Allow attaching tags with extra info to stats rows.

Stats responses are incredibly non-standard and no clients render
them correctly. This makes using /STATS a massive pain in the ass
for users.

However, now we have message tags we have a way to fix this. We can
send a <trailing> with the message details and for clients (probably
bots) that need to parse the response we can include the specific
details in message tags enabled by the new inspircd.org/stats-tags
capability.

The average user will get this:

    :<server> 210 <nick> <stats-char> :<stats-message>

If however they enable the newcapability they will get this instead:

    @inspircd.org/stats-foo=bar;inspircd.org/stats-baz=bax :<server> 210 <nick> <stats-char> :<stats-message>

include/modules/stats.h

@@ -57,6 +57,24 @@ public:
 		: Numeric(num)
 	{
 	}
+
+	/** Attaches a stats tag to the response.
+	 * @param stats The stats request that is being responded to.
+	 * @param name The name of the stats tag. Will be prefixed with `inspircd.org/stats-`.
+	 * @param value The value of the stats tag. Will be escaped before attaching.
+	 */
+	inline Row& AddTag(Stats::Context& stats, const std::string& name, const std::string& value);
+
+	/** Attaches multiple tags to the response.
+	 * @param stats The stats request that is being responded to.
+	 * @param tags An list of tags to attach. See AddTag for how this will be processed.
+	 */
+	inline Row& AddTags(Stats::Context& stats, std::initializer_list<std::pair<std::string, std::string>>&& tags)
+	{
+		for (const auto& [name, value] : tags)
+			AddTag(stats, name, value);
+		return *this;
+	}
 };
 
 class Stats::Context final
@@ -65,6 +83,9 @@ class Stats::Context final
 	 */
 	User* const source;
 
+	/** The provider for inspircd.org/stats-* tags. */
+	ClientProtocol::MessageTagProvider& tagprov;
+
 	/** List of reply rows
 	 */
 	std::vector<Row> rows;
@@ -78,12 +99,16 @@ public:
 	 * @param src Source user of the STATS request, can be a local or remote user
 	 * @param sym Symbol (letter) indicating the type of the request
 	 */
-	Context(User* src, char sym)
+	Context(ClientProtocol::MessageTagProvider& prov, User* src, char sym)
 		: source(src)
+		, tagprov(prov)
 		, symbol(sym)
 	{
 	}
 
+	/** Retrieves the provider of inspircd.org/stats-* tags. */
+	auto& GetTagProvider() const { return tagprov; }
+
 	/** Get the source user of the STATS request
 	 * @return Source user of the STATS request
 	 */
@@ -102,25 +127,35 @@ public:
 	/** Add a row to the reply list
 	 * @param row Reply to add
 	 */
-	void AddRow(const Row& row) { rows.push_back(row); }
+	Row& AddRow(const Row& row)
+	{
+		rows.push_back(row);
+		return rows.back();
+	}
 
 	template <typename... Param>
-	void AddRow(unsigned int numeric, Param&&... p)
+	Row& AddRow(unsigned int numeric, Param&&... p)
 	{
 		Row n(numeric);
 		n.push(std::forward<Param>(p)...);
-		AddRow(n);
+		return AddRow(n);
 	}
 
 	/** Adds a row to the stats response using a generic numeric.
 	 * @param p One or more fields to add to the response.
 	 */
 	template <typename... Param>
-	void AddGenericRow(Param&&... p)
+	Row& AddGenericRow(Param&&... p)
 	{
 		Row n(RPL_STATS);
 		n.push(GetSymbol());
 		n.push(std::forward<Param>(p)...);
-		AddRow(n);
+		return AddRow(n);
 	}
 };
+
+inline Stats::Row& Stats::Row::AddTag(Stats::Context& stats, const std::string& name, const std::string& value)
+{
+	Numeric::Numeric::AddTag("inspircd.org/stats-" + name, &stats.GetTagProvider(), ClientProtocol::Message::EscapeTag(value));
+	return *this;
+}

src/coremods/core_stats.cpp

@@ -28,6 +28,7 @@
 
 #include "inspircd.h"
 #include "xline.h"
+#include "modules/cap.h"
 #include "modules/stats.h"
 
 #ifdef _WIN32
@@ -36,10 +37,33 @@
 # include <sys/resource.h>
 #endif
 
+class StatsTagsProvider
+	: public ClientProtocol::MessageTagProvider
+{
+private:
+	Cap::Capability statscap;
+
+public:
+	StatsTagsProvider(Module* mod)
+		: ClientProtocol::MessageTagProvider(mod)
+		, statscap(mod, "inspircd.org/stats-tags")
+	{
+	}
+
+	bool ShouldSendTag(LocalUser* user, const ClientProtocol::MessageTagData& tagdata) override
+	{
+		return statscap.IsEnabled(user);
+	}
+};
+
+
 class CommandStats final
 	: public Command
 {
+private:
 	Events::ModuleEventProvider statsevprov;
+	StatsTagsProvider statstags;
+
 	void DoStats(Stats::Context& stats);
 
 public:
@@ -49,6 +73,7 @@ public:
 	CommandStats(Module* Creator)
 		: Command(Creator, "STATS", 1, 2)
 		, statsevprov(Creator, "event/stats")
+		, statstags(Creator)
 	{
 		syntax = { "<symbol> [<servername>]" };
 	}
@@ -333,7 +358,7 @@ CmdResult CommandStats::Handle(User* user, const Params& parameters)
 		return CmdResult::SUCCESS;
 	}
 
-	Stats::Context stats(user, parameters[0][0]);
+	Stats::Context stats(statstags, user, parameters[0][0]);
 	DoStats(stats);
 
 	for (const auto& row : stats.GetRows())