Merge bitcoin#17728: rpc: require second argument only for scantxoutset start action

7d26357 rpc: require second argument only for scantxoutset start action (Andrew Chow)

Pull request description:

  It was reported on [IRC](http://www.erisian.com.au/bitcoin-core-dev/log-2019-12-11.html#l-377) that `scantxoutset`'s API was broken in 0.19.0:

  ```
  <belcher> i think scantxoutset may have been broken in bitcoin core 0.19 ? regardless of what parameters i run it with (e.g. "scantxoutset abort", "scantxoutset status") it just returns the help doc, according to the release notes the only change was https://github.com/bitcoin/bitcoin/pull/16285/files but i dont see anything that wouldve broken it, it works fine in 0.18
  <belcher> im on regtest, in case its important
  <harding> I can confirm `scantxoutset abort` returns the help doc on latest master.  Waiting for 0.18.1 to start now to attempt to reproduce there.
  <harding> It looks like it's expecting a second parameter (even though that doesn't make sense with "abort").
  <jonatack> Same for me as well
  <harding> Can also confirm that `scantxoutset abort` returns the expected result on 0.18.1.
  ```

  As noted in the conversation, previously, the second argument of `scanobjects` is only required for the `start` action. `Stop` and `abort` actions did not and could work without them.

  It appears that this was broken by bitcoin#16240 which enforced the size of the arguments to match the listed required arguments.

  To fix this issue, this PR makes the `scanobjects` argument an optional argument. Then only in the `start` action do we check whether the `scanobjects` argument is there and throw an informative error about that. Also a test is added for this case.

ACKs for top commit:
  laanwj:
    ACK 7d26357
  promag:
    ACK 7d26357.

Tree-SHA512: 828bdfe47f4fffa5d00a2cf88db6cea4a2714d9c49276841ca5cbdd1603b87bb6862147b86edcf36d7b40314ddb80b1a07fd399faf288572c55cc788c5cf9526

Author: laanwj

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 @@ def run_test(self):
         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()