fix(server-party): co_managed_keygen 等待所有参与者加入后再开始 keygen

- Message Router GetSessionStatus 透传 participants 列表
- Server Party 新增 GetSessionStatusFull 方法获取完整会话状态
- participate_keygen.go 对 co_managed_keygen 类型轮询等待所有 N 个参与者加入
- 不影响原有 keygen/sign 功能（仅 co_managed_keygen 触发等待逻辑）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go

@@ -679,11 +679,24 @@ func (s *MessageRouterServer) GetSessionStatus(
 		return nil, err
 	}
 
+	// Convert participants from coordinator response
+	var participants []*pb.PartyInfo
+	if len(coordResp.Participants) > 0 {
+		participants = make([]*pb.PartyInfo, len(coordResp.Participants))
+		for i, p := range coordResp.Participants {
+			participants[i] = &pb.PartyInfo{
+				PartyId:    p.PartyId,
+				PartyIndex: p.PartyIndex,
+			}
+		}
+	}
+
 	return &pb.GetSessionStatusResponse{
-		SessionId:  req.SessionId,
-		Status:     coordResp.Status,
-		ThresholdN: coordResp.TotalParties,    // Use TotalParties as N
-		ThresholdT: coordResp.CompletedParties, // Return completed count in ThresholdT for info
+		SessionId:    req.SessionId,
+		Status:       coordResp.Status,
+		ThresholdN:   coordResp.TotalParties,    // Use TotalParties as N
+		ThresholdT:   coordResp.CompletedParties, // Return completed count in ThresholdT for info
+		Participants: participants,               // Include participants for co_managed_keygen
 	}, nil
 }
 

backend/mpc-system/services/server-party/adapters/output/grpc/message_router_client.go

@@ -851,3 +851,45 @@ func (c *MessageRouterClient) SubmitDelegateShare(
 		return nil
 	})
 }
+
+// GetSessionStatusFull gets the full session status including participants via Message Router
+// This is used for co_managed_keygen sessions to wait for all parties to join
+// Includes automatic retry with exponential backoff for transient failures
+func (c *MessageRouterClient) GetSessionStatusFull(
+	ctx context.Context,
+	sessionID uuid.UUID,
+) (*use_cases.SessionStatusInfo, error) {
+	req := &router.GetSessionStatusRequest{
+		SessionId: sessionID.String(),
+	}
+
+	return retry.Do(ctx, c.retryCfg, "GetSessionStatusFull", func() (*use_cases.SessionStatusInfo, error) {
+		resp := &router.GetSessionStatusResponse{}
+		err := c.getConn().Invoke(ctx, "/mpc.router.v1.MessageRouter/GetSessionStatus", req, resp)
+		if err != nil {
+			return nil, err
+		}
+
+		// Convert participants from response
+		participants := make([]use_cases.ParticipantInfo, len(resp.Participants))
+		for i, p := range resp.Participants {
+			participants[i] = use_cases.ParticipantInfo{
+				PartyID:    p.PartyId,
+				PartyIndex: int(p.PartyIndex),
+			}
+		}
+
+		logger.Debug("GetSessionStatusFull response",
+			zap.String("session_id", sessionID.String()),
+			zap.String("status", resp.Status),
+			zap.Int32("threshold_n", resp.ThresholdN),
+			zap.Int("participants_count", len(participants)))
+
+		return &use_cases.SessionStatusInfo{
+			Status:       resp.Status,
+			ThresholdN:   int(resp.ThresholdN),
+			ThresholdT:   int(resp.ThresholdT),
+			Participants: participants,
+		}, nil
+	})
+}

backend/mpc-system/services/server-party/application/use_cases/participate_keygen.go

@@ -41,6 +41,15 @@ type ParticipateKeygenOutput struct {
 type SessionCoordinatorClient interface {
 	JoinSession(ctx context.Context, sessionID uuid.UUID, partyID, joinToken string) (*SessionInfo, error)
 	ReportCompletion(ctx context.Context, sessionID uuid.UUID, partyID string, publicKey []byte) error
+	GetSessionStatusFull(ctx context.Context, sessionID uuid.UUID) (*SessionStatusInfo, error)
+}
+
+// SessionStatusInfo contains full session status information
+type SessionStatusInfo struct {
+	Status       string
+	ThresholdN   int
+	ThresholdT   int
+	Participants []ParticipantInfo
 }
 
