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/session.go

@@ -158,7 +158,7 @@ func (s *session) setupInteractiveMode(channel ssh.NewChannel) error {
 	}
 
 	go func() {
-		err = s.HandleGlobalRequest(reqs)
+		err := s.HandleGlobalRequest(reqs)
 		if err != nil {
 			log.Printf("global request handler error: %v", err)
 		}
@@ -200,23 +200,22 @@ func (s *session) waitForSessionEnd() error {
 }
 
 func (s *session) waitForTCPIPForward() *ssh.Request {
-	select {
-	case req, ok := <-s.initialReq:
-		if !ok {
-			log.Println("Forwarding request channel closed")
+	for {
+		select {
+		case req, ok := <-s.initialReq:
+			if !ok {
+				log.Println("Forwarding request channel closed")
+				return nil
+			}
+			if req.Type == "tcpip-forward" {
+				return req
+			}
+			log.Printf("Ignoring unexpected global request: %s", req.Type)
+			_ = req.Reply(false, nil)
+		case <-time.After(500 * time.Millisecond):
+			log.Println("No tcpip-forward request received within timeout")
 			return nil
 		}
-		if req.Type == "tcpip-forward" {
-			return req
-		}
-		if err := req.Reply(false, nil); err != nil {
-			log.Printf("Failed to reply to request: %v", err)
-		}
-		log.Printf("Expected tcpip-forward request, got: %s", req.Type)
-		return nil
-	case <-time.After(500 * time.Millisecond):
-		log.Println("No forwarding request received")
-		return nil
 	}
 }
 
@@ -379,8 +378,7 @@ func (s *session) HandleTCPForward(req *ssh.Request, addr string, portToBind uin
 	}
 
 	go func() {
-		err = tcpServer.Serve(listener)
-		if err != nil {
+		if err := tcpServer.Serve(listener); err != nil {
 			log.Printf("Failed serving tcp server: %s\n", err)
 		}
 	}()