[wallet] loadwallet shouldn't create new wallets.

A bug in the initial implementation of loadwallet meant that if the
arguement was a directory that didn't contain a wallet.dat file, a new
wallet would be created in that directory. Fix that so that if a
directory is passed in, it must contain a wallet.dat file.

Bug reported by promag (João Barbosa).

src/wallet/rpcwallet.cpp

@@ -3092,6 +3092,12 @@ static UniValue loadwallet(const JSONRPCRequest& request)
     fs::path wallet_path = fs::absolute(wallet_file, GetWalletDir());
     if (fs::symlink_status(wallet_path).type() == fs::file_not_found) {
         throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Wallet " + wallet_file + " not found.");
+    } else if (fs::is_directory(wallet_path)) {
+        // The given filename is a directory. Check that there's a wallet.dat file.
+        fs::path wallet_dat_file = wallet_path / "wallet.dat";
+        if (fs::symlink_status(wallet_dat_file).type() == fs::file_not_found) {
+            throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Directory " + wallet_file + " does not contain a wallet.dat file.");
+        }
     }
 
     std::string warning;

test/functional/wallet_multiwallet.py

@@ -211,6 +211,10 @@ class MultiWalletTest(BitcoinTestFramework):
         # Fail to load if wallet file is a symlink
         assert_raises_rpc_error(-4, "Wallet file verification failed: Invalid -wallet path 'w8_symlink'", self.nodes[0].loadwallet, 'w8_symlink')
 
+        # Fail to load if a directory is specified that doesn't contain a wallet
+        os.mkdir(wallet_dir('empty_wallet_dir'))
+        assert_raises_rpc_error(-18, "Directory empty_wallet_dir does not contain a wallet.dat file", self.nodes[0].loadwallet, 'empty_wallet_dir')
+
         self.log.info("Test dynamic wallet creation.")
 
         # Fail to create a wallet if it already exists.