ircd::mapi: Add a serial number to deal with removed modules which are still installed.

include/ircd/mods/mapi.h

@@ -30,7 +30,8 @@ namespace ircd::mapi
 	struct header;
 	struct metablock;
 	using magic_t = uint32_t;
-	using version_t = uint32_t;
+	using version_t = uint16_t;
+	using serial_t = uint16_t;
 	using meta_data = std::map<string_view, string_view, std::less<>>;
 	using init_func = std::function<void ()>;
 	using fini_func = std::function<void ()>;
@@ -63,6 +64,17 @@ IRCD_MAPI_VERSION
 	4
 };
 
+/// The serial number recorded by the module header. We increment this number
+/// after removing a module from the project because that module will still
+/// remain in the user's install directory. The removed module's serial nr
+/// will not be incremented anymore; libircd can ignore modules with serial
+/// numbers < this value.
+constexpr const ircd::mapi::serial_t
+IRCD_MAPI_SERIAL
+{
+	1
+};
+
 /// Module Header
 ///
 /// A static instance of this class must be included in an IRCd module with
@@ -74,6 +86,7 @@ struct ircd::mapi::header
 {
 	const magic_t magic {IRCD_MAPI_MAGIC};       // The magic must match
 	const version_t version {IRCD_MAPI_VERSION}; // Version indicator
+	const serial_t serial {IRCD_MAPI_SERIAL};    // Serial indicator
 	const int64_t timestamp {RB_DATECODE};       // Module's compile epoch
 	std::unique_ptr<metablock> meta;             // Non-standard-layout header data
 	mods::mod *self {nullptr};                   // Point to mod instance once loaded

ircd/mods.cc

@@ -192,6 +192,16 @@ try
 			"Unknown MAPI version [%u] expecting: [%u]", header->version, IRCD_MAPI_VERSION
 		};
 
+	if(header->serial < IRCD_MAPI_SERIAL)
+		throw error
+		{
+			"Module '%s' serial=%u is expired; expecting serial >= %u. This module was probably removed"
+			" from the project and you should delete it from the installation directory.",
+			name(),
+			header->serial,
+			IRCD_MAPI_SERIAL
+		};
+
 	// Tell the module where to find us.
 	header->self = this;