fs: improve promise based readFile performance for big files

This significantly reduces the peak memory for the promise
based readFile operation by reusing a single memory chunk after
each read and strinigifying that chunk immediately.

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>
PR-URL: https://github.com/nodejs/node/pull/44295
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 15 insertions, 3 deletions

diff --git a/test/parallel/test-fs-promises-readfile.js b/test/parallel/test-fs-promises-readfile.js
index a7fc46e5a65..fd1bfabf88d 100644
--- a/test/parallel/test-fs-promises-readfile.js
+++ b/test/parallel/test-fs-promises-readfile.js
@@ -1,3 +1,4 @@
+// Flags: --expose-internals
 'use strict';
 
 const common = require('../common');
@@ -6,15 +7,15 @@ const assert = require('assert');
 const path = require('path');
 const { writeFile, readFile } = require('fs').promises;
 const tmpdir = require('../common/tmpdir');
+const { internalBinding } = require('internal/test/binding');
+const fsBinding = internalBinding('fs');
 tmpdir.refresh();
 
 const fn = path.join(tmpdir.path, 'large-file');
 
 // Creating large buffer with random content
 const largeBuffer = Buffer.from(
-  Array.apply(null, { length: 16834 * 2 })
-    .map(Math.random)
-    .map((number) => (number * (1 << 8)))
+  Array.from({ length: 1024 ** 2 + 19 }, (_, index) => index)
 );
 
 async function createLargeFile() {
@@ -69,6 +70,16 @@ async function validateWrongSignalParam() {
 
 }
 
+async function validateZeroByteLiar() {
+  const originalFStat = fsBinding.fstat;
+  fsBinding.fstat = common.mustCall(
+    () => (/* stat fields */ [0, 1, 2, 3, 4, 5, 6, 7, 0 /* size */])
+  );
+  const readBuffer = await readFile(fn);
+  assert.strictEqual(readBuffer.toString(), largeBuffer.toString());
+  fsBinding.fstat = originalFStat;
+}
+
 (async () => {
   await createLargeFile();
   await validateReadFile();
@@ -76,4 +87,5 @@ async function validateWrongSignalParam() {
   await validateReadFileAbortLogicBefore();
   await validateReadFileAbortLogicDuring();
   await validateWrongSignalParam();
+  await validateZeroByteLiar();
 })().then(common.mustCall());