fix: limit concurrent HTTP requests in browser

Adds limit of concurrent HTTP requests sent to remote API
by dns and preload calls when running in web browser contexts.

Browsers limit connections per host (~6). This change mitigates the
problem of expensive and long running calls of one type exhausting
connection pool for other uses.

It additionally limits the number of DNS lookup calls by introducing
time-bound cache with eviction rules following what browser already do.

This is similar to:
libp2p/js-libp2p-delegated-content-routing#12

License: MIT
Signed-off-by: Marcin Rataj <lidel@lidel.org>

package.json

@@ -151,6 +151,7 @@
     "multihashes": "~0.4.14",
     "multihashing-async": "~0.6.0",
     "node-fetch": "^2.3.0",
+    "p-queue": "^6.1.0",
     "peer-book": "~0.9.0",
     "peer-id": "~0.12.3",
     "peer-info": "~0.15.0",

src/core/runtime/dns-browser.js

@@ -1,33 +1,57 @@
-/* global self */
+/* eslint-env browser */
 'use strict'
 
+const TLRU = require('../../utils/tlru')
+const { default: PQueue } = require('p-queue')
+
+// Avoid sending multiple queries for the same hostname by caching results
+const cache = new TLRU(1000)
+// TODO: /api/v0/dns does not return TTL yet: https://github.com/ipfs/go-ipfs/issues/5884
+// However we know browsers themselves cache DNS records for at least 1 minute,
+// which acts a provisional default ttl: https://stackoverflow.com/a/36917902/11518426
+const ttl = 60 * 1000
+
+// browsers limit concurrent connections per host,
+// we don't want preload calls to exhaust the limit (~6)
+const _httpQueue = new PQueue({ concurrency: 4 })
+
+function unpackResponse (domain, response, callback) {
+  if (response.Path) {
+    return callback(null, response.Path)
+  } else {
+    const err = new Error(response.Message)
+    return callback(err)
+  }
+}
+
 module.exports = (domain, opts, callback) => {
   if (typeof opts === 'function') {
     callback = opts
     opts = {}
   }
-
   opts = opts || {}
-
   domain = encodeURIComponent(domain)
-  let url = `https://ipfs.io/api/v0/dns?arg=${domain}`
 
+  if (cache.has(domain)) {
+    const response = cache.get(domain)
+    return unpackResponse(domain, response, callback)
+  }
+
+  let url = `https://ipfs.io/api/v0/dns?arg=${domain}`
   Object.keys(opts).forEach(prop => {
     url += `&${encodeURIComponent(prop)}=${encodeURIComponent(opts[prop])}`
   })
 
-  self.fetch(url, { mode: 'cors' })
+  _httpQueue.add(() => fetch(url, { mode: 'cors' })
     .then((response) => {
       return response.json()
     })
     .then((response) => {
-      if (response.Path) {
-        return callback(null, response.Path)
-      } else {
-        return callback(new Error(response.Message))
-      }
+      cache.set(domain, response, ttl)
+      return unpackResponse(domain, response, callback)
     })
     .catch((error) => {
       callback(error)
     })
+  )
 }

src/core/runtime/preload-browser.js

@@ -1,18 +1,23 @@
 /* eslint-env browser */
 'use strict'
 
+const { default: PQueue } = require('p-queue')
 const debug = require('debug')
 
 const log = debug('ipfs:preload')
 log.error = debug('ipfs:preload:error')
 
+// browsers limit concurrent connections per host,
+// we don't want preload calls to exhaust the limit (~6)
+const _httpQueue = new PQueue({ concurrency: 4 })
+
 module.exports = function preload (url, callback) {
   log(url)
 
   const controller = new AbortController()
   const signal = controller.signal
 
-  fetch(url, { signal })
+  _httpQueue.add(() => fetch(url, { signal })
     .then(res => {
       if (!res.ok) {
         log.error('failed to preload', url, res.status, res.statusText)
@@ -22,6 +27,7 @@ module.exports = function preload (url, callback) {
     })
     .then(() => callback())
     .catch(callback)
+  ).catch(callback)
 
   return {
     cancel: () => controller.abort()