Fix multiple options accepted for single-value options

Currently single-value options such as --tunnel will accept multiple
values, e.g. --tunnel "<...>" --tunnel "<...>". Confusingly for the user
only the last value will be used without any warning.

This commit fixes this by emitting an error message and exiting the
program if multiple option values were supplied when single value was
expected.

Author: p12tic

src/terminal/TerminalClientMain.cpp

@@ -29,6 +29,23 @@ void handleParseException(std::exception& e, cxxopts::Options& options) {
   exit(1);
 }
 
+template <class T, class DefaultT>
+T extractSingleOptionWithDefault(const cxxopts::ParseResult& result,
+                                 const cxxopts::Options& options,
+                                 const string& name, DefaultT defaultValue) {
+  auto count = result.count(name);
+  if (count == 0) {
+    return defaultValue;
+  }
+  if (count == 1) {
+    return result[name].as<T>();
+  }
+  CLOG(INFO, "stdout") << "Value for " << name
+                       << " must be specified only once\n";
+  CLOG(INFO, "stdout") << options.help({}) << endl;
+  exit(0);
+}
+
 int main(int argc, char** argv) {
   WinsockContext context;
   string tmpDir = GetTempDirectory();
@@ -222,10 +239,9 @@ int main(int argc, char** argv) {
     string host_alias = destinationHost;
 
     string jumphost =
-        result.count("jumphost") ? result["jumphost"].as<string>() : "";
-    int keepaliveDuration = result.count("keepalive")
-                                ? result["keepalive"].as<int>()
-                                : MAX_CLIENT_KEEP_ALIVE_DURATION;
+        extractSingleOptionWithDefault<string>(result, options, "jumphost", "");
+    int keepaliveDuration = extractSingleOptionWithDefault<int>(
+        result, options, "keepalive", MAX_CLIENT_KEEP_ALIVE_DURATION);
     if (keepaliveDuration < 1 ||
         keepaliveDuration > MAX_CLIENT_KEEP_ALIVE_DURATION) {
       CLOG(INFO, "stdout") << "Keep-alive duration must between 1 and "
@@ -361,10 +377,9 @@ int main(int argc, char** argv) {
     TelemetryService::get()->logToDatadog("Session Started", el::Level::Info,
                                           __FILE__, __LINE__);
     string tunnel_arg =
-        result.count("tunnel") ? result["tunnel"].as<string>() : "";
-    string r_tunnel_arg = result.count("reversetunnel")
-                              ? result["reversetunnel"].as<string>()
-                              : "";
+        extractSingleOptionWithDefault<string>(result, options, "tunnel", "");
+    string r_tunnel_arg = extractSingleOptionWithDefault<string>(
+        result, options, "reversetunnel", "");
     TerminalClient terminalClient(clientSocket, clientPipeSocket,
                                   socketEndpoint, id, passkey, console,
                                   is_jumphost, tunnel_arg, r_tunnel_arg,