vfs: read RealFSProvider files from open fd

Read RealFileHandle contents through the open file descriptor instead
of reopening the original real path. This keeps already-open VFS file
descriptors usable after the backing file is renamed.

Use positioned reads so readFileSync() and readFile() preserve the
handle's current offset.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr

lib/internal/vfs/providers/real.js

@@ -5,6 +5,7 @@ const {
   StringPrototypeStartsWith,
 } = primordials;
 
+const { Buffer } = require('buffer');
 const fs = require('fs');
 const path = require('path');
 const { VirtualProvider } = require('internal/vfs/provider');
@@ -34,6 +35,19 @@ class RealFileHandle extends VirtualFileHandle {
     }
   }
 
+  #readFileBuffer(size) {
+    return Buffer.allocUnsafe(size || 8192);
+  }
+
+  #readFileResult(buffer, bytesRead, options) {
+    buffer = buffer.subarray(0, bytesRead);
+    const encoding = typeof options === 'string' ? options : options?.encoding;
+    if (encoding && encoding !== 'buffer') {
+      buffer = buffer.toString(encoding);
+    }
+    return buffer;
+  }
+
   /**
    * @param {string} path The VFS path
    * @param {string} flags The open flags
@@ -79,12 +93,41 @@ class RealFileHandle extends VirtualFileHandle {
 
   readFileSync(options) {
     this.#checkClosed('read');
-    return fs.readFileSync(this.#realPath, options);
+    const size = fs.fstatSync(this.#fd).size;
+    const buffer = this.#readFileBuffer(size);
+    let bytesRead = 0;
+    while (bytesRead < buffer.byteLength) {
+      const read = fs.readSync(
+        this.#fd,
+        buffer,
+        bytesRead,
+        buffer.byteLength - bytesRead,
+        bytesRead,
+      );
+      if (read === 0) break;
+      bytesRead += read;
+    }
+
+    return this.#readFileResult(buffer, bytesRead, options);
   }
 
   async readFile(options) {
     this.#checkClosed('read');
-    return fs.promises.readFile(this.#realPath, options);
+    const size = (await this.stat()).size;
+    const buffer = this.#readFileBuffer(size);
+    let bytesRead = 0;
+    while (bytesRead < buffer.byteLength) {
+      const { bytesRead: read } = await this.read(
+        buffer,
+        bytesRead,
+        buffer.byteLength - bytesRead,
+        bytesRead,
+      );
+      if (read === 0) break;
+      bytesRead += read;
+    }
+
+    return this.#readFileResult(buffer, bytesRead, options);
   }
 
   writeFileSync(data, options) {

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

@@ -43,3 +43,32 @@ assert.strictEqual(
 }
 
 myVfs.unmount();
+
+// readFileSync via a RealFSProvider fd remains usable after the backing path
+// is renamed.
+{
+  const root = path.join('/tmp', 'vfs-real-readFileSync-' + process.pid);
+  const realMountPoint = path.join('/tmp', 'vfs-real-readFileSync-mount-' + process.pid);
+  fs.rmSync(root, { recursive: true, force: true });
+  fs.rmSync(realMountPoint, { recursive: true, force: true });
+  fs.mkdirSync(root, { recursive: true });
+  fs.mkdirSync(realMountPoint, { recursive: true });
+
+  const realVfs = vfs
+    .create(new vfs.RealFSProvider(root), { emitExperimentalWarning: false })
+    .mount(realMountPoint);
+  try {
+    fs.writeFileSync(path.join(root, 'a.txt'), 'still readable');
+    const fd = fs.openSync(path.join(realMountPoint, 'a.txt'), 'r');
+    try {
+      fs.renameSync(path.join(root, 'a.txt'), path.join(root, 'b.txt'));
+      assert.strictEqual(fs.readFileSync(fd, 'utf8'), 'still readable');
+    } finally {
+      fs.closeSync(fd);
+    }
+  } finally {
+    realVfs.unmount();
+    fs.rmSync(root, { recursive: true, force: true });
+    fs.rmSync(realMountPoint, { recursive: true, force: true });
+  }
+}

test/parallel/test-vfs-real-provider-handle.js

@@ -75,6 +75,23 @@ const myVfs = vfs.create(new vfs.RealFSProvider(root));
     await handle.close();
   }
 
+  // ===== readFile through an open real fd survives backing path rename =====
+  {
+    fs.writeFileSync(path.join(root, 'rename-read.txt'), 'still readable');
+    const syncHandle = await myVfs.provider.open('/rename-read.txt', 'r');
+    const asyncHandle = await myVfs.provider.open('/rename-read.txt', 'r');
+    fs.renameSync(path.join(root, 'rename-read.txt'),
+                  path.join(root, 'rename-read-renamed.txt'));
+    try {
+      assert.strictEqual(syncHandle.readFileSync('utf8'), 'still readable');
+      assert.strictEqual(await asyncHandle.readFile('utf8'), 'still readable');
+    } finally {
+      await syncHandle.close();
+      await asyncHandle.close();
+      fs.unlinkSync(path.join(root, 'rename-read-renamed.txt'));
+    }
+  }
+
   // ===== EBADF after close =====
   {
     await myVfs.promises.writeFile('/h.txt', 'hello');