fix: respect open file flags on write

lib/providers/real.js

@@ -72,12 +72,17 @@ class RealFileHandle extends VirtualFileHandle {
 
   writeFileSync(data, options) {
     this.#checkClosed();
-    fs.writeFileSync(this.#realPath, data, options);
+    fs.writeFileSync(this.#fd, data, options);
   }
 
   async writeFile(data, options) {
     this.#checkClosed();
-    return fs.promises.writeFile(this.#realPath, data, options);
+    return new Promise((resolve, reject) => {
+      fs.writeFile(this.#fd, data, options, (err) => {
+        if (err) reject(err);
+        else resolve();
+      });
+    });
   }
 
   statSync(options) {

test/real.test.js

@@ -0,0 +1,88 @@
+'use strict';
+
+const { describe, it, beforeEach, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { create, RealFSProvider } = require('../index.js');
+
+describe('RealFSProvider', () => {
+  let rootPath;
+  let vfs;
+
+  beforeEach(() => {
+    rootPath = fs.mkdtempSync(path.join(os.tmpdir(), 'vfs-real-'));
+    const provider = new RealFSProvider(rootPath);
+    vfs = create(provider, { moduleHooks: false });
+  });
+
+  afterEach(() => {
+    fs.rmSync(rootPath, { recursive: true, force: true });
+  });
+
+  it('writeFileSync and readFileSync', () => {
+    vfs.writeFileSync('/test.txt', 'hello');
+    const content = vfs.readFileSync('/test.txt', 'utf8');
+    assert.strictEqual(content, 'hello');
+
+    // Verify on real fs
+    assert.strictEqual(fs.readFileSync(path.join(rootPath, 'test.txt'), 'utf8'), 'hello');
+  });
+
+  it('appendFileSync appends data', () => {
+    vfs.writeFileSync('/append.txt', 'Hello');
+    vfs.appendFileSync('/append.txt', ' World');
+    const content = vfs.readFileSync('/append.txt', 'utf8');
+    assert.strictEqual(content, 'Hello World');
+
+    // Verify on real fs
+    assert.strictEqual(fs.readFileSync(path.join(rootPath, 'append.txt'), 'utf8'), 'Hello World');
+  });
+
+  it('appendFileSync creates file if not exists', () => {
+    vfs.appendFileSync('/new.txt', 'first');
+    const content = vfs.readFileSync('/new.txt', 'utf8');
+    assert.strictEqual(content, 'first');
+  });
+
+  it('appendFileSync appends multiple times', () => {
+    vfs.appendFileSync('/multi.txt', 'a\n');
+    vfs.appendFileSync('/multi.txt', 'b\n');
+    vfs.appendFileSync('/multi.txt', 'c\n');
+    const content = vfs.readFileSync('/multi.txt', 'utf8');
+    assert.strictEqual(content, 'a\nb\nc\n');
+  });
+
+  it('writeFileSync overwrites existing content', () => {
+    vfs.writeFileSync('/overwrite.txt', 'original');
+    vfs.writeFileSync('/overwrite.txt', 'replaced');
+    const content = vfs.readFileSync('/overwrite.txt', 'utf8');
+    assert.strictEqual(content, 'replaced');
+  });
+
+  it('mkdirSync and readdirSync', () => {
+    vfs.mkdirSync('/subdir', { recursive: true });
+    vfs.writeFileSync('/subdir/file.txt', 'nested');
+    const entries = vfs.readdirSync('/subdir');
+    assert.deepStrictEqual(entries, ['file.txt']);
+  });
+
+  it('promises.appendFile appends data', async () => {
+    vfs.writeFileSync('/async-append.txt', 'Hello');
+    await vfs.promises.appendFile('/async-append.txt', ' World');
+    const content = vfs.readFileSync('/async-append.txt', 'utf8');
+    assert.strictEqual(content, 'Hello World');
+  });
+
+  it('data persists across provider instances', () => {
+    vfs.mkdirSync('/session', { recursive: true });
+    vfs.writeFileSync('/session/data.txt', 'persistent');
+
+    // Create new provider from same rootPath
+    const provider2 = new RealFSProvider(rootPath);
+    const vfs2 = create(provider2, { moduleHooks: false });
+    const content = vfs2.readFileSync('/session/data.txt', 'utf8');
+    assert.strictEqual(content, 'persistent');
+  });
+});