Add GetQueryParameter helper function

Easily get the query parameter from the URI, with optional default value.

src/test/httpserver_tests.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) 2012-2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <httpserver.h>
+#include <test/util/setup_common.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(httpserver_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(test_query_parameters)
+{
+    std::string uri {};
+
+    // No parameters
+    uri = "localhost:8080/rest/headers/someresource.json";
+    BOOST_CHECK(!GetQueryParameterFromUri(uri.c_str(), "p1").has_value());
+
+    // Single parameter
+    uri = "localhost:8080/rest/endpoint/someresource.json?p1=v1";
+    BOOST_CHECK_EQUAL(GetQueryParameterFromUri(uri.c_str(), "p1").value(), "v1");
+    BOOST_CHECK(!GetQueryParameterFromUri(uri.c_str(), "p2").has_value());
+
+    // Multiple parameters
+    uri = "/rest/endpoint/someresource.json?p1=v1&p2=v2";
+    BOOST_CHECK_EQUAL(GetQueryParameterFromUri(uri.c_str(), "p1").value(), "v1");
+    BOOST_CHECK_EQUAL(GetQueryParameterFromUri(uri.c_str(), "p2").value(), "v2");
+
+    // If the query string contains duplicate keys, the first value is returned
+    uri = "/rest/endpoint/someresource.json?p1=v1&p1=v2";
+    BOOST_CHECK_EQUAL(GetQueryParameterFromUri(uri.c_str(), "p1").value(), "v1");
+
+    // Invalid query string syntax is the same as not having parameters
+    uri = "/rest/endpoint/someresource.json&p1=v1&p2=v2";
+    BOOST_CHECK(!GetQueryParameterFromUri(uri.c_str(), "p1").has_value());
+}
+BOOST_AUTO_TEST_SUITE_END()