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: #40715

PR-URL: #40716
Fixes: #40699
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>

Author: mihilmy

lib/internal/fs/promises.js

@@ -399,7 +399,7 @@ 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) {

test/parallel/test-fs-promises-file-handle-read.js

@@ -68,6 +68,24 @@ async function validateReadNoParams() {
 }
 
 let useConf = false;
+// 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() {
   for (const value of [false, true]) {
@@ -80,4 +98,5 @@ let useConf = false;
           .then(common.mustCall());
   }
   await validateReadNoParams();
+  await validateReadWithPositionZero();
 })().then(common.mustCall());