 // MessageRouterClient defines the interface for message router communication
@@ -115,6 +124,15 @@ func (uc *ParticipateKeygenUseCase) Execute(
 		return nil, ErrInvalidSession
 	}
 
+	// For co_managed_keygen: wait for all N participants to join before proceeding
+	// This is necessary because server parties join immediately but external party joins later
+	if sessionInfo.SessionType == "co_managed_keygen" {
+		sessionInfo, err = uc.waitForAllParticipants(ctx, input.SessionID, sessionInfo)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	// 2. Find self in participants and build party index map
 	var selfIndex int
 	partyIndexMap := make(map[string]int)
@@ -369,3 +387,77 @@ func (uc *ParticipateKeygenUseCase) getPartyRole() string {
 	}
 	return role
 }
+
+// waitForAllParticipants waits for all N participants to join the session
+// This is only used for co_managed_keygen sessions where server parties join first
+// and need to wait for the external party to join via invite code
+func (uc *ParticipateKeygenUseCase) waitForAllParticipants(
+	ctx context.Context,
+	sessionID uuid.UUID,
+	initialSessionInfo *SessionInfo,
+) (*SessionInfo, error) {
+	logger.Info("Waiting for all participants to join co_managed_keygen session",
+		zap.String("session_id", sessionID.String()),
+		zap.Int("expected_n", initialSessionInfo.ThresholdN),
+		zap.Int("current_participants", len(initialSessionInfo.Participants)))
+
+	// If already have all participants, return immediately
+	if len(initialSessionInfo.Participants) >= initialSessionInfo.ThresholdN {
+		logger.Info("All participants already joined",
+			zap.String("session_id", sessionID.String()))
+		return initialSessionInfo, nil
+	}
+
+	// Poll for session status until all participants join or timeout
+	pollInterval := 2 * time.Second
+	maxWaitTime := 5 * time.Minute
+	deadline := time.Now().Add(maxWaitTime)
+
+	for time.Now().Before(deadline) {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case <-time.After(pollInterval):
+			// Get full session status including participants
+			statusInfo, err := uc.sessionClient.GetSessionStatusFull(ctx, sessionID)
+			if err != nil {
+				logger.Warn("Failed to get session status, will retry",
+					zap.String("session_id", sessionID.String()),
+					zap.Error(err))
+				continue
+			}
+
+			logger.Debug("Polled session status",
+				zap.String("session_id", sessionID.String()),
+				zap.String("status", statusInfo.Status),
+				zap.Int("participants", len(statusInfo.Participants)),
+				zap.Int("expected_n", initialSessionInfo.ThresholdN))
+
+			// Check if session is in_progress (all parties joined and ready)
+			if statusInfo.Status == "in_progress" && len(statusInfo.Participants) >= initialSessionInfo.ThresholdN {
+				logger.Info("All participants joined, session is in_progress",
+					zap.String("session_id", sessionID.String()),
+					zap.Int("participants", len(statusInfo.Participants)))
+
+				// Update session info with full participants list
+				initialSessionInfo.Participants = statusInfo.Participants
+				return initialSessionInfo, nil
+			}
+
+			// Also accept if we have all N participants even if status hasn't changed
+			if len(statusInfo.Participants) >= initialSessionInfo.ThresholdN {
+				logger.Info("All participants joined",
+					zap.String("session_id", sessionID.String()),
+					zap.Int("participants", len(statusInfo.Participants)))
+
+				initialSessionInfo.Participants = statusInfo.Participants
+				return initialSessionInfo, nil
+			}
+		}
+	}
+
+	logger.Error("Timeout waiting for all participants",
+		zap.String("session_id", sessionID.String()),
+		zap.Int("expected_n", initialSessionInfo.ThresholdN))
+	return nil, ErrKeygenTimeout
+}