ircd:Ⓜ️:vm::fetch: Eliminate the remaining coarse sleep for prev fetch+eval.

matrix/vm_fetch.cc

@@ -13,7 +13,7 @@ namespace ircd::m::vm::fetch
 	static void prev_check(const event &, vm::eval &);
 	static bool prev_wait(const event &, vm::eval &);
 	static std::forward_list<ctx::future<m::fetch::result>> prev_fetch(const event &, vm::eval &, const room &);
-	static void prev_eval(const event &, vm::eval &, ctx::future<m::fetch::result> &);
+	static void prev_eval(const event &, vm::eval &, ctx::future<m::fetch::result> &, const system_point &);
 	static void prev(const event &, vm::eval &, const room &);
 	static std::forward_list<ctx::future<m::fetch::result>> state_fetch(const event &, vm::eval &, const room &);
 	static void state(const event &, vm::eval &, const room &);
@@ -22,9 +22,8 @@ namespace ircd::m::vm::fetch
 	static void auth(const event &, vm::eval &, const room &);
 	static void handle(const event &, vm::eval &);
 
-	extern conf::item<milliseconds> prev_fetch_check_interval;
+	extern conf::item<milliseconds> prev_preempt_time;
 	extern conf::item<milliseconds> prev_wait_time;
-	extern conf::item<size_t> prev_wait_count;
 	extern conf::item<size_t> prev_backfill_limit;
 	extern conf::item<seconds> event_timeout;
 	extern conf::item<seconds> state_timeout;
@@ -104,25 +103,18 @@ ircd::m::vm::fetch::prev_backfill_limit
 	{ "default",  128L                                  },
 };
 
-decltype(ircd::m::vm::fetch::prev_wait_count)
-ircd::m::vm::fetch::prev_wait_count
-{
-	{ "name",     "ircd.m.vm.fetch.prev.wait.count" },
-	{ "default",  4L                                },
-};
-
 decltype(ircd::m::vm::fetch::prev_wait_time)
 ircd::m::vm::fetch::prev_wait_time
 {
 	{ "name",     "ircd.m.vm.fetch.prev.wait.time" },
-	{ "default",  200L                             },
+	{ "default",  750L                             },
 };
 
-decltype(ircd::m::vm::fetch::prev_fetch_check_interval)
-ircd::m::vm::fetch::prev_fetch_check_interval
+decltype(ircd::m::vm::fetch::prev_preempt_time)
+ircd::m::vm::fetch::prev_preempt_time
 {
-	{ "name",     "ircd.m.vm.fetch.prev.fetch.check_interval" },
-	{ "default",  500L                                        },
+	{ "name",     "ircd.m.vm.fetch.prev.preempt.time" },
+	{ "default",  5000L                               },
 };
 
 //
@@ -508,11 +500,13 @@ ircd::m::vm::fetch::prev(const event &event,
 
 	if(!m::vm::fetch::enable)
 	{
+		// No fetches will take place; only check if satisfied.
 		prev_check(event, eval);
 		return;
 	}
 
-	auto futures
+	// Launch fetches for missing prev events
+	auto fetching
 	{
 		prev_fetch(event, eval, room)
 	};
@@ -526,43 +520,62 @@ ircd::m::vm::fetch::prev(const event &event,
 		at<"depth"_>(event),
 		prev_count,
 		prev_count - prev_exists,
-		std::distance(begin(futures), end(futures)),
-	};
-
-	auto fetching
-	{
-		ctx::when_all(begin(futures), end(futures))
-	};
-
-	const auto timeout
-	{
-		now<system_point>() + seconds(event_timeout)
-	};
-
-	const milliseconds &check_interval
-	{
-		prev_fetch_check_interval
+		std::distance(begin(fetching), end(fetching)),
 	};
 
 	// Rather than waiting for all of the events to arrive or for the entire
 	// timeout to expire, we check if the sought events made it to the server
-	// in the meantime. If so we can drop these requests and bail.
-	//TODO: Ideally should be replaced with listener/notification/hook on the
-	//TODO: events arriving rather than this coarse sleep cycles.
-	while(now<system_point>() < timeout)
-	{
-		// Wait for an interval to give this loop some iterations.
-		if(fetching.wait(check_interval, std::nothrow))
-			break;
+	// in the meantime. If so we can drop these fetches and bail.
+	std::optional<vm::notify::future> evaling[prev_count];
+	for(size_t i(0); i < prev_count; ++i)
+		evaling[i].emplace(prev.prev_event(i));
 
-		// Check for satisfaction.
-		if(prev.prev_events_exist() == prev_count)
-			return;
+	// Either all of the fetches are done and we can start evaluating or all
+	// of the events arrived elsehow and we don't need any of the fetches.
+	// XXX: Ideally this could be refactored with mix-and-match granularity
+	// but at this time it's unknown if there's practical benefit.
+	ctx::future<> when[]
+	{
+		ctx::when_all(begin(fetching), end(fetching)),
+		ctx::when_all(evaling, evaling + prev_count, []
+		(auto &optional) -> ctx::future<> &
+		{
+			return optional->value();
+		}),
+	};
+
+	// Represents one of the two outcomes.
+	auto future
+	{
+		ctx::when_any(begin(when), end(when))
+	};
+
+	const auto prev_wait_until
+	{
+		now<system_point>() + milliseconds(prev_preempt_time)
+	};
+
+	// Wait for one of the two outcomes.
+	const bool finished
+	{
+		future.wait_until(prev_wait_until, std::nothrow)
+	};
+
+	// Check for satisfaction.
+	if(prev.prev_events_exist() == prev_count)
+	{
+		assert(finished);
+		return;
 	}
 
-	// evaluate results
-	for(auto &future : futures)
-		prev_eval(event, eval, future);
+	const auto event_wait_until
+	{
+		now<system_point>() + seconds(event_timeout)
+	};
+
+	// If we're not satisfied we commit to evaluating the fetches.
+	for(auto &fetch : fetching)
+		prev_eval(event, eval, fetch, event_wait_until);
 
 	// check if result evals have satisfied this eval now; or throw
 	prev_check(event, eval);
@@ -571,12 +584,13 @@ ircd::m::vm::fetch::prev(const event &event,
 void
 ircd::m::vm::fetch::prev_eval(const event &event,
                               vm::eval &eval,
-                              ctx::future<m::fetch::result> &future)
+                              ctx::future<m::fetch::result> &future,
+                              const system_point &until)
 try
 {
 	m::fetch::result result
 	{
-		future.get()
+		future.get_until(until)
 	};
 
 	const json::object content