fix(nodebuilder/header): Cancel subscription when done (#2134)

Found while debugging swamp.

We never cancel subscription so when node attempts to shut down, there
are outstanding subs that need to be closed.

---------

Co-authored-by: Hlib Kanunnikov <hlibwondertan@gmail.com>

nodebuilder/header/service.go

@@ -79,6 +79,8 @@ func (s *Service) Subscribe(ctx context.Context) (<-chan *header.ExtendedHeader,
 	headerCh := make(chan *header.ExtendedHeader)
 	go func() {
 		defer close(headerCh)
+		defer subscription.Cancel()
+
 		for {
 			h, err := subscription.NextHeader(ctx)
 			if err != nil {

nodebuilder/tests/api_test.go

@@ -0,0 +1,57 @@
+package tests
+
+import (
+	"context"
+	"testing"
+
+	"github.com/libp2p/go-libp2p/core/host"
+	"github.com/libp2p/go-libp2p/core/peer"
+	"github.com/stretchr/testify/require"
+
+	"github.com/celestiaorg/celestia-node/nodebuilder"
+	"github.com/celestiaorg/celestia-node/nodebuilder/node"
+	"github.com/celestiaorg/celestia-node/nodebuilder/tests/swamp"
+)
+
+// TestHeaderSubscription ensures that the header subscription over RPC works
+// as intended and gets canceled successfully after rpc context cancellation.
+func TestHeaderSubscription(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), swamp.DefaultTestTimeout)
+	t.Cleanup(cancel)
+
+	sw := swamp.NewSwamp(t, swamp.WithBlockTime(btime))
+
+	// start a bridge node
+	bridge := sw.NewBridgeNode()
+	err := bridge.Start(ctx)
+	require.NoError(t, err)
+
+	cfg := nodebuilder.DefaultConfig(node.Light)
+	addrs, err := peer.AddrInfoToP2pAddrs(host.InfoFromHost(bridge.Host))
+	require.NoError(t, err)
+	cfg.Header.TrustedPeers = append(cfg.Header.TrustedPeers, addrs[0].String())
+
+	// start a light node that's connected to the bridge node
+	light := sw.NewNodeWithConfig(node.Light, cfg)
+	err = light.Start(ctx)
+	require.NoError(t, err)
+
+	// subscribe to headers via the light node's RPC header subscription
+	subctx, subcancel := context.WithCancel(ctx)
+	sub, err := light.HeaderServ.Subscribe(subctx)
+	require.NoError(t, err)
+	// listen for 5 headers
+	for i := 0; i < 5; i++ {
+		select {
+		case <-ctx.Done():
+			t.Fatal(ctx.Err())
+		case <-sub:
+		}
+	}
+	// cancel subscription via context
+	subcancel()
+
+	// stop the light node and expect no outstanding subscription errors
+	err = light.Stop(ctx)
+	require.NoError(t, err)
+}