Fix freeze on pad deletion

src/node/db/Pad.js

@@ -16,6 +16,7 @@ var readOnlyManager = require("./ReadOnlyManager");
 var crypto = require("crypto");
 var randomString = require("../utils/randomstring");
 var hooks = require('ep_etherpad-lite/static/js/pluginfw/hooks');
+var promises = require('../utils/promises')
 
 // serialization/deserialization attributes
 var attributeBlackList = ["id"];
@@ -482,14 +483,14 @@ Pad.prototype.remove = async function remove() {
   db.remove("readonly2pad:" + readonlyID);
 
   // delete all chat messages
-  for (let i = 0, n = this.chatHead; i <= n; ++i) {
-    db.remove("pad:" + padID + ":chat:" + i);
-  }
+  promises.timesLimit(this.chatHead + 1, 500, function (i) {
+    return db.remove("pad:" + padID + ":chat:" + i, null);
+  })
 
   // delete all revisions
-  for (let i = 0, n = this.head; i <= n; ++i) {
-    db.remove("pad:" + padID + ":revs:" + i);
-  }
+  promises.timesLimit(this.head + 1, 500, function (i) {
+    return db.remove("pad:" + padID + ":revs:" + i, null);
+  })
 
   // remove pad from all authors who contributed
   this.getAllAuthors().forEach(authorID => {

src/node/utils/promises.js

@@ -0,0 +1,32 @@
+/**
+ * Helpers to manipulate promises (like async but for promises).
+ */
+
+var timesLimit = function (ltMax, concurrency, promiseCreator) {
+  var done = 0
+  var current = 0
+
+  function addAnother () {
+    function _internalRun () {
+      done++
+
+      if (done < ltMax) {
+        addAnother()
+      }
+    }
+
+    promiseCreator(current)
+      .then(_internalRun)
+      .catch(_internalRun)
+
+    current++
+  }
+
+  for (var i = 0; i < concurrency && i < ltMax; i++) {
+    addAnother()
+  }
+}
+
+module.exports = {
+  timesLimit: timesLimit
+}