From 5a7ffcf499202d57266c71597c9ed7461ecfcdd8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Garc=C3=ADa=20Aubert?=
 <danielgarciaaubert@gmail.com>
Date: Tue, 15 Oct 2019 12:48:50 +0200
Subject: [PATCH] Be able to synchronize the TTL of cache-control header to
 expire in a coherent way

---
 lib/api/middlewares/cache-control-header.js   | 35 +++++++++++++++++--
 .../cache/cache-control-header-test.js        | 14 ++++++--
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/lib/api/middlewares/cache-control-header.js b/lib/api/middlewares/cache-control-header.js
index d15b259c..a72bdea3 100644
--- a/lib/api/middlewares/cache-control-header.js
+++ b/lib/api/middlewares/cache-control-header.js
@@ -1,14 +1,35 @@
 'use strict';
 
-const ONE_YEAR_IN_SECONDS = 60 * 60 * 24 * 365;
-const FIVE_MINUTES_IN_SECONDS = 60 * 5;
+const ONE_MINUTE_IN_SECONDS = 60;
+const THREE_MINUTE_IN_SECONDS = 60 * 3;
+const FIVE_MINUTES_IN_SECONDS = ONE_MINUTE_IN_SECONDS * 5;
+const TEN_MINUTES_IN_SECONDS = ONE_MINUTE_IN_SECONDS * 10;
+const FIFTEEN_MINUTES_IN_SECONDS = ONE_MINUTE_IN_SECONDS * 15;
+const THIRTY_MINUTES_IN_SECONDS = ONE_MINUTE_IN_SECONDS * 30;
+const ONE_HOUR_IN_SECONDS = ONE_MINUTE_IN_SECONDS * 60;
+const ONE_YEAR_IN_SECONDS = ONE_HOUR_IN_SECONDS * 24 * 365;
+
 const FALLBACK_TTL = global.environment.varnish.fallbackTtl || FIVE_MINUTES_IN_SECONDS;
 
+const validFallbackTTL = [
+    ONE_MINUTE_IN_SECONDS,
+    THREE_MINUTE_IN_SECONDS,
+    FIVE_MINUTES_IN_SECONDS,
+    TEN_MINUTES_IN_SECONDS,
+    FIFTEEN_MINUTES_IN_SECONDS,
+    THIRTY_MINUTES_IN_SECONDS,
+    ONE_HOUR_IN_SECONDS
+];
+
 module.exports = function setCacheControlHeader ({
     ttl = ONE_YEAR_IN_SECONDS,
     fallbackTtl = FALLBACK_TTL,
     revalidate = false
 } = {}) {
+    if (!validFallbackTTL.includes(fallbackTtl)) {
+        throw new Error(`Invalid fallback TTL value for Cache-Control header. Got ${fallbackTtl}, expected ${validFallbackTTL.join(', ')}`);
+    }
+
     return function setCacheControlHeaderMiddleware (req, res, next) {
         if (req.method !== 'GET') {
             return next();
@@ -27,7 +48,7 @@ module.exports = function setCacheControlHeader ({
             if (everyAffectedTableCanBeInvalidated(affectedTables)) {
                 directives.push(`max-age=${ttl}`);
             } else {
-                directives.push(`max-age=${fallbackTtl}`);
+                directives.push(`max-age=${computeNextTTL({ ttlInSeconds: fallbackTtl })}`);
             }
 
             if (revalidate) {
@@ -49,3 +70,11 @@ function everyAffectedTableCanBeInvalidated (affectedTables) {
         affectedTables.getTables(skipNotUpdatedAtTables, skipAnalysisCachedTables)
             .every(table => table.updated_at !== null);
 }
+
+function computeNextTTL ({ ttlInSeconds } = {}) {
+    const nowInSeconds = Math.ceil(Date.now() / 1000);
+    const secondsAfterPreviousTTLStep = nowInSeconds % ttlInSeconds;
+    const secondsToReachTheNextTTLStep = ttlInSeconds - secondsAfterPreviousTTLStep;
+
+    return secondsToReachTheNextTTLStep;
+}
diff --git a/test/acceptance/cache/cache-control-header-test.js b/test/acceptance/cache/cache-control-header-test.js
index d3b89863..d761b6ef 100644
--- a/test/acceptance/cache/cache-control-header-test.js
+++ b/test/acceptance/cache/cache-control-header-test.js
@@ -79,7 +79,12 @@ describe('cache-control header', function () {
                     return done(err);
                 }
 
-                assert.equal(res.headers['cache-control'], `public,max-age=${ttl}`);
+                const cacheControl = res.headers['cache-control'];
+                const [ type, maxAge ] = cacheControl.split(',');
+                const [ key, value ] = maxAge.split('=');
+
+                assert.ok(Number(value) <= ttl);
+
                 testClient.drain(done);
             });
         });
@@ -114,7 +119,12 @@ describe('cache-control header', function () {
                     return done(err);
                 }
 
-                assert.equal(res.headers['cache-control'], `public,max-age=${ttl}`);
+                const cacheControl = res.headers['cache-control'];
+                const [ type, maxAge ] = cacheControl.split(',');
+                const [ key, value ] = maxAge.split('=');
+
+                assert.ok(Number(value) <= ttl);
+
                 testClient.drain(done);
             });
         });