fix(memfs): resolve relative streams from virtual cwd

packages/fs-core/src/Superblock.ts

@@ -103,6 +103,10 @@ export class Superblock {
     this.root = root;
   }
 
+  private filenameToSteps(filename: string): string[] {
+    return filenameToSteps(filename, this.process.cwd());
+  }
+
   /** Fan-out of file system change events. Multiple consumers may subscribe. */
   public readonly changes = new FanOut<FsEvent>();
 
@@ -222,7 +226,7 @@ export class Superblock {
       steps = stepsOrFilenameOrLink.steps;
       filename = pathSep + steps.join(pathSep);
     } else if (typeof stepsOrFilenameOrLink === 'string') {
-      steps = filenameToSteps(stepsOrFilenameOrLink);
+      steps = this.filenameToSteps(stepsOrFilenameOrLink);
       filename = stepsOrFilenameOrLink;
     } else {
       steps = stepsOrFilenameOrLink;
@@ -263,7 +267,7 @@ export class Superblock {
       if (node.isSymlink() && (resolveSymlinks || i < steps.length - 1)) {
         const resolvedPath = isAbsolute(node.symlink) ? node.symlink : pathJoin(dirname(curr.getPath()), node.symlink); // Relative to symlink's parent
 
-        steps = filenameToSteps(resolvedPath).concat(steps.slice(i + 1));
+        steps = this.filenameToSteps(resolvedPath).concat(steps.slice(i + 1));
         curr = this.root;
         i = 0;
         continue;
@@ -351,7 +355,7 @@ export class Superblock {
 
   getLinkParentAsDirOrThrow(filenameOrSteps: string | string[], funcName?: string): Link {
     const steps: string[] = (
-      filenameOrSteps instanceof Array ? filenameOrSteps : filenameToSteps(filenameOrSteps)
+      filenameOrSteps instanceof Array ? filenameOrSteps : this.filenameToSteps(filenameOrSteps)
     ).slice(0, -1);
     const filename: string = pathSep + steps.join(pathSep);
     const link = this.getLinkOrThrow(filename, funcName);
@@ -524,7 +528,7 @@ export class Superblock {
     modeNum: number | undefined,
     resolveSymlinks: boolean = true,
   ): File {
-    const steps = filenameToSteps(filename);
+    const steps = this.filenameToSteps(filename);
     let link: Link | null;
     try {
       link = resolveSymlinks ? this.getResolvedLinkOrThrow(filename, 'open') : this.getLinkOrThrow(filename, 'open');
@@ -666,7 +670,7 @@ export class Superblock {
   };
 
   public readonly symlink = (targetFilename: string, pathFilename: string): Link => {
-    const pathSteps = filenameToSteps(pathFilename);
+    const pathSteps = this.filenameToSteps(pathFilename);
     // Check if directory exists, where we about to create a symlink.
     let dirLink;
     try {
@@ -745,7 +749,7 @@ export class Superblock {
   };
 
   public readonly mkdir = (filename: string, modeNum: number): void => {
-    const steps = filenameToSteps(filename);
+    const steps = this.filenameToSteps(filename);
     // This will throw if user tries to create root dir `fs.mkdirSync('/')`.
     if (!steps.length) throw createError(ERROR_CODE.EEXIST, 'mkdir', filename);
     const dir = this.getLinkParentAsDirOrThrow(filename, 'mkdir');
@@ -763,7 +767,7 @@ export class Superblock {
    */
   public readonly mkdirp = (filename: string, modeNum: number): string | undefined => {
     let created = false;
-    const steps = filenameToSteps(filename);
+    const steps = this.filenameToSteps(filename);
     let curr: Link | null = null;
     let i = steps.length;
     // Find the longest subpath of filename that still exists:

