rpc: require second argument only for scantxoutset start action

The second argument of scanobjects is only required for the start action.
Stop and abort actions do not need this.

src/rpc/blockchain.cpp

@@ -2055,7 +2055,7 @@ UniValue scantxoutset(const JSONRPCRequest& request)
             "                                      \"start\" for starting a scan\n"
             "                                      \"abort\" for aborting the current scan (returns true when abort was successful)\n"
             "                                      \"status\" for progress report (in %) of the current scan"},
-                    {"scanobjects", RPCArg::Type::ARR, RPCArg::Optional::NO, "Array of scan objects\n"
+                    {"scanobjects", RPCArg::Type::ARR, RPCArg::Optional::OMITTED, "Array of scan objects. Required for \"start\" action\n"
             "                                  Every scan object is either a string descriptor or an object:",
                         {
                             {"descriptor", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "An output descriptor"},
@@ -2115,6 +2115,11 @@ UniValue scantxoutset(const JSONRPCRequest& request)
         if (!reserver.reserve()) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Scan already in progress, use action \"abort\" or \"status\"");
         }
+
+        if (request.params.size() < 2) {
+            throw JSONRPCError(RPC_MISC_ERROR, "scanobjects argument is required for the start action");
+        }
+
         std::set<CScript> needles;
         std::map<CScript, std::string> descriptors;
         CAmount total_in = 0;

test/functional/rpc_scantxoutset.py

@@ -116,5 +116,12 @@ class ScantxoutsetTest(BitcoinTestFramework):
         assert_equal(descriptors(self.nodes[0].scantxoutset("start", [ "combo(tprv8ZgxMBicQKsPd7Uf69XL1XwhmjHopUGep8GuEiJDZmbQz6o58LninorQAfcKZWARbtRtfnLcJ5MQ2AtHcQJCCRUcMRvmDUjyEmNUWwx8UbK/1/1/0)"])), ["pkh([0c5f9a1e/1/1/0]03e1c5b6e650966971d7e71ef2674f80222752740fc1dfd63bbbd220d2da9bd0fb)#cxmct4w8"])
         assert_equal(descriptors(self.nodes[0].scantxoutset("start", [ {"desc": "combo(tpubD6NzVbkrYhZ4WaWSyoBvQwbpLkojyoTZPRsgXELWz3Popb3qkjcJyJUGLnL4qHHoQvao8ESaAstxYSnhyswJ76uZPStJRJCTKvosUCJZL5B/1/1/*)", "range": 1500}])), ['pkh([0c5f9a1e/1/1/0]03e1c5b6e650966971d7e71ef2674f80222752740fc1dfd63bbbd220d2da9bd0fb)#cxmct4w8', 'pkh([0c5f9a1e/1/1/1500]03832901c250025da2aebae2bfb38d5c703a57ab66ad477f9c578bfbcd78abca6f)#vchwd07g', 'pkh([0c5f9a1e/1/1/1]030d820fc9e8211c4169be8530efbc632775d8286167afd178caaf1089b77daba7)#z2t3ypsa'])
 
+        # Check that status and abort don't need second arg
+        assert_equal(self.nodes[0].scantxoutset("status"), None)
+        assert_equal(self.nodes[0].scantxoutset("abort"), False)
+
+        # Check that second arg is needed for start
+        assert_raises_rpc_error(-1, "scanobjects argument is required for the start action", self.nodes[0].scantxoutset, "start")
+
 if __name__ == '__main__':
     ScantxoutsetTest().main()