ircd:Ⓜ️:vm: Add options over existing state fetch decisions.

include/ircd/m/vm.h

@@ -302,6 +302,18 @@ struct ircd::m::vm::opts
 	/// are missing. -1 is auto / conf.
 	size_t fetch_prev_limit = -1;
 
+	/// Considers any missing prev_event as an indication of possible missing
+	/// state from a history we don't have; allowing a state acquisition. This
+	/// is not practical to apply by default as internal decisions are better.
+	bool fetch_state_any {false};
+
+	/// This option affects the behavior for a case where we are missing the
+	/// (depth - 1) prev_events reference, thus other resolved references
+	/// are not adjacent. Yet at the claimed depth there is no apparent gap
+	/// in the timeline. If true, we assume possible missing state in this
+	/// case, but by default that is far too unrealistic in practice.
+	bool fetch_state_shallow {false};
+
 	/// Evaluators can set this value to optimize the creation of the database
 	/// transaction where the event will be stored. This value should be set
 	/// to the amount of space the event consumes; the JSON-serialized size is

matrix/vm_fetch.cc

@@ -381,17 +381,35 @@ ircd::m::vm::fetch::state(const event &event,
                           const room &room)
 try
 {
-	const event::prev prev{event};
-	if(prev.prev_exist())
-		return;
-
-	const auto &[sounding_depth, sounding_idx]
+	const auto &opts{*eval.opts};
+	const event::prev prev
 	{
-		m::sounding(room.room_id)
+		event
 	};
 
-	if(at<"depth"_>(event) > sounding_depth)
-		return;
+	const bool prev_exist
+	{
+		prev.prev_exist()
+	};
+
+	if(opts.fetch_state_any)
+		if(prev_exist && prev.prev_events_count() == prev.prev_events_exist())
+			return;
+
+	if(likely(!opts.fetch_state_any))
+		if(prev_exist)
+			return;
+
+	if(likely(!opts.fetch_state_shallow))
+	{
+		const auto &[sounding_depth, sounding_idx]
+		{
+			m::sounding(room.room_id)
+		};
+
+		if(at<"depth"_>(event) > sounding_depth)
+			return;
+	}
 
 	log::dwarning
 	{