packages/fs-node/src/__tests__/volume.process.test.ts

@@ -41,4 +41,16 @@ describe('Volume with custom process', () => {
       expect(stat.gid).toBe(99);
     });
   });
+
+  describe('relative paths', () => {
+    it('resolves write operations against the custom process cwd', () => {
+      const customProcess = makeProcess({ cwd: () => '/app' });
+      const vol = Volume.fromJSON({ '/app': null }, '/', { process: customProcess });
+
+      vol.writeFileSync('data.txt', 'hello');
+
+      expect(vol.readFileSync('/app/data.txt', 'utf8')).toBe('hello');
+      expect(vol.readFileSync('data.txt', 'utf8')).toBe('hello');
+    });
+  });
 });

packages/memfs/src/__tests__/memfs.process.test.ts

@@ -1,4 +1,4 @@
-import { memfs } from '../index';
+import { fs as defaultFs, memfs } from '../index';
 import type { IProcess } from '../index';
 
 const makeProcess = (overrides: Partial<IProcess> = {}): IProcess => ({
@@ -50,4 +50,38 @@ describe('memfs() with custom process', () => {
     const p: IProcess = makeProcess();
     expect(typeof p.cwd).toBe('function');
   });
+
+  it('resolves relative createWriteStream paths against the default cwd', done => {
+    const { fs } = memfs();
+    const stream = fs.createWriteStream('downloaded.zip') as any;
+
+    stream.on('error', done);
+    stream.on('finish', () => {
+      expect(fs.readFileSync('/downloaded.zip', 'utf8')).toBe('content');
+      expect(fs.readFileSync('downloaded.zip', 'utf8')).toBe('content');
+      done();
+    });
+
+    stream.end('content');
+  });
+
+  it('resolves relative createWriteStream paths for the default fs export', done => {
+    const filename = '/default-create-write-stream.txt';
+    try {
+      defaultFs.unlinkSync(filename);
+    } catch (err) {
+      if ((err as any).code !== 'ENOENT') throw err;
+    }
+
+    const stream = defaultFs.createWriteStream('default-create-write-stream.txt') as any;
+
+    stream.on('error', done);
+    stream.on('finish', () => {
+      expect(defaultFs.readFileSync(filename, 'utf8')).toBe('content');
+      defaultFs.unlinkSync(filename);
+      done();
+    });
+
+    stream.end('content');
+  });
 });

packages/memfs/src/index.ts

@@ -19,8 +19,22 @@ const { F_OK, R_OK, W_OK, X_OK } = constants;
 export { DirectoryJSON, NestedDirectoryJSON, Volume };
 export type { IProcess };
 
+const getGlobalProcess = (): Partial<IProcess> => {
+  const maybeProcess = typeof globalThis !== 'undefined' ? (globalThis as any).process : undefined;
+  return maybeProcess ?? {};
+};
+
+const createProcess = (cwd: string, baseProcess: Partial<IProcess> = getGlobalProcess()): IProcess => ({
+  cwd: () => cwd,
+  platform: baseProcess.platform ?? 'linux',
+  emitWarning: baseProcess.emitWarning?.bind(baseProcess) ?? (() => {}),
+  env: baseProcess.env ?? {},
+  getuid: baseProcess.getuid?.bind(baseProcess),
+  getgid: baseProcess.getgid?.bind(baseProcess),
+});
+
 // Default volume.
-export const vol = new Volume();
+export const vol = Volume.fromNestedJSON({}, '/', { process: createProcess('/') });
 
 export interface IFs extends Volume {
   constants: typeof constants;
@@ -96,7 +110,8 @@ export const memfs = (
   // Superblock use that process's cwd(). Otherwise default to '/' so the
   // convenience function keeps its opinionated virtual-root default.
   const cwd = opts.cwd ?? (opts.process ? undefined : '/');
-  const vol = Volume.fromNestedJSON(json, cwd, { process: opts.process });
+  const process = cwd === undefined ? opts.process : createProcess(cwd, opts.process);
+  const vol = Volume.fromNestedJSON(json, cwd, { process });
   const fs = createFsFromVolume(vol);
   return { fs, vol };
 };