redis: Require a maxmemory-policy of volatile-ttl

This is the assumed expectation based on our Redis configuration in
production, but as with the last commit, its better to make this an
explicit assertion.  It ensures we don't accidentally misconfigure Redis
in the future and then later run into lost sessions and missed staleness
markers by surprise.

src/redis.js

@@ -50,6 +50,23 @@ if (PRODUCTION && !REVIEW_APP && !REDIS) {
   throw new Error("REDIS_URL required in production mode");
 }
 
+if (REDIS) {
+  /* Use "info memory" command instead of "config get maxmemory-policy" since
+   * Heroku Redis disables admin commands, which includes "config".
+   */
+  const memoryInfo = new Map(
+    (await REDIS.info("memory"))
+      .split(/\r\n/)
+      .map(line => line.match(/^(?<key>[^:]+):(?<value>.*)$/)?.groups)
+      .filter(matched => matched)
+      .map(({key, value}) => [key.replace(/_/g, "-"), value])
+  );
+  const maxmemoryPolicy = memoryInfo.get("maxmemory-policy");
+  if (maxmemoryPolicy !== "volatile-ttl") {
+    throw new Error(`Redis maxmemory-policy is "${maxmemoryPolicy}", but it *must* be "volatile-ttl".`);
+  }
+}
+
 
 function herokuRedisClient(urlStr) {
   const url = new URL(urlStr);