[rpc] Public interfaces to GetTransaction block until synced.

Now that the transaction index is updated asynchronously, in order to
preserve the current behavior of public interfaces, the code blocks
until the transaction index is caught up with the current state of the
blockchain.

src/rest.cpp

@@ -6,6 +6,7 @@
 #include <chain.h>
 #include <chainparams.h>
 #include <core_io.h>
+#include <index/txindex.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 #include <validation.h>
@@ -350,6 +351,10 @@ static bool rest_tx(HTTPRequest* req, const std::string& strURIPart)
     if (!ParseHashStr(hashStr, hash))
         return RESTERR(req, HTTP_BAD_REQUEST, "Invalid hash: " + hashStr);
 
+    if (g_txindex) {
+        g_txindex->BlockUntilSyncedToCurrentChain();
+    }
+
     CTransactionRef tx;
     uint256 hashBlock = uint256();
     if (!GetTransaction(hash, tx, Params().GetConsensus(), hashBlock, true))