Add refresh-project-summary periodic task.

It listens for events that might indicate that the project_summary
materialised view needs to be refreshed and schedules a refresh.

Refreshes are throttled to a maximum of one every throttlePeriod
milliseconds so that things don't get too crazy for instance when
importing a lot of data.

lib/www/server/periodic-tasks/tasks/index.js

@@ -1,4 +1,5 @@
 
 module.exports = [
-	require('./purge-notifications')
+	require('./purge-notifications'),
+	require('./refresh-project-summary')
 ];

lib/www/server/periodic-tasks/tasks/refresh-project-summary.js

@@ -0,0 +1,85 @@
+const db = require('../../lib/db');
+const { listen } = require('../../lib/db/notify');
+const { ALERT, ERROR, WARNING, NOTICE, INFO, DEBUG } = require('DOUGAL_ROOT/debug')(__filename);
+
+const timeout = 30*1000;
+
+async function init () {
+	INFO("Setting up task");
+
+	// Full list of channels in
+	// ../lib/db/channels
+	const channels = [
+		"project",
+		"preplot_lines", "preplot_points",
+		"raw_lines", "raw_shots",
+		"final_lines", "final_shots"
+	];
+
+	const throttlePeriod = 10*1000;
+
+	const projects = {};
+
+	listen (channels, (data) => {
+		// Something important has changed,
+		// set the dirty flag for the relevant project
+		const pid = data.payload?.pid ?? data.payload?.new?.pid ?? data.payload?.old?.pid;
+		if (pid) {
+			if (!pid in projects) {
+				projects[pid] = {
+					lastRefreshed: 0
+				};
+			}
+			if (!projects[pid].needsRefresh) {
+				projects[pid].needsRefresh = true;
+				DEBUG("Setting up refresh flag for %s: %j", pid, projects[pid]);
+			}
+		}
+	});
+
+	const task = async () => {
+		for (pid in projects) {
+			const project = projects[pid];
+			if (project.needsRefresh) {
+				const now = Date.now();
+				const lastRefreshAge = now - project.lastRefreshed;
+				if (lastRefreshAge > throttlePeriod) {
+					// Do the actual refresh
+					try {
+						DEBUG("Refreshing", pid);
+						await db.project.summary.refresh(pid);
+					} catch (err) {
+						if (err.status == 404) {
+							DEBUG("Project %s not found. Removing from refresh list", pid);
+							delete projects[pid];
+						} else {
+							ERROR(err);
+						}
+					}
+					project.needsRefresh = false;
+					project.lastRefreshed = now;
+				}
+			}
+		}
+	};
+
+	// Let us populate the project list and do a first refresh on startup
+	for (const project of await db.project.get()) {
+		projects[project.pid] = {
+			lastRefreshed: 0,
+			needsRefresh: true
+		}
+	}
+	task(); // No need to await
+
+	return task;
+}
+
+async function cleanup () {
+}
+
+module.exports = {
+	init,
+	timeout,
+	cleanup
+};