Cap listsinceblock target_confirmations param

Previously, listsinceblock would fail with error code -1 when the
target_confirmations exceeded the number of confirmations of the genesis
block. This commit allows target_confirmations to refer to a lastblock
hash with more confirmations than exist in the chain by setting the
lastblock hash to the genesis hash in this case. This allows for
`listsinceblock "" 6` to not fail if the block count is less than 5
which may happen on regtest.

Includes update to the functional test for listsinceblock to test for
this case.

Author: adaminsky

src/wallet/rpcwallet.cpp

@@ -1484,7 +1484,7 @@ static UniValue listsinceblock(const JSONRPCRequest& request)
                         {RPCResult::Type::ARR, "removed", "<structure is the same as \"transactions\" above, only present if include_removed=true>\n"
                             "Note: transactions that were re-added in the active chain will appear as-is in this array, and may thus have a positive confirmation count."
                         , {{RPCResult::Type::ELISION, "", ""},}},
-                        {RPCResult::Type::STR_HEX, "lastblock", "The hash of the block (target_confirmations-1) from the best block on the main chain. This is typically used to feed back into listsinceblock the next time you call it. So you would generally use a target_confirmations of say 6, so you will be continually re-notified of transactions until they've reached 6 confirmations plus any new ones"},
+                        {RPCResult::Type::STR_HEX, "lastblock", "The hash of the block (target_confirmations-1) from the best block on the main chain, or the genesis hash if the referenced block does not exist yet. This is typically used to feed back into listsinceblock the next time you call it. So you would generally use a target_confirmations of say 6, so you will be continually re-notified of transactions until they've reached 6 confirmations plus any new ones"},
                     }
                 },
                 RPCExamples{
@@ -1567,6 +1567,7 @@ static UniValue listsinceblock(const JSONRPCRequest& request)
     }
 
     uint256 lastblock;
+    target_confirms = std::min(target_confirms, wallet.GetLastBlockHeight() + 1);
     CHECK_NONFATAL(wallet.chain().findAncestorByHeight(wallet.GetLastBlockHash(), wallet.GetLastBlockHeight() + 1 - target_confirms, FoundBlock().hash(lastblock)));
 
     UniValue ret(UniValue::VOBJ);

test/functional/wallet_listsinceblock.py

@@ -36,6 +36,7 @@ def run_test(self):
         self.test_double_spend()
         self.test_double_send()
         self.double_spends_filtered()
+        self.test_targetconfirmations()
 
     def test_no_blockhash(self):
         self.log.info("Test no blockhash")
@@ -74,6 +75,27 @@ def test_invalid_blockhash(self):
         assert_raises_rpc_error(-8, "blockhash must be hexadecimal string (not 'Z000000000000000000000000000000000000000000000000000000000000000')", self.nodes[0].listsinceblock,
                                 "Z000000000000000000000000000000000000000000000000000000000000000")
 
+    def test_targetconfirmations(self):
+        '''
+        This tests when the value of target_confirmations exceeds the number of
+        blocks in the main chain. In this case, the genesis block hash should be
+        given for the `lastblock` property. If target_confirmations is < 1, then
+        a -8 invalid parameter error is thrown.
+        '''
+        self.log.info("Test target_confirmations")
+        blockhash, = self.nodes[2].generate(1)
+        blockheight = self.nodes[2].getblockheader(blockhash)['height']
+        self.sync_all()
+
+        assert_equal(
+            self.nodes[0].getblockhash(0),
+            self.nodes[0].listsinceblock(blockhash, blockheight + 1)['lastblock'])
+        assert_equal(
+            self.nodes[0].getblockhash(0),
+            self.nodes[0].listsinceblock(blockhash, blockheight + 1000)['lastblock'])
+        assert_raises_rpc_error(-8, "Invalid parameter",
+            self.nodes[0].listsinceblock, blockhash, 0)
+
     def test_reorg(self):
         '''
         `listsinceblock` did not behave correctly when handed a block that was