[REST] Handle UTXO retrieval when ignoring the mempool

Current REST API always returns empty UTXO when invoked without `/checkmempool/` URL part.

After the fix:
```
$ curl -s http://localhost:8332/rest/getutxos/0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098-0.json | jq
{
  "chainHeight": 514109,
  "chaintipHash": "0000000000000000001fe76d1445e8a6432fd2de04261dc9c5915311dc7ad6de",
  "bitmap": "1",
  "utxos": [
    {
      "height": 1,
      "value": 50,
      "scriptPubKey": {
        "asm": "0496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858ee OP_CHECKSIG",
        "hex": "410496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858eeac",
        "reqSigs": 1,
        "type": "pubkey",
        "addresses": [
          "12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX"
        ]
      }
    }
  ]
}
```

Before the fix:
```
$ curl -s http://localhost:8332/rest/getutxos/0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098-0.json | jq
{
  "chainHeight": 514109,
  "chaintipHash": "0000000000000000001fe76d1445e8a6432fd2de04261dc9c5915311dc7ad6de",
  "bitmap": "0",
  "utxos": []
}
```

2 files changed, 52 insertions, 25 deletions

diff --git a/src/rest.cpp b/src/rest.cpp
index f47b40343b..6501dab6bf 100644
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -487,26 +487,28 @@ static bool rest_getutxos(HTTPRequest* req, const std::string& strURIPart)
     std::vector<bool> hits;
     bitmap.resize((vOutPoints.size() + 7) / 8);
     {
-        LOCK2(cs_main, mempool.cs);
-
-        CCoinsView viewDummy;
-        CCoinsViewCache view(&viewDummy);
-
-        CCoinsViewCache& viewChain = *pcoinsTip;
-        CCoinsViewMemPool viewMempool(&viewChain, mempool);
-
-        if (fCheckMemPool)
-            view.SetBackend(viewMempool); // switch cache backend to db+mempool in case user likes to query mempool
-
-        for (size_t i = 0; i < vOutPoints.size(); i++) {
-            bool hit = false;
-            Coin coin;
-            if (view.GetCoin(vOutPoints[i], coin) && !mempool.isSpent(vOutPoints[i])) {
-                hit = true;
-                outs.emplace_back(std::move(coin));
+        auto process_utxos = [&vOutPoints, &outs, &hits](const CCoinsView& view, const CTxMemPool& mempool) {
+            for (const COutPoint& vOutPoint : vOutPoints) {
+                Coin coin;
+                bool hit = !mempool.isSpent(vOutPoint) && view.GetCoin(vOutPoint, coin);
+                hits.push_back(hit);
+                if (hit) outs.emplace_back(std::move(coin));
             }
+        };
+
+        if (fCheckMemPool) {
+            // use db+mempool as cache backend in case user likes to query mempool
+            LOCK2(cs_main, mempool.cs);
+            CCoinsViewCache& viewChain = *pcoinsTip;
+            CCoinsViewMemPool viewMempool(&viewChain, mempool);
+            process_utxos(viewMempool, mempool);
+        } else {
+            LOCK(cs_main);  // no need to lock mempool!
+            process_utxos(*pcoinsTip, CTxMemPool());
+        }
 
-            hits.push_back(hit);
+        for (size_t i = 0; i < hits.size(); ++i) {
+            const bool hit = hits[i];
             bitmapStringRepresentation.append(hit ? "1" : "0"); // form a binary string representation (human-readable for json output)
             bitmap[i / 8] |= ((uint8_t)hit) << (i % 8);
         }
diff --git a/test/functional/interface_rest.py b/test/functional/interface_rest.py
index c6cb4c54cd..6f585f6825 100755
--- a/test/functional/interface_rest.py
+++ b/test/functional/interface_rest.py
@@ -85,7 +85,7 @@ class RESTTest (BitcoinTestFramework):
         #######################################
         # GETUTXOS: query an unspent outpoint #
         #######################################
-        json_request = '/checkmempool/'+txid+'-'+str(n)
+        json_request = '/'+txid+'-'+str(n)
         json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
         json_obj = json.loads(json_string)
 
@@ -100,7 +100,7 @@ class RESTTest (BitcoinTestFramework):
         #################################################
         # GETUTXOS: now query an already spent outpoint #
         #################################################
-        json_request = '/checkmempool/'+vintx+'-0'
+        json_request = '/'+vintx+'-0'
         json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
         json_obj = json.loads(json_string)
 
@@ -117,7 +117,7 @@ class RESTTest (BitcoinTestFramework):
         ##################################################
         # GETUTXOS: now check both with the same request #
         ##################################################
-        json_request = '/checkmempool/'+txid+'-'+str(n)+'/'+vintx+'-0'
+        json_request = '/'+txid+'-'+str(n)+'/'+vintx+'-0'
         json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
         json_obj = json.loads(json_string)
         assert_equal(len(json_obj['utxos']), 1)
@@ -151,23 +151,48 @@ class RESTTest (BitcoinTestFramework):
         txid = self.nodes[0].sendtoaddress(self.nodes[1].getnewaddress(), 0.1)
         json_string = http_get_call(url.hostname, url.port, '/rest/tx/'+txid+self.FORMAT_SEPARATOR+"json")
         json_obj = json.loads(json_string)
-        vintx = json_obj['vin'][0]['txid'] # get the vin to later check for utxo (should be spent by then)
+        # get the spent output to later check for utxo (should be spent by then)
+        spent = '{}-{}'.format(json_obj['vin'][0]['txid'], json_obj['vin'][0]['vout'])
         # get n of 0.1 outpoint
         n = 0
         for vout in json_obj['vout']:
             if vout['value'] == 0.1:
                 n = vout['n']
+        spending = '{}-{}'.format(txid, n)
 
-        json_request = '/'+txid+'-'+str(n)
+        json_request = '/'+spending
         json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
         json_obj = json.loads(json_string)
-        assert_equal(len(json_obj['utxos']), 0) #there should be an outpoint because it has just added to the mempool
+        assert_equal(len(json_obj['utxos']), 0) #there should be no outpoint because it has just added to the mempool
 
-        json_request = '/checkmempool/'+txid+'-'+str(n)
+        json_request = '/checkmempool/'+spending
         json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
         json_obj = json.loads(json_string)
         assert_equal(len(json_obj['utxos']), 1) #there should be an outpoint because it has just added to the mempool
 
+        json_request = '/'+spent
+        json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
+        json_obj = json.loads(json_string)
+        assert_equal(len(json_obj['utxos']), 1) #there should be an outpoint because its spending tx is not confirmed
+
+        json_request = '/checkmempool/'+spent
+        json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
+        json_obj = json.loads(json_string)
+        assert_equal(len(json_obj['utxos']), 0) #there should be no outpoint because it has just spent (by mempool tx)
+
+        self.nodes[0].generate(1)
+        self.sync_all()
+
+        json_request = '/'+spending
+        json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
+        json_obj = json.loads(json_string)
+        assert_equal(len(json_obj['utxos']), 1) #there should be an outpoint because it was mined
+
+        json_request = '/checkmempool/'+spending
+        json_string = http_get_call(url.hostname, url.port, '/rest/getutxos'+json_request+self.FORMAT_SEPARATOR+'json')
+        json_obj = json.loads(json_string)
+        assert_equal(len(json_obj['utxos']), 1) #there should be an outpoint because it was mined
+
         #do some invalid requests
         json_request = '{"checkmempool'
         response = http_post_call(url.hostname, url.port, '/rest/getutxos'+self.FORMAT_SEPARATOR+'json', json_request, True)