[playground] circumvent mongodb-download out-of-date URLs

[mongodb-prebuilt](https://github.com/winfinit/mongodb-prebuilt) uses
[mongodb-download](https://github.com/mongodb-js/mongodb-download) internally
to download mongodb binaries. Unfortunately, the URLs on
https://fastdl.mongodb.org used for these binaries have changed and
`mongodb-download` doesn't seem up-to-date, cf.
mongodb-js/mongodb-download#36 and
mongodb-js/mongodb-prebuilt#59
To bypass this issue, you can manually specify the download URL by setting the
`MONGODB_DL_URI` environment variable. For instance, you can use
the following command line:
`MONGODB_DL_URI=https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-debian92-4.0.13.tgz node playground/server.js --automongo`
or
`MONGODB_DL_URI=https://fastdl.mongodb.org/osx/mongodb-osx-ssl-x86_64-4.0.13.tgz node playground/server.js --automongo`

README.md

@@ -237,5 +237,19 @@ node playground/server.js --defaultDataset my-dataset.csv --reset
 If you don't have mongodb installed, you can use the `--automongo` flag from the
 command line. It will use
 [mongodb-prebuilt](https://github.com/winfinit/mongodb-prebuilt) to download
-(the first time) and run mongo `3.6.12` and then listen on the port guessed from
+(the first time) and run mongo `4.0.13` and then listen on the port guessed from
 the `--dburi` flag.
+
+> [mongodb-prebuilt](https://github.com/winfinit/mongodb-prebuilt) uses
+> [mongodb-download](https://github.com/mongodb-js/mongodb-download) internally
+> to download mongodb binaries. Unfortunately, the URLs on
+> https://fastdl.mongodb.org used for these binaries have changed and
+> `mongodb-download` doesn't seem up-to-date, cf.
+> https://github.com/mongodb-js/mongodb-download/issues/36 and
+> https://github.com/mongodb-js/mongodb-prebuilt/issues/59
+> To bypass this issue, you can manually specify the download URL by setting the
+> `MONGODB_DL_URI` environment variable. For instance, you can use
+> the following command line:
+> `MONGODB_DL_URI=https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-debian92-4.0.13.tgz node playground/server.js --automongo`
+> or
+> `MONGODB_DL_URI=https://fastdl.mongodb.org/osx/mongodb-osx-ssl-x86_64-4.0.13.tgz node playground/server.js --automongo`

playground/mongo-server.js

@@ -2,6 +2,23 @@
 const tmp = require('tmp');
 const { MongodHelper } = require('mongodb-prebuilt');
 
+/**
+ * `mongodb-download` package uses wrong download URLS for mongo binaries
+ * (cf. https://github.com/mongodb-js/mongodb-download/issues/36)
+ * Until the issue is fixed, users can set the `MONGODB_DL_URI` environment
+ * variable to specify the download URL. In that case, we monkeypatch `mongodb-download`
+ * to use the user specified URL.
+ */
+if (process.env.MONGODB_DL_URI !== undefined) {
+  const url = require('url');
+  const { MongoDBDownload } = require('mongodb-download');
+  MongoDBDownload.prototype.getDownloadURI = function() {
+    return new Promise(function(resolve,) {
+      resolve(url.parse(process.env.MONGODB_DL_URI));
+    });
+  }
+}
+
 tmp.setGracefulCleanup(); // required to be able to remove directory correctly
 
 /**
@@ -18,7 +35,7 @@ function portFromuri(dburi) {
   return match[2];
 }
 
-async function startMongo({ dburi = 'mongodb://localhost:27017', version = '3.6.12' } = {}) {
+async function startMongo({ dburi = 'mongodb://localhost:27017', version = '4.0.13' } = {}) {
   const port = portFromuri(dburi);
   if (port === null) {
     throw new Error(