vfs: support writeFileSync with virtual fds

Route fs.writeFileSync() and fs.appendFileSync() calls with VFS-owned
file descriptors through the virtual file handle instead of falling
back to the native fs binding. This preserves descriptor semantics for
both memory-backed VFS files and RealFSProvider handles.
Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #64165Fixes: #64164
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: trivikr

lib/internal/vfs/setup.js

@@ -9,6 +9,7 @@ const {
 }=primordials;
 
 const{ Buffer }=require('buffer');
+const{ isArrayBufferView }=require('internal/util/types');
 const{ resolve, sep }=require('path');
 const{ fileURLToPath,URL}=require('internal/url');
 const{ kEmptyObject }=require('internal/util');
@@ -46,6 +47,31 @@ function noopFd(fd) {
 returnundefined;
 }
 
+functiontoWriteBuffer(data,options){
+if(Buffer.isBuffer(data))returndata;
+if(isArrayBufferView(data)){
+returnBuffer.from(data.buffer,data.byteOffset,data.byteLength);
+}
+constencoding=typeofoptions==='string' ? options : options?.encoding;
+returnBuffer.from(data,encoding||'utf8');
+}
+
+functionwriteFileSyncFd(fd,data,options){
+constvfd=getVirtualFd(fd);
+if(vfd===undefined)returnundefined;
+
+constbuffer=toWriteBuffer(data,options);
+letoffset=0;
+letlength=buffer.byteLength;
+while(length>0){
+constwritten=vfd.entry.writeSync(buffer,offset,length,null);
+offset+=written;
+length-=written;
+}
+
+returntrue;
+}
+
 // Registry of active VFS instances.
 constactiveVFSList=[];
 
@@ -288,10 +314,14 @@ function createVfsHandlers() {
 
 // ==================== Sync path-based write ops ====================
 
-writeFileSync: (path,data,options)=>
-vfsOpVoid(path,(vfs,n)=>vfs.writeFileSync(n,data,options)),
-appendFileSync: (path,data,options)=>
-vfsOpVoid(path,(vfs,n)=>vfs.appendFileSync(n,data,options)),
+writeFileSync(path,data,options){
+if(typeofpath==='number')returnwriteFileSyncFd(path,data,options);
+returnvfsOpVoid(path,(vfs,n)=>vfs.writeFileSync(n,data,options));
+},
+appendFileSync(path,data,options){
+if(typeofpath==='number')returnwriteFileSyncFd(path,data,options);
+returnvfsOpVoid(path,(vfs,n)=>vfs.appendFileSync(n,data,options));
+},
 mkdirSync: (path,options)=>
 vfsOp(path,(vfs,n)=>({result: vfs.mkdirSync(n,options)})),
 rmdirSync: (path)=>vfsOpVoid(path,(vfs,n)=>vfs.rmdirSync(n)),

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

@@ -26,4 +26,63 @@ assert.strictEqual(fs.readFileSync(target, 'utf8'), 'fresh more');
 fs.writeFileSync(target,Buffer.from('binary'));
 assert.strictEqual(fs.readFileSync(target,'utf8'),'binary');
 
+// writeFileSync via a VFS fd writes through the open descriptor.
+{
+constfdTarget=path.join(mountPoint,'src/fd.txt');
+constfd=fs.openSync(fdTarget,'w+');
+try{
+fs.writeFileSync(fd,'hello');
+fs.writeFileSync(fd,newUint8Array(Buffer.from(' world')));
+assert.strictEqual(fs.readFileSync(fdTarget,'utf8'),'hello world');
+}finally{
+fs.closeSync(fd);
+}
+}
+
+// appendFileSync via a VFS fd follows normal fd write semantics.
+{
+constfdTarget=path.join(mountPoint,'src/append-fd.txt');
+fs.writeFileSync(fdTarget,'start');
+constfd=fs.openSync(fdTarget,'a');
+try{
+fs.appendFileSync(fd,' end');
+assert.strictEqual(fs.readFileSync(fdTarget,'utf8'),'start end');
+}finally{
+fs.closeSync(fd);
+}
+}
+
 myVfs.unmount();
+
+// writeFileSync via a RealFSProvider fd remains tied to the open descriptor
+// after the backing path is renamed.
+{
+constroot=path.join('/tmp','vfs-real-writeFileSync-'+process.pid);
+constrealMountPoint=path.join('/tmp','vfs-real-writeFileSync-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});
+
+constrealVfs=vfs
+.create(newvfs.RealFSProvider(root),{emitExperimentalWarning: false})
+.mount(realMountPoint);
+try{
+constmountedFile=path.join(realMountPoint,'a.txt');
+fs.writeFileSync(path.join(root,'a.txt'),'old');
+constfd=fs.openSync(mountedFile,'r+');
+try{
+fs.renameSync(path.join(root,'a.txt'),path.join(root,'b.txt'));
+fs.writeFileSync(path.join(root,'a.txt'),'new');
+fs.writeFileSync(fd,'updated');
+assert.strictEqual(fs.readFileSync(path.join(root,'b.txt'),'utf8'),'updated');
+assert.strictEqual(fs.readFileSync(path.join(root,'a.txt'),'utf8'),'new');
+}finally{
+fs.closeSync(fd);
+}
+}finally{
+realVfs.unmount();
+fs.rmSync(root,{recursive: true,force: true});
+fs.rmSync(realMountPoint,{recursive: true,force: true});
+}
+}