Ensure streams are always drained on close or error

services/filestore/app/js/FileController.js

@@ -61,9 +61,6 @@ function getFile(req, res, next) {
     }
 
     pipeline(fileStream, res, err => {
-      if (!fileStream.destroyed) {
-        fileStream.destroy()
-      }
       if (err && err.code === 'ERR_STREAM_PREMATURE_CLOSE') {
         res.end()
       } else if (err) {

services/filestore/app/js/PersistorHelper.js

@@ -23,6 +23,7 @@ class ObserverStream extends Stream.Transform {
     if (options.hash) {
       this.hash = crypto.createHash(options.hash)
     }
+
     if (options.metric) {
       const onEnd = () => {
         metrics.count(options.metric, this.bytes)
@@ -98,13 +99,29 @@ async function verifyMd5(persistor, bucket, key, sourceMd5, destMd5 = null) {
 function getReadyPipeline(...streams) {
   return new Promise((resolve, reject) => {
     const lastStream = streams.slice(-1)[0]
-    let resolvedOrErrored = false
 
+    // in case of error or stream close, we must ensure that we drain the
+    // previous stream so that it can clean up its socket (if it has one)
+    const drainPreviousStream = function(previousStream) {
+      // this stream is no longer reliable, so don't pipe anything more into it
+      previousStream.unpipe(this)
+      previousStream.resume()
+    }
+
+    // handler to resolve when either:
+    // - an error happens, or
+    // - the last stream in the chain is readable
+    // for example, in the case of a 4xx error an error will occur and the
+    // streams will not become readable
     const handler = function(err) {
-      if (!resolvedOrErrored) {
-        resolvedOrErrored = true
-
+      // remove handler from all streams because we don't want to do this on
+      // later errors
       lastStream.removeListener('readable', handler)
+      for (const stream of streams) {
+        stream.removeListener('error', handler)
+      }
+
+      // return control to the caller
       if (err) {
         reject(
           wrapError(err, 'error before stream became ready', {}, ReadError)
@@ -113,20 +130,30 @@ function getReadyPipeline(...streams) {
         resolve(lastStream)
       }
     }
-      if (err) {
+
+    // ensure the handler fires when the last strem becomes readable
+    lastStream.on('readable', handler)
+
     for (const stream of streams) {
-          if (!stream.destroyed) {
-            stream.destroy()
-          }
-        }
-      }
+      // when a stream receives a pipe, set up the drain handler to drain the
+      // connection if an error occurs or the stream is closed
+      stream.on('pipe', previousStream => {
+        stream.on('error', x => {
+          drainPreviousStream(previousStream)
+        })
+        stream.on('close', () => {
+          drainPreviousStream(previousStream)
+        })
+      })
+      // add the handler function to resolve this method on error if we can't
+      // set up the pipeline
+      stream.on('error', handler)
     }
 
+    // begin the pipeline
     for (let index = 0; index < streams.length - 1; index++) {
-      streams[index + 1].on('close', () => streams[index].destroy())
+      streams[index].pipe(streams[index + 1])
     }
-    pipeline(...streams).catch(handler)
-    lastStream.on('readable', handler)
   })
 }