Add GetQueryParameter helper function

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

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);