rpc: add method to test for subscription support (ethereum#25942)

This adds two ways to check for subscription support. First, one can now check
whether the transport method (HTTP/WS/etc.) is capable of subscriptions using
the new Client.SupportsSubscriptions method.

Second, the error returned by Subscribe can now reliably be tested using this
pattern:
    
    sub, err := client.Subscribe(...)
    if errors.Is(err, rpc.ErrNotificationsUnsupported) {
        // no subscription support
    }

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/client.go

@@ -538,6 +538,13 @@ func (c *Client) Subscribe(ctx context.Context, namespace string, channel interf
 	return op.sub, nil
 }
 
+// SupportsSubscriptions reports whether subscriptions are supported by the client
+// transport. When this returns false, Subscribe and related methods will return
+// ErrNotificationsUnsupported.
+func (c *Client) SupportsSubscriptions() bool {
+	return !c.isHTTP
+}
+
 func (c *Client) newMessage(method string, paramsIn ...interface{}) (*jsonrpcMessage, error) {
 	msg := &jsonrpcMessage{Version: vsn, ID: c.nextID(), Method: method}
 	if paramsIn != nil { // prevent sending "params":null

rpc/errors.go

@@ -58,12 +58,13 @@ var (
 )
 
 const (
-	errcodeDefault                  = -32000
-	errcodeNotificationsUnsupported = -32001
-	errcodeTimeout                  = -32002
-	errcodeResponseTooLarge         = -32003
-	errcodePanic                    = -32603
-	errcodeMarshalError             = -32603
+	errcodeDefault          = -32000
+	errcodeTimeout          = -32002
+	errcodeResponseTooLarge = -32003
+	errcodePanic            = -32603
+	errcodeMarshalError     = -32603
+
+	legacyErrcodeNotificationsUnsupported = -32001
 )
 
 const (
@@ -80,6 +81,34 @@ func (e *methodNotFoundError) Error() string {
 	return fmt.Sprintf("the method %s does not exist/is not available", e.method)
 }
 
+type notificationsUnsupportedError struct{}
+
+func (e notificationsUnsupportedError) Error() string {
+	return "notifications not supported"
+}
+
+func (e notificationsUnsupportedError) ErrorCode() int { return -32601 }
+
+// Is checks for equivalence to another error. Here we define that all errors with code
+// -32601 (method not found) are equivalent to notificationsUnsupportedError. This is
+// done to enable the following pattern:
+//
+//	sub, err := client.Subscribe(...)
+//	if errors.Is(err, rpc.ErrNotificationsUnsupported) {
+//		// server doesn't support subscriptions
+//	}
+func (e notificationsUnsupportedError) Is(other error) bool {
+	if other == (notificationsUnsupportedError{}) {
+		return true
+	}
+	rpcErr, ok := other.(Error)
+	if ok {
+		code := rpcErr.ErrorCode()
+		return code == -32601 || code == legacyErrcodeNotificationsUnsupported
+	}
+	return false
+}
+
 type subscriptionNotFoundError struct{ namespace, subscription string }
 
 func (e *subscriptionNotFoundError) ErrorCode() int { return -32601 }

rpc/handler.go

@@ -530,10 +530,7 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 // handleSubscribe processes *_subscribe method calls.
 func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage {
 	if !h.allowSubscribe {
-		return msg.errorResponse(&internalServerError{
-			code:    errcodeNotificationsUnsupported,
-			message: ErrNotificationsUnsupported.Error(),
-		})
+		return msg.errorResponse(ErrNotificationsUnsupported)
 	}
 
 	// Subscription method name is first argument.

rpc/subscription.go

@@ -32,8 +32,17 @@ import (
 )
 
 var (
-	// ErrNotificationsUnsupported is returned when the connection doesn't support notifications
-	ErrNotificationsUnsupported = errors.New("notifications not supported")
+	// ErrNotificationsUnsupported is returned by the client when the connection doesn't
+	// support notifications. You can use this error value to check for subscription
+	// support like this:
+	//
+	//	sub, err := client.EthSubscribe(ctx, channel, "newHeads", true)
+	//	if errors.Is(err, rpc.ErrNotificationsUnsupported) {
+	//		// Server does not support subscriptions, fall back to polling.
+	//	}
+	//
+	ErrNotificationsUnsupported = notificationsUnsupportedError{}
+
 	// ErrSubscriptionNotFound is returned when the notification for the given id is not found
 	ErrSubscriptionNotFound = errors.New("subscription not found")
 )