refactor: extract internal/session and eliminate I/O globals

Move serial port, trzsz filter, and pipe lifecycle into
internal/session.SerialSession. Replace 8 global I/O vars
(serialPort, trzszFilter, stdinPipe, stdoutPipe, clientIn,
clientOut, termch, termchOnce) with single sess variable.
Delete utils.go.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/session/session.go

@@ -0,0 +1,138 @@
+// Package session manages the serial port connection and its associated pipes.
+package session
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"runtime"
+	"sync"
+
+	"github.com/trzsz/trzsz-go/trzsz"
+	"go.bug.st/serial"
+	"golang.org/x/term"
+
+	"github.com/jixishi/SerialTerminalForWindowsTerminal/internal/config"
+)
+
+// SerialSession owns the serial port, trzsz filter, and pipe pair.
+type SerialSession struct {
+	Port        serial.Port
+	TrzszFilter *trzsz.TrzszFilter
+	StdinPipe   *io.PipeWriter
+	StdoutPipe  *io.PipeReader
+	ClientIn    *io.PipeReader
+	ClientOut   *io.PipeWriter
+
+	termCh    chan os.Signal
+	closeOnce sync.Once
+}
+
+// Open creates a SerialSession by opening the serial port and initializing trzsz.
+func Open(cfg *config.Config) (*SerialSession, error) {
+	mode := &serial.Mode{
+		BaudRate: cfg.BaudRate,
+		StopBits: serial.StopBits(cfg.StopBits),
+		DataBits: cfg.DataBits,
+		Parity:   serial.Parity(cfg.ParityBit),
+	}
+	port, err := serial.Open(cfg.PortName, mode)
+	if err != nil {
+		return nil, err
+	}
+
+	fd := int(os.Stdin.Fd())
+	width, _, err := term.GetSize(fd)
+	if err != nil {
+		if runtime.GOOS != "windows" {
+			port.Close()
+			return nil, fmt.Errorf("term get size failed: %w", err)
+		}
+		width = 80
+	}
+
+	clientIn, stdinPipe := io.Pipe()
+	stdoutPipe, clientOut := io.Pipe()
+	trzszFilter := trzsz.NewTrzszFilter(clientIn, clientOut, port, port,
+		trzsz.TrzszOptions{TerminalColumns: int32(width), EnableZmodem: true})
+	trzsz.SetAffectedByWindows(false)
+
+	s := &SerialSession{
+		Port:        port,
+		TrzszFilter: trzszFilter,
+		StdinPipe:   stdinPipe,
+		StdoutPipe:  stdoutPipe,
+		ClientIn:    clientIn,
+		ClientOut:   clientOut,
+		termCh:      make(chan os.Signal, 1),
+	}
+
+	go func() {
+		for range s.termCh {
+			w, _, err := term.GetSize(fd)
+			if err != nil {
+				fmt.Printf("term get size failed: %s\n", err)
+				continue
+			}
+			trzszFilter.SetTerminalColumns(int32(w))
+		}
+	}()
+
+	return s, nil
+}
+
+// Write writes data to the stdin pipe (toward serial port, through trzsz).
+func (s *SerialSession) Write(data []byte) (int, error) {
+	return s.StdinPipe.Write(data)
+}
+
+// Read reads data from the stdout pipe (from serial port, through trzsz).
+func (s *SerialSession) Read(buf []byte) (int, error) {
+	return s.StdoutPipe.Read(buf)
+}
+
+// SendCtrl sends a control character directly to the serial port (bypasses trzsz).
+func (s *SerialSession) SendCtrl(letter byte) (int, error) {
+	if letter >= 'A' && letter <= 'Z' {
+		letter = letter + ('a' - 'A')
+	}
+	control := []byte{letter & 0x1f}
+	return s.Port.Write(control)
+}
+
+// Close tears down the session: stops term signals, closes trzsz, then serial port.
+func (s *SerialSession) Close() {
+	s.closeOnce.Do(func() {
+		if s.termCh != nil {
+			signal.Stop(s.termCh)
+			close(s.termCh)
+		}
+		if s.Port != nil {
+			if err := s.Port.Close(); err != nil {
+				fmt.Fprint(os.Stderr, err)
+				fmt.Fprint(os.Stderr, "\n")
+			}
+		}
+	})
+}
+
+// CheckPortAvailability returns the list of available ports and verifies the named port exists.
+func CheckPortAvailability(name string) ([]string, error) {
+	ports, err := serial.GetPortsList()
+	if err != nil {
+		return nil, err
+	}
+	if len(ports) == 0 {
+		return nil, fmt.Errorf("no serial ports found")
+	}
+	if name == "" {
+		return ports, fmt.Errorf("port name not specified")
+	}
+	for _, port := range ports {
+		if port == name {
+			return ports, nil
+		}
+	}
+	return ports, fmt.Errorf("port " + name + " is not available")
+}