smb: test closing failed dial connections

Followup to:

74f9f182aa smb: fix TCP connection leak when connection setup fails - #9678

backend/smb/smb_internal_test.go

@@ -1,7 +1,50 @@
 // Unit tests for internal SMB functions
 package smb
 
-import "testing"
+import (
+	"context"
+	"io"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDialClosesConnectionOnSetupError(t *testing.T) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	defer func() { require.NoError(t, listener.Close()) }()
+
+	type acceptResult struct {
+		conn net.Conn
+		err  error
+	}
+	accepted := make(chan acceptResult, 1)
+	go func() {
+		conn, err := listener.Accept()
+		accepted <- acceptResult{conn: conn, err: err}
+	}()
+
+	f := &Fs{opt: Options{Pass: "invalid"}}
+	_, err = f.dial(context.Background(), "tcp", listener.Addr().String())
+	require.Error(t, err)
+
+	var result acceptResult
+	select {
+	case result = <-accepted:
+	case <-time.After(time.Second):
+		t.Fatal("timed out waiting for server to accept connection")
+	}
+	require.NoError(t, result.err)
+	defer func() { require.NoError(t, result.conn.Close()) }()
+	require.NoError(t, result.conn.SetReadDeadline(time.Now().Add(time.Second)))
+
+	buffer := make([]byte, 1)
+	n, err := result.conn.Read(buffer)
+	require.Zero(t, n)
+	require.ErrorIs(t, err, io.EOF)
+}
 
 // TestIsPathDir tests the isPathDir function logic
 func TestIsPathDir(t *testing.T) {