use an LRU cache for the getPolyfillMeta function to avoid too many f…

…ilesystem operations

lib/sources.js

@@ -5,6 +5,8 @@ const fs = require("graceful-fs");
 const denodeify = require("denodeify");
 const readFile = denodeify(fs.readFile);
 const readdir = denodeify(fs.readdir);
+const LRUCache = require('mnemonist/lru-cache');
+const polyfillMetaCache = new LRUCache(1000);
 
 const polyfillDirectory = path.join(__dirname, "../polyfills/__dist");
 /**
@@ -13,12 +15,16 @@ const polyfillDirectory = path.join(__dirname, "../polyfills/__dist");
  * @returns {Promise<Object|undefined>} A promise which resolves with the metadata or with `undefined` if no metadata exists for the polyfill.
  */
 function getPolyfillMeta(featureName) {
-	return readFile(
+	let meta = polyfillMetaCache.get(featureName);
+	if (meta === undefined) {
+		meta = readFile(
 			path.join(polyfillDirectory, featureName, "meta.json"),
 			"UTF-8"
-		)
-		.then(JSON.parse)
+		).then(JSON.parse)
 		.catch(() => undefined);
+		polyfillMetaCache.set(featureName, meta);
+}
+	return meta;
 }
 
 /**