http: Implement HTTPResponse class

HTTP Response message:
https://datatracker.ietf.org/doc/html/rfc1945#section-6

Status line (first line of response):
https://datatracker.ietf.org/doc/html/rfc1945#section-6.1

Status code definitions:
https://datatracker.ietf.org/doc/html/rfc1945#section-9

src/httpserver.cpp

@@ -808,4 +808,14 @@ std::string HTTPHeaders::Stringify() const
 
     return out;
 }
+
+std::string HTTPResponse::StringifyHeaders() const
+{
+    return strprintf("HTTP/%d.%d %d %s\r\n%s",
+                     m_version.major,
+                     m_version.minor,
+                     m_status,
+                     HTTPStatusReasonString(m_status),
+                     m_headers.Stringify());
+}
 } // namespace http_bitcoin

src/httpserver.h

@@ -10,6 +10,7 @@
 #include <span>
 #include <string>
 
+#include <rpc/protocol.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 
@@ -234,6 +235,29 @@ private:
      */
     std::vector<std::pair<std::string, std::string>> m_headers;
 };
+
+struct HTTPVersion {
+    /**
+     * Default HTTP protocol version 1.1 is used by error responses
+     * when a request is unreadable.
+     */
+    /// @{
+    uint8_t major{1};
+    uint8_t minor{1};
+    /// @}
+};
+
+
+class HTTPResponse
+{
+public:
+    HTTPVersion m_version;
+
+    HTTPStatusCode m_status{HTTP_INTERNAL_SERVER_ERROR};
+    HTTPHeaders m_headers;
+
+    std::string StringifyHeaders() const;
+};
 } // namespace http_bitcoin
 
 #endif // BITCOIN_HTTPSERVER_H

src/rpc/protocol.h

@@ -20,6 +20,27 @@ enum HTTPStatusCode : int
     HTTP_SERVICE_UNAVAILABLE   = 503,
 };
 
+//! Mapping of HTTP status codes to short string explanation.
+//! Copied from libevent http.c success_phrases[] and client_error_phrases[]
+inline std::string_view HTTPStatusReasonString(HTTPStatusCode code)
+{
+    switch (code) {
+    case HTTP_OK: return "OK";
+    case HTTP_NO_CONTENT: return "No Content";
+    case HTTP_BAD_REQUEST: return "Bad Request";
+    case HTTP_UNAUTHORIZED: return "Unauthorized";
+    case HTTP_FORBIDDEN: return "Forbidden";
+    case HTTP_NOT_FOUND: return "Not Found";
+    case HTTP_BAD_METHOD: return "Method Not Allowed";
+    case HTTP_INTERNAL_SERVER_ERROR: return "Internal Server Error";
+    case HTTP_SERVICE_UNAVAILABLE: return "Service Unavailable";
+    }
+
+    // Reason phrases are optional and may be replaced by local variants.
+    // https://httpwg.org/specs/rfc9110.html#rfc.section.15.1
+    return "";
+}
+
 //! Bitcoin RPC error codes
 enum RPCErrorCode
 {

src/test/httpserver_tests.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <httpserver.h>
+#include <rpc/protocol.h>
 #include <test/util/common.h>
 #include <test/util/setup_common.h>
 #include <util/string.h>
@@ -10,6 +11,7 @@
 #include <boost/test/unit_test.hpp>
 
 using http_bitcoin::HTTPHeaders;
+using http_bitcoin::HTTPResponse;
 using http_bitcoin::MAX_HEADERS_SIZE;
 
 BOOST_FIXTURE_TEST_SUITE(httpserver_tests, BasicTestingSetup)
@@ -144,4 +146,23 @@ BOOST_AUTO_TEST_CASE(http_headers_tests)
         BOOST_CHECK_EQUAL(headers.FindFirst("key"), "value");
     }
 }
+
+BOOST_AUTO_TEST_CASE(http_response_tests)
+{
+    // Typical HTTP 1.1 response headers
+    HTTPHeaders headers{};
+    headers.Write("Content-Length", "41");
+
+    // Response points to headers which already exist because some of them
+    // are set before we even know what the response will be.
+    HTTPResponse res;
+    res.m_version = {.major = 1, .minor = 1};
+    res.m_status = HTTP_OK;
+    res.m_headers = std::move(headers);
+    BOOST_CHECK_EQUAL(
+        res.StringifyHeaders(),
+        "HTTP/1.1 200 OK\r\n"
+        "Content-Length: 41\r\n"
+        "\r\n");
+}
 BOOST_AUTO_TEST_SUITE_END()