Add tests for ConnPool close and concurrency issues

This commit introduces two new tests: `TestConnPool_Close` and `TestConnPool_Concurrency`. The former ensures the proper closing of connection pool resources, while the latter checks for concurrency issues by creating and closing multiple connections in parallel.

connpool_test.go

@@ -255,6 +255,106 @@ func TestPoolConn_MarkUnusable(t *testing.T) {
 	}
 }
 
+func TestConnPool_Close(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 15
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	pool, err := newConnPool(serverPort)
+	if err != nil {
+		t.Errorf("failed to create connection pool: %s", err)
+	}
+	pool.Close()
+
+	castPool := pool.(*connPool)
+
+	if castPool.conns != nil {
+		t.Error("closing pool failed: conns channel should be nil")
+	}
+	if castPool.dialCtxFunc != nil {
+		t.Error("closing pool failed: dialCtxFunc should be nil")
+	}
+	if castPool.dialContext != nil {
+		t.Error("closing pool failed: dialContext should be nil")
+	}
+	if castPool.dialAddress != "" {
+		t.Error("closing pool failed: dialAddress should be empty")
+	}
+	if castPool.dialNetwork != "" {
+		t.Error("closing pool failed: dialNetwork should be empty")
+	}
+
+	conn, err := pool.Get()
+	if err == nil {
+		t.Errorf("closing pool failed: getting new connection should return an error")
+	}
+	if conn != nil {
+		t.Errorf("closing pool failed: getting new connection should return a nil-connection")
+	}
+	if pool.Size() != 0 {
+		t.Errorf("closing pool failed: pool size should be 0, but got: %d", pool.Size())
+	}
+}
+
+func TestConnPool_Concurrency(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 16
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	pool, err := newConnPool(serverPort)
+	if err != nil {
+		t.Errorf("failed to create connection pool: %s", err)
+	}
+	defer pool.Close()
+	pipe := make(chan net.Conn)
+
+	getWg := sync.WaitGroup{}
+	closeWg := sync.WaitGroup{}
+	for i := 0; i < 30; i++ {
+		getWg.Add(1)
+		closeWg.Add(1)
+		go func() {
+			conn, err := pool.Get()
+			if err != nil {
+				t.Errorf("failed to get new connection from pool: %s", err)
+			}
+			pipe <- conn
+			getWg.Done()
+		}()
+
+		go func() {
+			conn := <-pipe
+			if conn == nil {
+				return
+			}
+			if err = conn.Close(); err != nil {
+				t.Errorf("failed to close connection: %s", err)
+			}
+			closeWg.Done()
+		}()
+		getWg.Wait()
+		closeWg.Wait()
+	}
+}
+
 func newConnPool(port int) (Pool, error) {
 	netDialer := net.Dialer{}
 	return NewConnPool(context.Background(), 5, 30, netDialer.DialContext, "tcp",