Prevent multiple concurrent requests for package info

server/PackageInfo.js

@@ -0,0 +1,68 @@
+require('isomorphic-fetch')
+const PackageInfoCache = require('./PackageInfoCache')
+const createMutex = require('./createMutex')
+
+const RegistryURL = process.env.NPM_REGISTRY_URL || 'https://registry.npmjs.org'
+
+function fetchPackageInfo(packageName) {
+  console.log(`Fetching info for package ${packageName} from NPM`)
+
+  let encodedPackageName
+  if (packageName.charAt(0) === '@') {
+    encodedPackageName = `@${encodeURIComponent(packageName.substring(1))}`
+  } else {
+    encodedPackageName = encodeURIComponent(packageName)
+  }
+
+  const url = `${RegistryURL}/${encodedPackageName}`
+
+  return fetch(url, {
+    headers: { 'Accept': 'application/json' }
+  }).then(function (response) {
+    return response.status === 404 ? null : response.json()
+  })
+}
+
+const PackageNotFound = 'PackageNotFound'
+
+// This mutex prevents multiple concurrent requests to
+// the registry for the same package info.
+const fetchMutex = createMutex(function (packageName, callback) {
+  fetchPackageInfo(packageName).then(function (value) {
+    if (value == null) {
+      // Cache 404s for 5 minutes. This prevents us from making
+      // unnecessary requests to the registry for bad package names.
+      // In the worst case, a brand new package's info will be
+      // available within 5 minutes.
+      PackageInfoCache.set(packageName, PackageNotFound, 300, function () {
+        callback(null, value)
+      })
+    } else {
+      // Cache valid package info for one minute.
+      PackageInfoCache.set(packageName, value, 60, function () {
+        callback(null, value)
+      })
+    }
+  }, function (error) {
+    // Do not cache errors.
+    PackageInfoCache.del(packageName, function () {
+      callback(error)
+    })
+  })
+})
+
+function getPackageInfo(packageName, callback) {
+  PackageInfoCache.get(packageName, function (error, value) {
+    if (error) {
+      callback(error)
+    } else if (value) {
+      callback(null, value === PackageNotFound ? null : value)
+    } else {
+      fetchMutex(packageName, callback)
+    }
+  })
+}
+
+module.exports = {
+  get: getPackageInfo
+}