Add GetQueryParameter helper function

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

src/Makefile.test.include

@@ -95,6 +95,7 @@ BITCOIN_TESTS =\
   test/fs_tests.cpp \
   test/getarg_tests.cpp \
   test/hash_tests.cpp \
+  test/httpserver_tests.cpp \
   test/i2p_tests.cpp \
   test/interfaces_tests.cpp \
   test/key_io_tests.cpp \

src/httpserver.cpp

@@ -23,6 +23,7 @@
 
 #include <deque>
 #include <memory>
+#include <optional>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string>
@@ -30,11 +31,12 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 
-#include <event2/thread.h>
 #include <event2/buffer.h>
 #include <event2/bufferevent.h>
-#include <event2/util.h>
+#include <event2/http.h>
 #include <event2/keyvalq_struct.h>
+#include <event2/thread.h>
+#include <event2/util.h>
 
 #include <support/events.h>
 
@@ -639,6 +641,37 @@ HTTPRequest::RequestMethod HTTPRequest::GetRequestMethod() const
     }
 }
 
+std::optional<std::string> HTTPRequest::GetQueryParameter(const std::string& key) const
+{
+    const char* uri{evhttp_request_get_uri(req)};
+
+    return GetQueryParameterFromUri(uri, key);
+}
+
+std::optional<std::string> GetQueryParameterFromUri(const char* uri, const std::string& key)
+{
+    evhttp_uri* uri_parsed{evhttp_uri_parse(uri)};
+    const char* query{evhttp_uri_get_query(uri_parsed)};
+    std::optional<std::string> result;
+
+    if (query) {
+        // Parse the query string into a key-value queue and iterate over it
+        struct evkeyvalq params_q;
+        evhttp_parse_query_str(query, &params_q);
+
+        for (struct evkeyval* param{params_q.tqh_first}; param != nullptr; param = param->next.tqe_next) {
+            if (param->key == key) {
+                result = param->value;
+                break;
+            }
+        }
+        evhttp_clear_headers(&params_q);
+    }
+    evhttp_uri_free(uri_parsed);
+
+    return result;
+}
+
 void RegisterHTTPHandler(const std::string &prefix, bool exactMatch, const HTTPRequestHandler &handler)
 {
     LogPrint(BCLog::HTTP, "Registering HTTP handler for %s (exactmatch %d)\n", prefix, exactMatch);

src/httpserver.h

@@ -5,8 +5,9 @@
 #ifndef BITCOIN_HTTPSERVER_H
 #define BITCOIN_HTTPSERVER_H
 
-#include <string>
 #include <functional>
+#include <optional>
+#include <string>
 
 static const int DEFAULT_HTTP_THREADS=4;
 static const int DEFAULT_HTTP_WORKQUEUE=16;
@@ -83,6 +84,17 @@ public:
      */
     RequestMethod GetRequestMethod() const;
 
+    /** Get the query parameter value from request uri for a specified key, or std::nullopt if the
+     * key is not found.
+     *
+     * If the query string contains duplicate keys, the first value is returned. Many web frameworks
+     * would instead parse this as an array of values, but this is not (yet) implemented as it is
+     * currently not needed in any of the endpoints.
+     *
+     * @param[in] key represents the query parameter of which the value is returned
+     */
+    std::optional<std::string> GetQueryParameter(const std::string& key) const;
+
     /**
      * Get the request header specified by hdr, or an empty string.
      * Return a pair (isPresent,string).
@@ -115,6 +127,20 @@ public:
     void WriteReply(int nStatus, const std::string& strReply = "");
 };
 
+/** Get the query parameter value from request uri for a specified key, or std::nullopt if the key
+ * is not found.
+ *
+ * If the query string contains duplicate keys, the first value is returned. Many web frameworks
+ * would instead parse this as an array of values, but this is not (yet) implemented as it is
+ * currently not needed in any of the endpoints.
+ *
+ * Helper function for HTTPRequest::GetQueryParameter.
+ *
+ * @param[in] uri is the entire request uri
+ * @param[in] key represents the query parameter of which the value is returned
+ */
+std::optional<std::string> GetQueryParameterFromUri(const char* uri, const std::string& key);
+
 /** Event handler closure.
  */
 class HTTPClosure

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()