Revert back to using our own file streaming

The outage was caused by a race condition caused by the deployment
scripts copying files from staging to production. During this time the
server could read a partial file, which would then be cached immutably
for 1 year. Currently we're fixing that by having the deployment scripts
stop the backend service during the copy.

src/server.js

@@ -336,6 +336,17 @@ app.get('/*', asyncHandler(async (req, res, next) => {
   }
   const varyAcceptEncoding = fileEncodings.length > 0;
 
+  const sendFileHeaders = () => {
+    stats.handleServedFile(pathName);
+    res.setHeader('Content-Type', fileType.type);
+    if (contentEncoding !== null) {
+      res.setHeader('Content-Encoding', contentEncoding);
+    }
+    if (varyAcceptEncoding) {
+      res.setHeader('Vary', 'Accept-Encoding');
+    }
+  };
+
   if (requiresSpecialRewriting) {
     let fileContents = await readFile(filePath, 'utf-8');
 
@@ -376,23 +387,25 @@ app.get('/*', asyncHandler(async (req, res, next) => {
       fileContents = fileContents.replace('</head>', newHead + '</head>');
     }
 
-    res.setHeader('Content-Type', fileType.type);
-    if (varyAcceptEncoding) {
-      res.setHeader('Vary', 'Accept-Encoding');
-    }
+    sendFileHeaders();
     res.send(fileContents);
   } else {
-    const headers = {};
-    headers['Content-Type'] = fileType.type;
-    if (contentEncoding !== null) {
-      headers['Content-Encoding'] = contentEncoding;
-    }
-    if (varyAcceptEncoding) {
-      headers['Vary'] = 'Accept-Encoding';
-    }
-
-    res.sendFile(filePath, {
-      headers
+    const readStream = fs.createReadStream(filePath, {
+      highWaterMark: 1024 * 64
+    });
+    readStream.on('open', () => {
+      sendFileHeaders();
+      res.setHeader('Content-Length', fileStat.size);
+      readStream.pipe(res);
+      res.on('close', () => {
+        readStream.destroy();
+      });
+      res.on('error', (err) => {
+        readStream.destroy(err);
+      });
+    });
+    readStream.on('error', (err) => {
+      next(err);
     });
   }
 }));