Better error for “wrong number of params” in RPC

Doesn’t rely on `fHelp` throwing an exception, and adds a test.

qa/pull-tester/rpc-tests.py

@@ -146,6 +146,7 @@
     'wallet_zero_value.py',
     'threeofthreerestore.py',
     'show_help.py',
+    'errors.py'
 ]
 
 ZMQ_SCRIPTS = [

qa/rpc-tests/errors.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+# Copyright (c) 2014-2016 The Bitcoin Core developers
+# Copyright (c) 2018-2023 The Zcash developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://www.opensource.org/licenses/mit-license.php .
+
+#
+# Test RPC call failure cases. Tests should mostly correspond to code in rpc/server.cpp.
+#
+
+from test_framework.authproxy import JSONRPCException
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import start_nodes
+
+class BlockchainTest(BitcoinTestFramework):
+    """
+    Test RPC call failure cases.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self.num_nodes = 2
+
+    def setup_network(self, split=False):
+        self.nodes = start_nodes(self.num_nodes, self.options.tmpdir)
+        self.is_network_split = False
+        self.sync_all()
+
+    def run_test(self):
+        node = self.nodes[0]
+
+        try:
+            node.gettxoutsetinfo(1)
+        except JSONRPCException as e:
+            errorString = e.error['message']
+        assert("Too many parameters for method `gettxoutsetinfo`. Needed exactly 0, but received 1" in errorString)
+
+if __name__ == '__main__':
+    BlockchainTest().main()

src/rpc/client.cpp

@@ -23,13 +23,17 @@ std::string FormatConversionFailure(const std::string& strMethod, const Conversi
             },
             [&](const WrongNumberOfParams& err) {
                 return tinyformat::format(
-                        "%s for method, %s. Needed between %u and %u, but received %u",
+                        "%s for method `%s`. Needed %s, but received %u",
                         err.providedParams < err.requiredParams
                         ? "Not enough parameters"
                         : "Too many parameters",
                         strMethod,
-                        err.requiredParams,
-                        err.requiredParams + err.optionalParams,
+                        err.optionalParams == 0
+                        ? tinyformat::format("exactly %u", err.requiredParams)
+                        : tinyformat::format(
+                                "from %u to %u",
+                                err.requiredParams,
+                                err.requiredParams + err.optionalParams),
                         err.providedParams);
             },
             [](const UnparseableParam& err) {

src/rpc/server.cpp

@@ -495,12 +495,33 @@ UniValue CRPCTable::execute(const std::string &strMethod, const UniValue &params
         if (paramRange != rpcCvtTable.end()) {
             auto numRequired = paramRange->second.first.size();
             auto numOptional = paramRange->second.second.size();
-            return pcmd->actor(
-                    params,
-                    params.size() < numRequired || numRequired + numOptional < params.size());
+            if (params.size() < numRequired || numRequired + numOptional < params.size()) {
+                std::string helpMsg;
+                try {
+                    // help gets thrown – if it doesn’t throw, then no help message
+                    pcmd->actor(params, true);
+                } catch (const std::runtime_error& err) {
+                    helpMsg = std::string("\n\n") + err.what();
+                }
+                throw JSONRPCError(
+                    RPC_INVALID_PARAMS,
+                    strprintf(
+                            "%s for method `%s`. Needed %s, but received %u%s",
+                            params.size() < numRequired
+                            ? "Not enough parameters"
+                            : "Too many parameters",
+                            strMethod,
+                            numOptional == 0
+                            ? strprintf("exactly %u", numRequired)
+                            : strprintf("from %u to %u", numRequired, numRequired + numOptional),
+                            params.size(),
+                            helpMsg));
+            } else {
+                return pcmd->actor(params, false);
+            }
         } else {
             throw JSONRPCError(
-                    RPC_METHOD_NOT_FOUND,
+                    RPC_INTERNAL_ERROR,
                     "Parameters for "
                     + strMethod
                     + " not found – this is an internal error, please report it.");