Persist DHT nodes

Persists DHT nodes to disk every 15 minutes.
Loads persisted nodes on construction.
Enabled by default.

Default save file is dht.json,
under crossplatform app data folder provided by app-data-folder.

Adds option dhtState to configure:
* false disables
* true enables with default path
* String specifies path to custom save file

docs/api.md

@@ -53,12 +53,13 @@ If `opts` is specified, then the default options (shown below) will be overridde
 
 ```js
 {
-  maxConns: Number,        // Max number of connections per torrent (default=55)
-  nodeId: String|Buffer,   // DHT protocol node ID (default=randomly generated)
-  peerId: String|Buffer,   // Wire protocol peer ID (default=randomly generated)
-  tracker: Boolean|Object, // Enable trackers (default=true), or options object for Tracker
-  dht: Boolean|Object,     // Enable DHT (default=true), or options object for DHT
-  webSeeds: Boolean        // Enable BEP19 web seeds (default=true)
+  maxConns: Number,         // Max number of connections per torrent (default=55)
+  nodeId: String|Buffer,    // DHT protocol node ID (default=randomly generated)
+  peerId: String|Buffer,    // Wire protocol peer ID (default=randomly generated)
+  tracker: Boolean|Object,  // Enable trackers (default=true), or options object for Tracker
+  dht: Boolean|Object,      // Enable DHT (default=true), or options object for DHT
+  dhtState: Boolean|String, // Persist DHT nodes (default=true), or save file path
+  webSeeds: Boolean         // Enable BEP19 web seeds (default=true)
 }
 ```
 

index.js

@@ -2,15 +2,18 @@
 
 module.exports = WebTorrent
 
+var appDataFolder = require('app-data-folder')
 var Buffer = require('safe-buffer').Buffer
 var concat = require('simple-concat')
 var createTorrent = require('create-torrent')
 var debug = require('debug')('webtorrent')
 var DHT = require('bittorrent-dht/client') // browser exclude
 var EventEmitter = require('events').EventEmitter
 var extend = require('xtend')
+var fs = require('fs')
 var inherits = require('inherits')
 var loadIPSet = require('load-ip-set') // browser exclude
+var mkdirp = require('mkdirp')
 var parallel = require('run-parallel')
 var parseTorrent = require('parse-torrent')
 var path = require('path')
@@ -78,6 +81,9 @@ function WebTorrent (opts) {
   }
   self.nodeIdBuffer = Buffer.from(self.nodeId, 'hex')
 
+  // Default opts.dhtState to true
+  if (!('dhtState' in opts)) opts.dhtState = true
+
   self._debugId = self.peerId.toString('hex').substring(0, 7)
 
   self.destroyed = false
@@ -122,9 +128,102 @@ function WebTorrent (opts) {
   self._downloadSpeed = speedometer()
   self._uploadSpeed = speedometer()
 
+  // DHT state save location
+  var dhtSaveFile
+
+  var savingDhtState = false
+  function saveDhtState () {
+    if (savingDhtState) return
+    if (!self.dht) return // Quell after destroy
+    savingDhtState = true
+    var dhtState = self.dht.toJSON()
+    var dhtStateJson = JSON.stringify(dhtState)
+    mkdirp(
+      path.dirname(dhtSaveFile),
+      function handleDhtSaveDirCreated (err) {
+        if (err) {
+          savingDhtState = false
+          return
+        }
+        fs.writeFile(
+          dhtSaveFile,
+          dhtStateJson,
+          function handleDhtStateWritten () {
+            savingDhtState = false
+          }
+        )
+      }
+    )
+  }
+
+  function readDhtState (file) {
+    try {
+      return fs.readFileSync(file)
+    } catch (e) {
+      switch (e.code) {
+        case 'EACCES':
+        case 'EISDIR':
+        case 'ENOENT':
+        case 'EPERM':
+          return null
+        default:
+          throw e
+      }
+    }
+  }
+
+  function parseDhtState (dhtStateJson) {
+    try {
+      return JSON.parse(dhtStateJson)
+    } catch (e) {
+      if (e instanceof SyntaxError) return null
+      else throw e
+    }
+  }
+
+  function loadDhtState (file) {
+    var dhtStateJson = readDhtState(file)
+    if (!dhtStateJson) return null
+    var dhtState = parseDhtState(dhtStateJson)
+    if (!dhtState) return null
+    return dhtState
+  }
+
+  function loadDhtNodes (file) {
+    var dhtState = loadDhtState(file)
+    if (!dhtState) return null
+    if (!('nodes' in dhtState)) return null
+    var nodes = dhtState.nodes
+    if (!Array.isArray(nodes)) return null
+    if (nodes.length === 0) return null // Don't load an empty nodes list
+    return nodes
+  }
+
   if (opts.dht !== false && typeof DHT === 'function' /* browser exclude */) {
+    var dhtOpts = extend({ nodeId: self.nodeId }, opts.dht)
+
+    if (opts.dhtState) {
+      // Construct state save location
+      dhtSaveFile =
+        opts.dhtState === true
+          ? path.join(appDataFolder('webtorrent'), 'dht.json')
+          : opts.dhtState
+
+      if (!('bootstrap' in dhtOpts)) {
+        // Load persisted state
+        var nodes = loadDhtNodes(dhtSaveFile)
+        if (nodes) dhtOpts.bootstrap = nodes
+      }
+    }
+
     // use a single DHT instance for all torrents, so the routing table can be reused
-    self.dht = new DHT(extend({ nodeId: self.nodeId }, opts.dht))
+    self.dht = new DHT(dhtOpts)
+
+    if (opts.dhtState) {
+      // Persist state periodically
+      var saveInterval = 15 * 60 * 1000 // 15 minutes
+      self.saveDhtStateTimer = setInterval(saveDhtState, saveInterval)
+    }
 
     self.dht.once('error', function (err) {
       self._destroy(err)
@@ -428,6 +527,7 @@ WebTorrent.prototype._destroy = function (err, cb) {
 
   if (self.dht) {
     tasks.push(function (cb) {
+      clearInterval(self.saveDhtStateTimer)
       self.dht.destroy(cb)
     })
   }

package.json

@@ -27,6 +27,7 @@
   },
   "dependencies": {
     "addr-to-ip-port": "^1.4.2",
+    "app-data-folder": "^1.0.0",
     "bitfield": "^2.0.0",
     "bittorrent-dht": "^8.0.0",
     "bittorrent-protocol": "^2.1.5",
@@ -40,6 +41,7 @@
     "load-ip-set": "^1.2.7",
     "memory-chunk-store": "^1.2.0",
     "mime": "^2.2.0",
+    "mkdirp": "^0.5.1",
     "multistream": "^2.0.5",
     "package-json-versionify": "^1.0.2",
     "parse-numeric-range": "^0.0.2",