http: Implement HTTPRequest class

HTTP Request message:
https://datatracker.ietf.org/doc/html/rfc1945#section-5

Request Line aka Control Line aka first line:
https://datatracker.ietf.org/doc/html/rfc1945#section-5.1

See message_read_status() in libevent http.c for how
`MORE_DATA_EXPECTED` is handled there

src/httpserver.h

@@ -8,9 +8,11 @@
 #include <functional>
 #include <optional>
 #include <span>
+#include <stdexcept>
 #include <string>
 
 #include <rpc/protocol.h>
+#include <util/byte_units.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 
@@ -192,6 +194,10 @@ private:
 };
 
 namespace http_bitcoin {
+using util::LineReader;
+
+//! Shortest valid request line, used by libevent in evhttp_parse_request_line()
+constexpr size_t MIN_REQUEST_LINE_LENGTH = std::string_view("GET / HTTP/1.0").size();
 
 //! Maximum size of each headers line in an HTTP request,
 //! also the maximum size of all headers total.
@@ -199,6 +205,15 @@ namespace http_bitcoin {
 //! And libevent http.c evhttp_parse_headers_()
 constexpr size_t MAX_HEADERS_SIZE{8192};
 
+//! Maximum size of an HTTP request body
+constexpr uint64_t MAX_BODY_SIZE{32_MiB};
+
+//! Thrown when a request body exceeds MAX_BODY_SIZE
+//! so the server can reply with more specific code 413 (content too large) vs general 400 (bad request)
+struct ContentTooLargeError : std::runtime_error {
+    using std::runtime_error::runtime_error;
+};
+
 class HTTPHeaders
 {
 public:
@@ -258,6 +273,30 @@ public:
 
     std::string StringifyHeaders() const;
 };
+
+class HTTPRequest
+{
+public:
+    std::string m_method;
+    std::string m_target;
+    HTTPVersion m_version;
+    HTTPHeaders m_headers;
+    std::string m_body;
+
+    /**
+     * Methods that attempt to parse HTTP request fields line-by-line
+     * from a receive buffer.
+     * @param[in]   reader  A LineReader object constructed over a span of data.
+     * @returns     true    If the request field was parsed.
+     *              false   If there was not enough data in the buffer to complete the field.
+     * @throws      std::runtime_error if data is invalid.
+     */
+    /// @{
+    bool LoadControlData(LineReader& reader);
+    bool LoadHeaders(LineReader& reader);
+    bool LoadBody(LineReader& reader);
+    /// @}
+};
 } // namespace http_bitcoin
 
 #endif // BITCOIN_HTTPSERVER_H