From e1d3e9dc75fcae4bedbf2f9ce1e5aced2238a5e2 Mon Sep 17 00:00:00 2001
From: Omar El-Mihilmy <mihilmy@yahoo.com>
Date: Wed, 3 Nov 2021 12:22:59 -0700
Subject: [PATCH] fs: nullish coalescing to respect zero positional reads

When the file read position is moved passing zero is
not respected and `null` is used instead. PR fixes the
issues by using nullish coalescing which will return
the rhs only when the lhs is `null` or `undefined`;
respecting the zero.

Fixes: https://github.com/nodejs/node/issues/40715
Refs: https://github.com/nodejs/node/pull/40716
---
 lib/internal/fs/promises.js                   |  2 +-
 .../test-fs-promises-file-handle-read.js      | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/internal/fs/promises.js b/lib/internal/fs/promises.js
index b9c3b7cb407159..93b88b4cf8077c 100644
--- a/lib/internal/fs/promises.js
+++ b/lib/internal/fs/promises.js
@@ -521,7 +521,7 @@ async function read(handle, bufferOrOptions, offset, length, position) {
     }
     offset = bufferOrOptions.offset || 0;
     length = buffer.byteLength;
-    position = bufferOrOptions.position || null;
+    position = bufferOrOptions.position ?? null;
   }
 
   if (offset == null) {
diff --git a/test/parallel/test-fs-promises-file-handle-read.js b/test/parallel/test-fs-promises-file-handle-read.js
index 44ef2c6b815a3d..2eec89296a3c20 100644
--- a/test/parallel/test-fs-promises-file-handle-read.js
+++ b/test/parallel/test-fs-promises-file-handle-read.js
@@ -68,6 +68,27 @@ async function validateReadNoParams() {
   await fileHandle.read();
 }
 
+/**
+ * Validates that the zero position is respected after the position has been
+ * moved. The test iterates over the xyz chars twice making sure that the values
+ * are read from the correct position.
+ */
+async function validateReadWithPositionZero() {
+  const opts = { useConf: true };
+  const filePath = fixtures.path('x.txt');
+  const fileHandle = await open(filePath, 'r');
+  const expectedSequence = ['x', 'y', 'z'];
+
+  for (let i = 0; i < expectedSequence.length * 2; i++) {
+    const len = 1;
+    const pos = i % 3;
+    const buf = Buffer.alloc(len);
+    const { bytesRead } = await read(fileHandle, buf, 0, len, pos, opts);
+    assert.strictEqual(bytesRead, len);
+    assert.strictEqual(buf.toString(), expectedSequence[pos]);
+  }
+}
+
 
 (async function() {
   tmpdir.refresh();
@@ -78,4 +99,5 @@ async function validateReadNoParams() {
   await validateLargeRead({ useConf: false });
   await validateLargeRead({ useConf: true });
   await validateReadNoParams();
+  await validateReadWithPositionZero();
 })().then(common.mustCall());