From fd90538fcc63fa55be516fa29c53ca82c5c98a23 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Thu, 18 Jun 2020 22:15:29 +0200
Subject: blog: Fix rendering race condition with multiple clients

---
 modules/blog/blog.js | 43 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 38 insertions(+), 5 deletions(-)

diff --git a/modules/blog/blog.js b/modules/blog/blog.js
index 0b7785b..b6085d9 100644
--- a/modules/blog/blog.js
+++ b/modules/blog/blog.js
@@ -9,7 +9,23 @@ let moddir = null;
 let repodir = null;
 const repoRemote = "https://git.tomsmeding.com/blog";
 
+// Cache for rendered posts; null if currently being rendered
 let templateCache = new Map();
+// Clients that requested a post that is currently being rendered; values are
+// callbacks that take (http status code, rendered html [= null if status != 200])
+let renderWatchers = new Map();
+
+function triggerRenderWatchers(path, statusCode, rendered) {
+	const list = renderWatchers.get(path);
+	if (list !== undefined) {
+		try {
+			for (const callback of list) callback(statusCode, rendered);
+		} catch(e) {
+			console.error("While triggering render watchers:", e);
+		}
+		renderWatchers.delete(path);
+	}
+}
 
 function fetch(url) {
 	return new Promise((resolve, reject) => {
@@ -111,21 +127,38 @@ module.exports = (app, io, _moddir) => {
 			return;
 		}
 
-		const path = req.path.slice(6).replace(/\.html$/, "");
-
-		if (templateCache.has(path)) {
-			res.send(templateCache.get(path));
+		const path = req.path
+				.slice(6)
+				.replace(/\/[\/]*/g, "/")
+				.replace(/\.html$/, "");
+
+		const fromCache = templateCache.get(path);
+
+		if (fromCache != null) {  // neither null nor undefined
+			res.send(fromCache);
+		} else if (fromCache !== undefined) {
+			// Is currently being renderered for another client
+			if (!renderWatchers.has(path)) renderWatchers.set(path, []);
+			renderWatchers.get(path).push((statusCode, rendered) => {
+				if (statusCode == 200) res.send(rendered);
+				else res.sendStatus(statusCode);
+			});
 		} else {
+			// Indicate that this path is currently being rendered
+			templateCache.set(path, null);
+
 			generateTemplate(repodir, path ? path + ".html" : undefined)
 				.then(rendered => {
-					// TODO: fix rendering race condition
 					templateCache.set(path, rendered);
+					triggerRenderWatchers(path, 200, rendered);
 					res.send(rendered);
 				})
 				.catch(err => {
 					if (err.code && err.code == "ENOENT") {
+						triggerRenderWatchers(path, 400, null);
 						res.sendStatus(404);
 					} else {
+						triggerRenderWatchers(path, 500, null);
 						console.error(err);
 						res.sendStatus(500);
 					}
-- 
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