refactor: split large functions for maintainability

Split functions exceeding 50 lines into smaller, focused helpers:

- DirectTurboWriter.WriteLineData (~97 lines) split into:
  - WriteLineData (dispatcher, 9 lines)
  - writeServerlessLine (serverless mode, 48 lines)
  - writeNetworkLine (network mode, 40 lines)

- TurboNetworkWriter.WriteLineData (~60 lines) split into:
  - WriteLineData (builds protocol line, 33 lines)
  - sendToTurboChannel (channel send with retry, 28 lines)

- Server.handleRequests (~67 lines) split into:
  - handleRequests (request loop, 23 lines)
  - handleShellRequest (shell session setup, 57 lines)

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

1 files changed, 61 insertions, 46 deletions

diff --git a/internal/server/server.go b/internal/server/server.go
index 8b581b1..38b042f 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -170,52 +170,7 @@ func (s *Server) handleRequests(ctx context.Context, sshConn gossh.Conn,
 
 		switch req.Type {
 		case "shell":
-			var handler handlers.Handler
-			switch user.Name {
-			case config.HealthUser:
-				handler = handlers.NewHealthHandler(user)
-			default:
-				handler = handlers.NewServerHandler(user, s.catLimiter, s.tailLimiter)
-			}
-			terminate := func() {
-				handler.Shutdown()
-				sshConn.Close()
-			}
-
-			go func() {
-				defer terminate()
-				// Broken pipe, cancel
-				if _, err := io.Copy(channel, handler); err != nil {
-					dlog.Server.Trace(user, fmt.Errorf("channel->handler: %w", err))
-				}
-			}()
-			go func() {
-				defer terminate()
-				// Broken pipe, cancel
-				if _, err := io.Copy(handler, channel); err != nil {
-					dlog.Server.Trace(user, fmt.Errorf("handler->channel: %w", err))
-				}
-			}()
-			go func() {
-				select {
-				case <-ctx.Done():
-				case <-handler.Done():
-				}
-				terminate()
-			}()
-			go func() {
-				if err := sshConn.Wait(); err != nil && err != io.EOF {
-					dlog.Server.Error(user, err)
-				}
-				s.stats.decrementConnections()
-				dlog.Server.Info(user, "Good bye Mister!")
-				terminate()
-			}()
-
-			// Only serving shell type
-			if err := req.Reply(true, nil); err != nil {
-				dlog.Server.Trace(user, fmt.Errorf("reply(true): %w", err))
-			}
+			s.handleShellRequest(ctx, sshConn, channel, user, req)
 		default:
 			if err := req.Reply(false, nil); err != nil {
 				dlog.Server.Trace(user, fmt.Errorf("reply(false): %w", err))
@@ -227,6 +182,66 @@ func (s *Server) handleRequests(ctx context.Context, sshConn gossh.Conn,
 	return nil
 }
 
+// handleShellRequest sets up the shell session with handler goroutines for I/O,
+// context cancellation, and connection lifecycle management.
+func (s *Server) handleShellRequest(ctx context.Context, sshConn gossh.Conn,
+	channel gossh.Channel, user *user.User, req *gossh.Request) {
+
+	// Create the appropriate handler based on user type
+	var handler handlers.Handler
+	switch user.Name {
+	case config.HealthUser:
+		handler = handlers.NewHealthHandler(user)
+	default:
+		handler = handlers.NewServerHandler(user, s.catLimiter, s.tailLimiter)
+	}
+
+	terminate := func() {
+		handler.Shutdown()
+		sshConn.Close()
+	}
+
+	// Start goroutine to copy data from channel to handler
+	go func() {
+		defer terminate()
+		if _, err := io.Copy(channel, handler); err != nil {
+			dlog.Server.Trace(user, fmt.Errorf("channel->handler: %w", err))
+		}
+	}()
+
+	// Start goroutine to copy data from handler to channel
+	go func() {
+		defer terminate()
+		if _, err := io.Copy(handler, channel); err != nil {
+			dlog.Server.Trace(user, fmt.Errorf("handler->channel: %w", err))
+		}
+	}()
+
+	// Start goroutine to handle context or handler completion
+	go func() {
+		select {
+		case <-ctx.Done():
+		case <-handler.Done():
+		}
+		terminate()
+	}()
+
+	// Start goroutine to handle connection lifecycle and cleanup
+	go func() {
+		if err := sshConn.Wait(); err != nil && err != io.EOF {
+			dlog.Server.Error(user, err)
+		}
+		s.stats.decrementConnections()
+		dlog.Server.Info(user, "Good bye Mister!")
+		terminate()
+	}()
+
+	// Reply to indicate shell request was accepted
+	if err := req.Reply(true, nil); err != nil {
+		dlog.Server.Trace(user, fmt.Errorf("reply(true): %w", err))
+	}
+}
+
 // Callback for SSH authentication.
 func (s *Server) Callback(c gossh.ConnMetadata,
 	authPayload []byte) (*gossh.Permissions, error) {