util.async: Add some more comments for clarity

util/async.lua

@@ -7,6 +7,8 @@ local function runner_continue(thread)
 	end
 	local ok, state, runner = coroutine.resume(thread);
 	if not ok then
+		-- Running the coroutine failed, which means we have to find the runner manually,
+		-- in order to inform the error handler
 		local level = 0;
 		while debug.getinfo(thread, level, "") do level = level + 1; end
 		ok, runner = debug.getlocal(thread, level-1, 1);
@@ -99,37 +101,48 @@ local function runner(func, watchers, data)
 	, runner_mt);
 end
 
+-- Add a task item for the runner to process
 function runner_mt:run(input)
 	if input ~= nil then
 		table.insert(self.queue, input);
 	end
 	if self.state ~= "ready" then
+		-- The runner is busy. Indicate that the task item has been
+		-- queued, and return information about the current runner state
 		return true, self.state, #self.queue;
 	end
 
 	local q, thread = self.queue, self.thread;
 	if not thread or coroutine.status(thread) == "dead" then
+		-- Create a new coroutine for this runner
 		thread = runner_create_thread(self.func, self);
 		self.thread = thread;
 	end
 
+	-- Process task item(s) while the queue is not empty, and we're not blocked
 	local n, state, err = #q, self.state, nil;
 	self.state = "running";
 	while n > 0 and state == "ready" do
 		local consumed;
+		-- Loop through queue items, and attempt to run them
 		for i = 1,n do
 			local input = q[i];
 			local ok, new_state = coroutine.resume(thread, input);
 			if not ok then
+				-- There was an error running the coroutine, save the error, mark runner as ready to begin again
 				consumed, state, err = i, "ready", debug.traceback(thread, new_state);
 				self.thread = nil;
 				break;
 			elseif new_state == "wait" then
+				 -- Runner is blocked on waiting for a task item to complete
 				consumed, state = i, "waiting";
 				break;
 			end
 		end
+		-- Loop ended - either queue empty because all tasks passed without blocking (consumed == nil)
+		-- or runner is blocked/errored, and consumed will contain the number of tasks processed so far
 		if not consumed then consumed = n; end
+		-- Remove consumed items from the queue array
 		if q[n+1] ~= nil then
 			n = #q;
 		end
@@ -138,6 +151,7 @@ function runner_mt:run(input)
 		end
 		n = #q;
 	end
+	-- Runner processed all items it can, so save current runner state
 	self.state = state;
 	if err or state ~= self.notified_state then
 		if err then
@@ -151,6 +165,7 @@ function runner_mt:run(input)
 	return true, state, n;
 end
 
+-- Add a task item to the queue without invoking the runner, even if it is idle
 function runner_mt:enqueue(input)
 	table.insert(self.queue, input);
 end