rpcserver: Improve JSON-RPC compatibility

Avoid compatibility issues with software that relies on the behavior of
bitcoind's JSON-RPC implementation.

The JSON-RPC 1.0 spec defines that notifications must have their "id"
set to null and states that notifications do not have a response.

A JSON-RPC 2.0 notification is a request with "json-rpc":"2.0", and
without an "id" member. The specification states that notifications
must not be responded to. JSON-RPC 2.0 permits the null value as a
valid request id, therefore such requests are not notifications.

Bitcoin Core serves requests with "id":null or even an absent "id", and
responds to such requests with "id":null in the response.

Btcd does not respond to any request without and "id" or with "id":null,
regardless the indicated JSON-RPC protocol version.

In order to avoid compatibility issues with software relying on
Core's behavior, this commit implements "quirks mode" as follows:
 - quirks mode can be enabled via configuration (disabled by default)
 - If no JSON-RPC version is indicated in the request, accept and
respond to request with "id":null
 - If no JSON-RPC version is indicated in the request, accept and
respond to requests without an "id" member
 - In both cases above, use "id":null in the response
 - Do not respond to request without an "id" or with "id":null when
JSON-RPC version is indicated in the request (process as notification)

config.go

@@ -107,6 +107,7 @@ type config struct {
 	RPCMaxClients        int           `long:"rpcmaxclients" description:"Max number of RPC clients for standard connections"`
 	RPCMaxWebsockets     int           `long:"rpcmaxwebsockets" description:"Max number of RPC websocket connections"`
 	RPCMaxConcurrentReqs int           `long:"rpcmaxconcurrentreqs" description:"Max number of concurrent RPC requests that may be processed concurrently"`
+	RPCQuirks            bool          `long:"rpcquirks" description:"Mirror some JSON-RPC quirks of Bitcoin Core -- NOTE: Discouraged unless interoperability issues need to be worked around"`
 	DisableRPC           bool          `long:"norpc" description:"Disable built-in RPC server -- NOTE: The RPC server is disabled by default if no rpcuser/rpcpass or rpclimituser/rpclimitpass is specified"`
 	DisableTLS           bool          `long:"notls" description:"Disable TLS for the RPC server -- NOTE: This is only allowed if the RPC server is bound to localhost"`
 	DisableDNSSeed       bool          `long:"nodnsseed" description:"Disable DNS seeding for peers"`

doc.go

@@ -50,6 +50,9 @@ Application Options:
       --rpcmaxclients=      Max number of RPC clients for standard connections
                             (10)
       --rpcmaxwebsockets=   Max number of RPC websocket connections (25)
+      --rpcquirks           Mirror some JSON-RPC quirks of Bitcoin Core -- NOTE:
+                            Discouraged unless interoperability issues need to
+                            be worked around
       --norpc               Disable built-in RPC server -- NOTE: The RPC server
                             is disabled by default if no rpcuser/rpcpass or
                             rpclimituser/rpclimitpass is specified

rpcserver.go

@@ -3978,9 +3978,25 @@ func (s *rpcServer) jsonRPCRead(w http.ResponseWriter, r *http.Request, isAdmin
 		}
 	}
 	if jsonErr == nil {
-		// Requests with no ID (notifications) must not have a response
-		// per the JSON-RPC spec.
-		if request.ID == nil {
+		// The JSON-RPC 1.0 spec defines that notifications must have their "id"
+		// set to null and states that notifications do not have a response.
+		//
+		// A JSON-RPC 2.0 notification is a request with "json-rpc":"2.0", and
+		// without an "id" member. The specification states that notifications
+		// must not be responded to. JSON-RPC 2.0 permits the null value as a
+		// valid request id, therefore such requests are not notifications.
+		//
+		// Bitcoin Core serves requests with "id":null or even an absent "id",
+		// and responds to such requests with "id":null in the response.
+		//
+		// Btcd does not respond to any request without and "id" or "id":null,
+		// regardless the indicated JSON-RPC protocol version unless RPC quirks
+		// are enabled. With RPC quirks enabled, such requests will be responded
+		// to if the reqeust does not indicate JSON-RPC version.
+		//
+		// RPC quirks can be enabled by the user to avoid compatibility issues
+		// with software relying on Core's behavior.
+		if request.ID == nil && !(cfg.RPCQuirks && request.Jsonrpc == "") {
 			return
 		}
 

rpcwebsocket.go

@@ -950,9 +950,25 @@ out:
 			continue
 		}
 
-		// Requests with no ID (notifications) must not have a response per the
-		// JSON-RPC spec.
-		if request.ID == nil {
+		// The JSON-RPC 1.0 spec defines that notifications must have their "id"
+		// set to null and states that notifications do not have a response.
+		//
+		// A JSON-RPC 2.0 notification is a request with "json-rpc":"2.0", and
+		// without an "id" member. The specification states that notifications
+		// must not be responded to. JSON-RPC 2.0 permits the null value as a
+		// valid request id, therefore such requests are not notifications.
+		//
+		// Bitcoin Core serves requests with "id":null or even an absent "id",
+		// and responds to such requests with "id":null in the response.
+		//
+		// Btcd does not respond to any request without and "id" or "id":null,
+		// regardless the indicated JSON-RPC protocol version unless RPC quirks
+		// are enabled. With RPC quirks enabled, such requests will be responded
+		// to if the reqeust does not indicate JSON-RPC version.
+		//
+		// RPC quirks can be enabled by the user to avoid compatibility issues
+		// with software relying on Core's behavior.
+		if request.ID == nil && !(cfg.RPCQuirks && request.Jsonrpc == "") {
 			if !c.authenticated {
 				break out
 			}

sample-btcd.conf

@@ -207,6 +207,10 @@
 ; Specify the maximum number of concurrent RPC websocket clients.
 ; rpcmaxwebsockets=25
 
+; Mirror some JSON-RPC quirks of Bitcoin Core -- NOTE: Discouraged unless
+; interoperability issues need to be worked around
+; rpcquirks=1
+
 ; Use the following setting to disable the RPC server even if the rpcuser and
 ; rpcpass are specified above.  This allows one to quickly disable the RPC
 ; server without having to remove credentials from the config file.