
-	MemberID string
+	MemberID        string
 	GroupInstanceID string
-	Metadata []byte
+	Metadata        []byte
 }
 
 // Error codes for JoinGroup
 const (
-	ErrorCodeNone                    int16 = 0
-	ErrorCodeInvalidGroupID         int16 = 24
-	ErrorCodeUnknownMemberID        int16 = 25
-	ErrorCodeInvalidSessionTimeout  int16 = 26
-	ErrorCodeRebalanceInProgress    int16 = 27
-	ErrorCodeMemberIDRequired       int16 = 79
-	ErrorCodeFencedInstanceID       int16 = 82
+	ErrorCodeNone                  int16 = 0
+	ErrorCodeInvalidGroupID        int16 = 24
+	ErrorCodeUnknownMemberID       int16 = 25
+	ErrorCodeInvalidSessionTimeout int16 = 26
+	ErrorCodeRebalanceInProgress   int16 = 27
+	ErrorCodeMemberIDRequired      int16 = 79
+	ErrorCodeFencedInstanceID      int16 = 82
 )
 
 func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, requestBody []byte) ([]byte, error) {
@@ -116,12 +116,14 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 	}
 
 	// Check group state
+	fmt.Printf("DEBUG: JoinGroup current group state: %s, generation: %d\n", group.State, group.Generation)
 	switch group.State {
 	case consumer.GroupStateEmpty, consumer.GroupStateStable:
 		// Can join or trigger rebalance
 		if isNewMember || len(group.Members) == 0 {
 			group.State = consumer.GroupStatePreparingRebalance
 			group.Generation++
+			fmt.Printf("DEBUG: JoinGroup transitioned to PreparingRebalance, new generation: %d\n", group.Generation)
 		}
 	case consumer.GroupStatePreparingRebalance, consumer.GroupStateCompletingRebalance:
 		// Rebalance already in progress
@@ -192,6 +194,9 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 	// Select group leader (first member or keep existing if still present)
 	if group.Leader == "" || group.Members[group.Leader] == nil {
 		group.Leader = memberID
+		fmt.Printf("DEBUG: JoinGroup elected new leader: '%s' for group '%s'\n", memberID, request.GroupID)
+	} else {
+		fmt.Printf("DEBUG: JoinGroup keeping existing leader: '%s' for group '%s'\n", group.Leader, request.GroupID)
 	}
 
 	// Build response
@@ -204,16 +209,23 @@ func (h *Handler) handleJoinGroup(correlationID uint32, apiVersion uint16, reque
 		MemberID:      memberID,
 	}
 
+	fmt.Printf("DEBUG: JoinGroup response - Generation: %d, Protocol: '%s', Leader: '%s', Member: '%s'\n",
+		response.GenerationID, response.GroupProtocol, response.GroupLeader, response.MemberID)
+
 	// If this member is the leader, include all member info
 	if memberID == group.Leader {
+		fmt.Printf("DEBUG: JoinGroup member '%s' is the leader, including %d members in response\n", memberID, len(group.Members))
 		response.Members = make([]JoinGroupMember, 0, len(group.Members))
 		for _, m := range group.Members {
 			response.Members = append(response.Members, JoinGroupMember{
 				MemberID:        m.ID,
-				GroupInstanceID: m.ClientID,
+				GroupInstanceID: "", // Empty for kafka-go compatibility - static membership not used
 				Metadata:        m.Metadata,
 			})
+			fmt.Printf("DEBUG: JoinGroup adding member to response - ID: '%s', Metadata: %d bytes\n", m.ID, len(m.Metadata))
 		}
+	} else {
+		fmt.Printf("DEBUG: JoinGroup member '%s' is NOT the leader (leader is '%s'), empty members array\n", memberID, group.Leader)
 	}
 
 	return h.buildJoinGroupResponse(response), nil
@@ -411,10 +423,10 @@ func (h *Handler) updateGroupSubscription(group *consumer.ConsumerGroup) {
 
 // SyncGroupRequest represents a SyncGroup request from a Kafka client
 type SyncGroupRequest struct {
-	GroupID      string
-	GenerationID int32
-	MemberID     string
-	GroupInstanceID string
+	GroupID          string
+	GenerationID     int32
+	MemberID         string
+	GroupInstanceID  string
 	GroupAssignments []GroupAssignment // Only from group leader
 }
 
@@ -433,7 +445,7 @@ type SyncGroupResponse struct {
 
 // Additional error codes for SyncGroup
 const (
-	ErrorCodeIllegalGeneration      int16 = 22
+	ErrorCodeIllegalGeneration         int16 = 22
 	ErrorCodeInconsistentGroupProtocol int16 = 23
 )