fs: handle long files reading in fs.promises

PR-URL: #19643
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/fs/promises.js

@@ -147,15 +147,17 @@ async function readFileHandle(filehandle, options) {
 
   const chunks = [];
   const chunkSize = Math.min(size, 16384);
-  const buf = Buffer.alloc(chunkSize);
   let totalRead = 0;
+  let endOfFile = false;
   do {
+    const buf = Buffer.alloc(chunkSize);
     const { bytesRead, buffer } =
-      await read(filehandle, buf, 0, buf.length);
-    totalRead = bytesRead;
-    if (totalRead > 0)
-      chunks.push(buffer.slice(0, totalRead));
-  } while (totalRead === chunkSize);
+      await read(filehandle, buf, 0, chunkSize, totalRead);
+    totalRead += bytesRead;
+    endOfFile = bytesRead !== chunkSize;
+    if (bytesRead > 0)
+      chunks.push(buffer.slice(0, bytesRead));
+  } while (!endOfFile);
 
   const result = Buffer.concat(chunks);
   if (options.encoding) {

test/parallel/test-fs-promises-readfile.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const common = require('../common');
+
+const assert = require('assert');
+const path = require('path');
+const { writeFile, readFile } = require('fs/promises');
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const fn = path.join(tmpdir.path, 'large-file');
+
+common.crashOnUnhandledRejection();
+
+// Creating large buffer with random content
+const buffer = Buffer.from(
+  Array.apply(null, { length: 16834 * 2 })
+    .map(Math.random)
+    .map((number) => (number * (1 << 8)))
+);
+
+// Writing buffer to a file then try to read it
+writeFile(fn, buffer)
+  .then(() => readFile(fn))
+  .then((readBuffer) => {
+    assert.strictEqual(readBuffer.equals(buffer), true);
+  })
+  .then(common.mustCall());