[client] Refactor free port function (#2455)

Rely on net.ListenUDP to get an available port for wireguard in case the configured one is in use

---------

Co-authored-by: Viktor Liu <17948409+lixmal@users.noreply.github.com>

client/internal/connect_test.go

@@ -7,51 +7,55 @@ import (
 
 func Test_freePort(t *testing.T) {
 	tests := []struct {
-		name    string
-		port    int
-		want    int
-		wantErr bool
+		name        string
+		port        int
+		want        int
+		shouldMatch bool
 	}{
 		{
-			name:    "available",
-			port:    51820,
-			want:    51820,
-			wantErr: false,
+			name:        "not provided, fallback to default",
+			port:        0,
+			want:        51820,
+			shouldMatch: true,
 		},
 		{
-			name:    "notavailable",
-			port:    51830,
-			want:    51831,
-			wantErr: false,
+			name:        "provided and available",
+			port:        51821,
+			want:        51821,
+			shouldMatch: true,
 		},
 		{
-			name:    "noports",
-			port:    65535,
-			want:    0,
-			wantErr: true,
+			name:        "provided and not available",
+			port:        51830,
+			want:        51830,
+			shouldMatch: false,
 		},
 	}
+	c1, err := net.ListenUDP("udp", &net.UDPAddr{Port: 51830})
+	if err != nil {
+		t.Errorf("freePort error = %v", err)
+	}
+	defer func(c1 *net.UDPConn) {
+		_ = c1.Close()
+	}(c1)
+
 	for _, tt := range tests {
 
-		c1, err := net.ListenUDP("udp", &net.UDPAddr{Port: 51830})
-		if err != nil {
-			t.Errorf("freePort error = %v", err)
-		}
-		c2, err := net.ListenUDP("udp", &net.UDPAddr{Port: 65535})
-		if err != nil {
-			t.Errorf("freePort error = %v", err)
-		}
 		t.Run(tt.name, func(t *testing.T) {
 			got, err := freePort(tt.port)
-			if (err != nil) != tt.wantErr {
-				t.Errorf("freePort() error = %v, wantErr %v", err, tt.wantErr)
-				return
+
+			if err != nil {
+				t.Errorf("got an error while getting free port: %v", err)
 			}
-			if got != tt.want {
-				t.Errorf("freePort() = %v, want %v", got, tt.want)
+
+			if tt.shouldMatch && got != tt.want {
+				t.Errorf("got a different port %v, want %v", got, tt.want)
+			}
+
+			if !tt.shouldMatch && got == tt.want {
+				t.Errorf("got the same port %v, want a different port", tt.want)
 			}
 		})
-		c1.Close()
-		c2.Close()
+
 	}
 }