ethclient: ensure returned subscription is nil on error (#26976)

Author: norwnd

ethclient/ethclient.go

@@ -320,7 +320,14 @@ func (ec *Client) SyncProgress(ctx context.Context) (*ethereum.SyncProgress, err
 // SubscribeNewHead subscribes to notifications about the current blockchain head
 // on the given channel.
 func (ec *Client) SubscribeNewHead(ctx context.Context, ch chan<- *types.Header) (ethereum.Subscription, error) {
-	return ec.c.EthSubscribe(ctx, ch, "newHeads")
+	sub, err := ec.c.EthSubscribe(ctx, ch, "newHeads")
+	if err != nil {
+		// Defensively prefer returning nil interface explicitly on error-path, instead
+		// of letting default golang behavior wrap it with non-nil interface that stores
+		// nil concrete type value.
+		return nil, err
+	}
+	return sub, nil
 }
 
 // State Access
@@ -389,7 +396,14 @@ func (ec *Client) SubscribeFilterLogs(ctx context.Context, q ethereum.FilterQuer
 	if err != nil {
 		return nil, err
 	}
-	return ec.c.EthSubscribe(ctx, ch, "logs", arg)
+	sub, err := ec.c.EthSubscribe(ctx, ch, "logs", arg)
+	if err != nil {
+		// Defensively prefer returning nil interface explicitly on error-path, instead
+		// of letting default golang behavior wrap it with non-nil interface that stores
+		// nil concrete type value.
+		return nil, err
+	}
+	return sub, nil
 }
 
 func toFilterArg(q ethereum.FilterQuery) (interface{}, error) {