vfs: handle current-position sentinel in memory files

Treat -1 as the current file position in MemoryFileHandle read and
write operations. This matches the value passed by fs.readSync() after
normalizing a null position.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #64163
Fixes: #64162
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: trivikr

lib/internal/vfs/file_handle.js

@@ -28,6 +28,10 @@ const kMode = Symbol('kMode');
 const kPosition = Symbol('kPosition');
 const kClosed = Symbol('kClosed');
 
+function isCurrentPosition(position) {
+  return position === null || position === undefined || position === -1;
+}
+
 /**
  * Base class for virtual file handles.
  * Provides the interface that file handles must implement.
@@ -467,8 +471,8 @@ class MemoryFileHandle extends VirtualFileHandle {
 
     // Get content (resolves dynamic content providers)
     const content = this.content;
-    const readPos = position !== null && position !== undefined ?
-      Number(position) : this.position;
+    const useCurrentPosition = isCurrentPosition(position);
+    const readPos = useCurrentPosition ? this.position : Number(position);
     const available = content.length - readPos;
 
     if (available <= 0) {
@@ -479,7 +483,7 @@ class MemoryFileHandle extends VirtualFileHandle {
     content.copy(buffer, offset, readPos, readPos + bytesToRead);
 
     // Update position if not using explicit position
-    if (position === null || position === undefined) {
+    if (useCurrentPosition) {
       this.position = readPos + bytesToRead;
     }
 
@@ -512,10 +516,10 @@ class MemoryFileHandle extends VirtualFileHandle {
     this.#checkWritable();
 
     // In append mode, always write at the end
+    const useCurrentPosition = isCurrentPosition(position);
     const writePos = this.#isAppend() ?
       this.#size :
-      (position !== null && position !== undefined ?
-        Number(position) : this.position);
+      (useCurrentPosition ? this.position : Number(position));
     const data = buffer.subarray(offset, offset + length);
 
     // Expand buffer if needed (geometric doubling for amortized O(1) appends)
@@ -544,7 +548,7 @@ class MemoryFileHandle extends VirtualFileHandle {
     }
 
     // Update position if not using explicit position
-    if (position === null || position === undefined) {
+    if (useCurrentPosition) {
       this.position = writePos + length;
     }
 

test/parallel/test-vfs-fd.js

@@ -80,6 +80,26 @@ const vfs = require('node:vfs');
   myVfs.closeSync(fd);
 }
 
+// Test readSync with Node's current-position sentinel
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/file.txt', 'hello world');
+
+  const fd = myVfs.openSync('/file.txt');
+  const buffer1 = Buffer.alloc(5);
+  const buffer2 = Buffer.alloc(6);
+
+  let bytesRead = myVfs.readSync(fd, buffer1, 0, 5, -1);
+  assert.strictEqual(bytesRead, 5);
+  assert.strictEqual(buffer1.toString(), 'hello');
+
+  bytesRead = myVfs.readSync(fd, buffer2, 0, 6, -1);
+  assert.strictEqual(bytesRead, 6);
+  assert.strictEqual(buffer2.toString(), ' world');
+
+  myVfs.closeSync(fd);
+}
+
 // Test readSync with explicit position
 {
   const myVfs = vfs.create();

test/parallel/test-vfs-file-handle.js

@@ -82,6 +82,12 @@ myVfs.writeFileSync('/file.txt', 'hello world');
   myVfs.writeSync(fd, buf, 0, 3, 0);
   myVfs.closeSync(fd);
 
+  const fd3 = myVfs.openSync('/sync-current.txt', 'w');
+  myVfs.writeSync(fd3, Buffer.from('abc'), 0, 3, -1);
+  myVfs.writeSync(fd3, Buffer.from('def'), 0, 3, -1);
+  myVfs.closeSync(fd3);
+  assert.strictEqual(myVfs.readFileSync('/sync-current.txt', 'utf8'), 'abcdef');
+
   const fd2 = myVfs.openSync('/sync.txt', 'r');
   const out = Buffer.alloc(3);
   myVfs.readSync(fd2, out, 0, 3, 0);

test/parallel/test-vfs-fs-openSync.js

@@ -30,6 +30,19 @@ myVfs.mount(mountPoint);
   fs.closeSync(fd);
 }
 
+// readSync with position null uses and advances the current file position
+{
+  const fd = fs.openSync(path.join(mountPoint, 'src/hello.txt'), 'r');
+  const b1 = Buffer.alloc(5);
+  const b2 = Buffer.alloc(6);
+
+  assert.strictEqual(fs.readSync(fd, b1, 0, 5, null), 5);
+  assert.strictEqual(fs.readSync(fd, b2, 0, 6, null), 6);
+  assert.strictEqual(b1.toString(), 'hello');
+  assert.strictEqual(b2.toString(), ' world');
+  fs.closeSync(fd);
+}
+
 // openSync + writeSync (buffer) + closeSync
 {
   const fd = fs.openSync(path.join(mountPoint, 'src/wfd.txt'), 'w');