fix(session): resolve race conditions (#151)

## Summary

Comprehensive fixes for race conditions, concurrency bugs, and a critical session handling bug across the session package and its child packages.

## Changes

### Bug Fix: waitForTCPIPForward fails on non-tcpip-forward global requests

- Changed `waitForTCPIPForward()` from a single select to a for loop that drains non-tcpip-forward requests gracefully
- Non-tcpip-forward requests now receive a false reply and the function continues waiting for the actual forward request
- The 500ms timeout resets after each request to handle slow clients with multiple early requests
- Updated and added tests to cover the new behavior: Wrong_Request_Type_Then_Timeout, Multiple_Non-Forward_Requests_Then_Success, Timeout_After_Non-Forward_Requests

### Race Condition Fixes

- Added sync.RWMutex to the forwarder struct to protect concurrent field access
- Fixed data race in forwarder setters (`SetType()`, `SetForwardedPort()`, `SetListener()`) by adding `Lock()`/`Unlock()` protection
- Fixed data race in forwarder getters (`TunnelType()`, `ForwardedPort()`, `Listener()`) by adding `RLock()`/`RUnlock()` protection
- Added sync.RWMutex to the slug struct to protect concurrent field access
- Fixed data race in `slug.Set()` by adding `Lock()`/`Unlock()` protection
- Fixed data race in `slug.String()` by adding `RLock()`/`RUnlock()` protection

### Concurrency Safety Improvements

- Changed `OpenForwardedChannel()` to use `f.ForwardedPort()` getter instead of direct field access
- Fixed `Close()` in forwarder to use `f.Listener()` getter consistently instead of accessing `f.listener` directly
- Fixed data race in `HandleTCPForward()` where the goroutine running `tcpServer.Serve(listener)` wrote to the outer err variable from the enclosing function scope
- Changed err = `tcpServer.Serve(listener)` to `if err := tcpServer.Serve(listener)` so the goroutine uses a local variable

Reviewed-on: #151
Co-authored-by: Bagas <bagas@fossy.my.id>
Co-committed-by: Bagas <bagas@fossy.my.id>

internal/session/forwarder/forwarder.go

@@ -28,6 +28,7 @@ type Forwarder interface {
 	Close() error
 }
 type forwarder struct {
+	mu            sync.RWMutex
 	listener      net.Listener
 	tunnelType    types.TunnelType
 	forwardedPort uint16
@@ -60,7 +61,7 @@ func (f *forwarder) copyWithBuffer(dst io.Writer, src io.Reader) (written int64,
 }
 
 func (f *forwarder) OpenForwardedChannel(ctx context.Context, origin net.Addr) (ssh.Channel, <-chan *ssh.Request, error) {
-	payload := createForwardedTCPIPPayload(origin, f.forwardedPort)
+	payload := createForwardedTCPIPPayload(origin, f.ForwardedPort())
 	type channelResult struct {
 		channel ssh.Channel
 		reqs    <-chan *ssh.Request
@@ -141,32 +142,44 @@ func (f *forwarder) HandleConnection(dst io.ReadWriter, src ssh.Channel) {
 }
 
 func (f *forwarder) SetType(tunnelType types.TunnelType) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.tunnelType = tunnelType
 }
 
 func (f *forwarder) TunnelType() types.TunnelType {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
 	return f.tunnelType
 }
 
 func (f *forwarder) ForwardedPort() uint16 {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
 	return f.forwardedPort
 }
 
 func (f *forwarder) SetForwardedPort(port uint16) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.forwardedPort = port
 }
 
 func (f *forwarder) SetListener(listener net.Listener) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	f.listener = listener
 }
 
 func (f *forwarder) Listener() net.Listener {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
 	return f.listener
 }
 
 func (f *forwarder) Close() error {
-	if f.Listener() != nil {
-		return f.listener.Close()
+	if listener := f.Listener(); listener != nil {
+		return listener.Close()
 	}
 	return nil
 }