ircd:Ⓜ️:dbs: Add event_bad column for blacklisting.

include/ircd/m/dbs.h

@@ -27,6 +27,7 @@ namespace ircd::m::dbs
 
 	// Event metadata columns
 	extern db::column event_idx;       // event_id => event_idx
+	extern db::column event_bad;       // event_id => event_idx
 	extern db::index room_events;      // room_id | depth, event_idx => state_root
 	extern db::index room_joined;      // room_id | origin, member => event_idx
 	extern db::index room_state;       // room_id | type, state_key => event_idx
@@ -107,6 +108,9 @@ namespace ircd::m::dbs::desc
 	// events index
 	extern const database::descriptor events__event_idx;
 
+	// events blacklist
+	extern const database::descriptor events__event_bad;
+
 	// room events sequence
 	extern const db::prefix_transform events__room_events__pfx;
 	extern const db::comparator events__room_events__cmp;

ircd/m/dbs.cc

@@ -26,6 +26,11 @@ decltype(ircd::m::dbs::event_idx)
 ircd::m::dbs::event_idx
 {};
 
+/// Linkage for a reference to the event_bad column.
+decltype(ircd::m::dbs::event_bad)
+ircd::m::dbs::event_bad
+{};
+
 /// Linkage for a reference to the state_node column.
 decltype(ircd::m::dbs::state_node)
 ircd::m::dbs::state_node
@@ -71,6 +76,7 @@ ircd::m::dbs::init::init(std::string dbopts)
 
 	// Cache the columns for the metadata
 	event_idx = db::column{*events, desc::events__event_idx.name};
+	event_bad = db::column{*events, desc::events__event_bad.name};
 	state_node = db::column{*events, desc::events__state_node.name};
 	room_events = db::index{*events, desc::events__room_events.name};
 	room_joined = db::index{*events, desc::events__room_joined.name};
@@ -494,6 +500,57 @@ ircd::m::dbs::desc::events__event_idx
 	false,
 };
 
+const ircd::database::descriptor
+ircd::m::dbs::desc::events__event_bad
+{
+	// name
+	"_event_bad",
+
+	// explanation
+	R"(### developer note:
+
+	The key is an event_id. The mere existence of the key indicates we do not
+	have this event; it will not be a key found in the event_idx and will not
+	have its own idx number and we know nothing else about this event.
+	
+	However the value is an optional index number of *another* event which is
+	recommended to be used when this missing event was essential. For example,
+	if other servers accept the bad event and then continue a room DAG it is
+	essential we know the last good prev reference to have an unbroken trace.
+	Such a prev reference will be recorded in this value for said use.
+
+	If this column has no value or a zero value it is being used to blacklist
+	an event_id for some other reason.
+
+	)",
+
+	// typing (key, value)
+	{
+		typeid(string_view), typeid(uint64_t)
+	},
+
+	// options
+	{},
+
+	// comparator
+	{},
+
+	// prefix transform
+	{},
+
+	// cache size
+	16_MiB, //TODO: conf
+
+	// cache size for compressed assets
+	8_MiB, //TODO: conf
+
+	// bloom filter bits
+	16,
+
+	// expect queries hit
+	false,
+};
+
 /// Prefix transform for the events__room_events. The prefix here is a room_id
 /// and the suffix is the depth+event_id concatenation.
 /// for efficient sequences
@@ -1712,6 +1769,10 @@ ircd::m::dbs::desc::events
 	// Mapping of event_id to index number.
 	events__event_idx,
 
+	// event_id => uint64_t
+	// Mapping of faulty event_id to possible alternative event_idx.
+	events__event_bad,
+
 	// (room_id, (depth, event_idx)) => (state_root)
 	// Sequence of all events for a room, ever.
 	events__room_events,