rpc: JSON-RPC 2.0 should not respond to "notifications"

For JSON-RPC 2.0 requests we need to distinguish between
a missing "id" field and "id":null. This is accomplished
by making the JSONRPCRequest id property a
std::optional<UniValue> with a default value of
UniValue::VNULL.

A side-effect of this change for non-2.0 requests is that request which do not
specify an "id" field will no longer return "id": null in the response.

src/httprpc.cpp

@@ -211,6 +211,12 @@ static bool HTTPReq_JSONRPC(const std::any& context, HTTPRequest* req)
             const bool catch_errors{jreq.m_json_version == JSONRPCVersion::V2};
             reply = JSONRPCExec(jreq, catch_errors);
 
+            if (jreq.IsNotification()) {
+                // Even though we do execute notifications, we do not respond to them
+                req->WriteReply(HTTP_NO_CONTENT);
+                return true;
+            }
+
         // array of requests
         } else if (valRequest.isArray()) {
             // Check authorization for each request's method
@@ -235,15 +241,32 @@ static bool HTTPReq_JSONRPC(const std::any& context, HTTPRequest* req)
             reply = UniValue::VARR;
             for (size_t i{0}; i < valRequest.size(); ++i) {
                 // Batches never throw HTTP errors, they are always just included
-                // in "HTTP OK" responses.
+                // in "HTTP OK" responses. Notifications never get any response.
+                UniValue response;
                 try {
                     jreq.parse(valRequest[i]);
-                    reply.push_back(JSONRPCExec(jreq, /*catch_errors=*/true));
+                    response = JSONRPCExec(jreq, /*catch_errors=*/true);
                 } catch (UniValue& e) {
-                    reply.push_back(JSONRPCReplyObj(NullUniValue, std::move(e), jreq.id, jreq.m_json_version));
+                    response = JSONRPCReplyObj(NullUniValue, std::move(e), jreq.id, jreq.m_json_version);
                 } catch (const std::exception& e) {
-                    reply.push_back(JSONRPCReplyObj(NullUniValue, JSONRPCError(RPC_PARSE_ERROR, e.what()), jreq.id, jreq.m_json_version));
+                    response = JSONRPCReplyObj(NullUniValue, JSONRPCError(RPC_PARSE_ERROR, e.what()), jreq.id, jreq.m_json_version);
                 }
+                if (!jreq.IsNotification()) {
+                    reply.push_back(std::move(response));
+                }
+            }
+            // Return no response for an all-notification batch, but only if the
+            // batch request is non-empty. Technically according to the JSON-RPC
+            // 2.0 spec, an empty batch request should also return no response,
+            // However, if the batch request is empty, it means the request did
+            // not contain any JSON-RPC version numbers, so returning an empty
+            // response could break backwards compatibility with old RPC clients
+            // relying on previous behavior. Return an empty array instead of an
+            // empty response in this case to favor being backwards compatible
+            // over complying with the JSON-RPC 2.0 spec in this case.
+            if (reply.size() == 0 && valRequest.size() > 0) {
+                req->WriteReply(HTTP_NO_CONTENT);
+                return true;
             }
         }
         else

src/rpc/protocol.h

@@ -10,6 +10,7 @@
 enum HTTPStatusCode
 {
     HTTP_OK                    = 200,
+    HTTP_NO_CONTENT            = 204,
     HTTP_BAD_REQUEST           = 400,
     HTTP_UNAUTHORIZED          = 401,
     HTTP_FORBIDDEN             = 403,

src/rpc/request.cpp

@@ -37,7 +37,7 @@ UniValue JSONRPCRequestObj(const std::string& strMethod, const UniValue& params,
     return request;
 }
 
-UniValue JSONRPCReplyObj(UniValue result, UniValue error, UniValue id, JSONRPCVersion jsonrpc_version)
+UniValue JSONRPCReplyObj(UniValue result, UniValue error, std::optional<UniValue> id, JSONRPCVersion jsonrpc_version)
 {
     UniValue reply(UniValue::VOBJ);
     // Add JSON-RPC version number field in v2 only.
@@ -52,7 +52,7 @@ UniValue JSONRPCReplyObj(UniValue result, UniValue error, UniValue id, JSONRPCVe
         if (jsonrpc_version == JSONRPCVersion::V1_LEGACY) reply.pushKV("result", NullUniValue);
         reply.pushKV("error", std::move(error));
     }
-    reply.pushKV("id", std::move(id));
+    if (id.has_value()) reply.pushKV("id", std::move(id.value()));
     return reply;
 }
 
@@ -172,7 +172,11 @@ void JSONRPCRequest::parse(const UniValue& valRequest)
     const UniValue& request = valRequest.get_obj();
 
     // Parse id now so errors from here on will have the id
-    id = request.find_value("id");
+    if (request.exists("id")) {
+        id = request.find_value("id");
+    } else {
+        id = std::nullopt;
+    }
 
     // Check for JSON-RPC 2.0 (default 1.1)
     m_json_version = JSONRPCVersion::V1_LEGACY;

src/rpc/request.h

@@ -7,6 +7,7 @@
 #define BITCOIN_RPC_REQUEST_H
 
 #include <any>
+#include <optional>
 #include <string>
 
 #include <univalue.h>
@@ -17,7 +18,7 @@ enum class JSONRPCVersion {
 };
 
 UniValue JSONRPCRequestObj(const std::string& strMethod, const UniValue& params, const UniValue& id);
-UniValue JSONRPCReplyObj(UniValue result, UniValue error, UniValue id, JSONRPCVersion jsonrpc_version);
+UniValue JSONRPCReplyObj(UniValue result, UniValue error, std::optional<UniValue> id, JSONRPCVersion jsonrpc_version);
 UniValue JSONRPCError(int code, const std::string& message);
 
 /** Generate a new RPC authentication cookie and write it to disk */
@@ -32,7 +33,7 @@ std::vector<UniValue> JSONRPCProcessBatchReply(const UniValue& in);
 class JSONRPCRequest
 {
 public:
-    UniValue id;
+    std::optional<UniValue> id = UniValue::VNULL;
     std::string strMethod;
     UniValue params;
     enum Mode { EXECUTE, GET_HELP, GET_ARGS } mode = EXECUTE;
@@ -43,6 +44,7 @@ public:
     JSONRPCVersion m_json_version = JSONRPCVersion::V1_LEGACY;
 
     void parse(const UniValue& valRequest);
+    [[nodiscard]] bool IsNotification() const { return !id.has_value() && m_json_version == JSONRPCVersion::V2; };
 };
 
 #endif // BITCOIN_RPC_REQUEST_H