fix: FileHandle.truncate

Author: merceyz

packages/yarnpkg-fslib/sources/patchFs/FileHandle.ts

@@ -223,9 +223,13 @@ export class FileHandle<P extends Path> {
     }
   }
 
-  // FIXME: Missing FakeFS version
-  truncate(len?: number): Promise<void> {
-    throw new Error(`Method not implemented.`);
+  async truncate(len?: number): Promise<void> {
+    try {
+      this[kRef](this.truncate);
+      return await this[kBaseFs].ftruncatePromise(this.fd, len);
+    } finally {
+      this[kUnref]();
+    }
   }
 
   // FIXME: Missing FakeFS version

packages/yarnpkg-fslib/tests/patchedFs.test.ts

@@ -468,4 +468,19 @@ describe(`patchedFs`, () => {
       await fd.close();
     });
   });
+
+  it(`should support FileHandle.truncate`, async () => {
+    const patchedFs = extendFs(fs, new PosixFS(new NodeFS()));
+
+    await xfs.mktempPromise(async dir => {
+      const filepath = npath.join(npath.fromPortablePath(dir), `foo.txt`);
+      await patchedFs.promises.writeFile(filepath, `foo`);
+
+      const fd = await patchedFs.promises.open(filepath, `r+`);
+      await fd.truncate(1);
+      await fd.close();
+
+      await expect(patchedFs.promises.readFile(filepath, `utf8`)).resolves.toEqual(`f`);
+    });
+  });
 });