ethclient: add HTTP fallback for subscriptions

ethclient/ethclient.go

@@ -329,9 +329,9 @@ func (ec *Client) SyncProgress(ctx context.Context) (*ethereum.SyncProgress, err
 func (ec *Client) SubscribeNewHead(ctx context.Context, ch chan<- *types.Header) (ethereum.Subscription, error) {
 	sub, err := ec.c.EthSubscribe(ctx, ch, "newHeads")
 	if err != nil {
-		// Defensively prefer returning nil interface explicitly on error-path, instead
+		if errors.Is(err, rpc.ErrNotificationsUnsupported) {
-		// of letting default golang behavior wrap it with non-nil interface that stores
+			return ec.subscribeNewHeadPolling(ctx, ch)
-		// nil concrete type value.
+		}
 		return nil, err
 	}
 	return sub, nil
@@ -405,9 +405,9 @@ func (ec *Client) SubscribeFilterLogs(ctx context.Context, q ethereum.FilterQuer
 	}
 	sub, err := ec.c.EthSubscribe(ctx, ch, "logs", arg)
 	if err != nil {
-		// Defensively prefer returning nil interface explicitly on error-path, instead
+		if errors.Is(err, rpc.ErrNotificationsUnsupported) {
-		// of letting default golang behavior wrap it with non-nil interface that stores
+			return ec.subscribeFilterLogsPolling(ctx, q, ch)
-		// nil concrete type value.
+		}
 		return nil, err
 	}
 	return sub, nil

ethclient/httpsub.go

@@ -0,0 +1,150 @@
+// Copyright 2023 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+package ethclient
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/event"
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+// subscribeNewHeadPolling creates a block filter and polls it.
+// This is the fallback for SubscribeNewHead on HTTP connections.
+func (ec *Client) subscribeNewHeadPolling(ctx context.Context, ch chan<- *types.Header) (event.Subscription, error) {
+	var id string
+	err := ec.c.CallContext(ctx, &id, "eth_newBlockFilter")
+	if err != nil {
+		return nil, err
+	}
+	sub := newFilterSub(ec.c, id, ch)
+	return sub, nil
+}
+
+// subscribeFilterLogs subscribes to a log filter.
+// This is the fallback for SubscribeFilterLogs on HTTP connections.
+func (ec *Client) subscribeFilterLogsPolling(ctx context.Context, q ethereum.FilterQuery, ch chan<- types.Log) (ethereum.Subscription, error) {
+	arg, err := toFilterArg(q)
+	if err != nil {
+		return nil, err
+	}
+	var id string
+	err = ec.c.CallContext(ctx, &id, "eth_newFilter", arg)
+	if err != nil {
+		return nil, err
+	}
+	sub := newFilterSub(ec.c, id, ch)
+	return sub, nil
+}
+
+// filterSub implements event.Subscription with the polling filter API.
+type filterSub[Result any] struct {
+	// The channel to send the results to
+	resultc   chan<- Result
+	errc      chan error
+	closed    chan struct{}
+	unsubOnce sync.Once
+
+	id     string
+	client *rpc.Client
+}
+
+func newFilterSub[Result any](client *rpc.Client, id string, resultc chan<- Result) *filterSub[Result] {
+	sub := &filterSub[Result]{
+		resultc: resultc,
+		errc:    make(chan error, 1),
+		closed:  make(chan struct{}),
+		client:  client,
+	}
+	go sub.poll()
+	return sub
+}
+
+// Unsubscribe cancels the event subscription.
+func (s *filterSub[Result]) Unsubscribe() {
+	s.closeWithError(nil)
+}
+
+// Err returns the subscription error channel.
+func (s *filterSub[Result]) Err() <-chan error {
+	return s.errc
+}
+
+func (s *filterSub[Result]) poll() {
+	var timer = time.NewTicker(10 * time.Second)
+	for {
+		err := s.getChanges()
+		if err != nil {
+			s.closeWithError(err)
+			return
+		}
+
+		// Wait for next time.
+		select {
+		case <-timer.C:
+		case <-s.closed:
+			return
+		}
+	}
+}
+
+func (s *filterSub[Result]) closeWithError(err error) {
+	s.unsubOnce.Do(func() {
+		close(s.closed)
+		unsubErr := s.uninstallFilter()
+		if unsubErr != nil && err == nil {
+			err = unsubErr
+		}
+		if err != nil {
+			select {
+			case s.errc <- err:
+			default:
+			}
+		}
+		close(s.errc)
+	})
+}
+
+// getChanges calls eth_getFilterChanges and delivers the results.
+func (s *filterSub[Result]) getChanges() error {
+	ctx, done := context.WithTimeout(context.Background(), 10*time.Second)
+	defer done()
+
+	var results []Result
+	err := s.client.CallContext(ctx, &results, "eth_getFilterChanges", s.id)
+	if err != nil {
+		return err
+	}
+	for _, result := range results {
+		select {
+		case s.resultc <- result:
+		case <-s.closed:
+			return nil
+		}
+	}
+	return nil
+}
+
+// uninstallFilter removes the filter on the server side.
+func (s *filterSub[Result]) uninstallFilter() error {
+	ctx, done := context.WithTimeout(context.Background(), 10*time.Second)
+	defer done()
+	return s.client.CallContext(ctx, nil, "eth_uninstallFilter", s.id)
+}