multipart: better backpressure: waiting for drain before resuming fil…

…e content

src/utils/multipart.js

@@ -8,7 +8,7 @@ const NEW_LINE_BUFFER = Buffer.from(NEW_LINE)
 
 class Multipart extends Transform {
   constructor (options) {
-    super(Object.assign({}, options, { objectMode: true }))
+    super(Object.assign({}, options, { objectMode: true, highWaterMark: 1 }))
 
     this._boundary = this._generateBoundary()
     this._files = []
@@ -90,7 +90,11 @@ class Multipart extends Transform {
     })
 
     content.on('data', (data) => {
-      this.push(data)
+      const drained = this.push(data)
+      if (!drained) {
+        content.pause()
+        this.once('drain', () => content.resume())
+      }
     })
   }
 

src/utils/send-files-stream.js

@@ -146,6 +146,10 @@ module.exports = (send, path) => {
       response.pipe(convertedResponse)
     })
 
+    // signal the multipart that the underlying stream has drained and that
+    // it can continue producing data..
+    request.on('drain', () => multipart.emit('drain'))
+
     multipart.pipe(request)
 
     callback(null, retStream)