output/httpd: add config option "dscp"

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1401

doc/plugins.rst

@@ -994,6 +994,13 @@ It is highly recommended to configure a fixed format, because a stream cannot sw
      - Binds the HTTP server to the specified port.
    * - **bind_to_address ADDR**
      - Binds the HTTP server to the specified address (IPv4, IPv6 or local socket). Multiple addresses in parallel are not supported.
+   * - **dscp_class CLASS**
+     - Sets a DSCP (`Differentiated Services Code Point
+       <https://en.wikipedia.org/wiki/Differentiated_services>`__)
+       class for outgoing traffic.  This can either be a name
+       (``CS*``, ``LE``, ``AF*``, ``EF``) or numeric (decimal or
+       hexadecimal).  A reasonable choice for this setting is ``CS3``
+       ("broadcast video").
    * - **encoder NAME**
      - Chooses an encoder plugin. A list of encoder plugins can be found in the encoder plugin reference :ref:`encoder_plugins`.
    * - **max_clients MC**

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -28,11 +28,13 @@
 #include "Page.hxx"
 #include "IcyMetaDataServer.hxx"
 #include "event/Call.hxx"
+#include "net/DscpParser.hxx"
 #include "util/Domain.hxx"
 #include "util/DeleteDisposer.hxx"
 #include "config/Net.hxx"
 
 #include <cassert>
+#include <stdexcept>
 
 #include <string.h>
 
@@ -49,6 +51,15 @@ HttpdOutput::HttpdOutput(EventLoop &_loop, const ConfigBlock &block)
 	 website(block.GetBlockValue("website", "Set website in config")),
 	 clients_max(block.GetBlockValue("max_clients", 0U))
 {
+	if (const auto *p = block.GetBlockParam("dscp_class"))
+		p->With([this](const char *s){
+			const int value = ParseDscpClass(s);
+			if (value < 0)
+				throw std::runtime_error("Not a valid DSCP class");
+
+			ServerSocket::SetDscpClass(value);
+		});
+
 	/* set up bind_to_address */
 
 	ServerSocketAddGeneric(*this, block.GetBlockValue("bind_to_address"), block.GetBlockValue("port", 8000U));