Retry when querying Azure Pipelines' artifacts

We encountered a problem where Azure Pipelines would return a 503. This
is obviously a transient error, and we want to be gentle and nice about
it and just retry a couple times, with a nice and easy exponential
back-off.

Sadly, `node-fetch` does not provide that functionality out of the box.
But there is a small wrapper around it that does. We just need to add
Typescript declarations, and use it, and we're all set.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.vscode/settings.json

@@ -10,6 +10,7 @@
         "vstfs://.*"
     ],
     "cSpell.words": [
+        "Backoff",
         "MSYSTEM",
         "Pacman",
         "autodrain",

src/downloader.ts

@@ -4,7 +4,7 @@ import {Readable} from 'stream'
 import unzipper from 'unzipper'
 import {spawn} from 'child_process'
 import {delimiter} from 'path'
-import fetch from 'node-fetch'
+import fetch from '@adobe/node-fetch-retry'
 
 const gitForWindowsUsrBinPath = 'C:/Program Files/Git/usr/bin'
 const gitForWindowsMINGW64BinPath = 'C:/Program Files/Git/mingw64/bin'

typings/@adobe/node-fetch-retry/index.d.ts

@@ -0,0 +1,26 @@
+// Type definitions for node-fetch 1.1.0
+// Project: https://github.com/adobe/node-fetch-retry
+
+/// <reference types="node" />
+
+declare module '@adobe/node-fetch-retry' {
+  import {RequestInfo, RequestInit, Response} from 'node-fetch'
+
+  interface RequestInitWithRetry extends RequestInit {
+    retryOptions?: {
+      retryMaxDuration?: number
+      retryInitialDelay?: number
+      retryBackoff?: number
+      retryOnHttpResponse?: (response: Response) => boolean
+      socketTimeout?: number
+      forceSocketTimeout?: boolean
+    }
+  }
+
+  function fetch(
+    url: RequestInfo,
+    init?: RequestInitWithRetry
+  ): Promise<Response>
+
+  export default fetch
+}