René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"vfs: mounted fs.readSync(fd, ..., null) treats position as -1 and throws","articleBody":"### Version\n\nmain\n\n### Platform\n\n```text\nmacOS 26.5.0\n```\n\n### Subsystem\n\nvfs\n\n### What steps will reproduce the bug?\n\n```js\nimport fs from 'node:fs';\nimport os from 'node:os';\nimport path from 'node:path';\nimport vfs from 'node:vfs';\n\nconst mountPoint = fs.mkdtempSync(path.join(os.tmpdir(), 'vfs-mount-'));\nconst mounted = vfs\n  .create({ emitExperimentalWarning: false })\n  .mount(mountPoint);\n\ntry {\n  const file = path.join(mountPoint, 'file.txt');\n\n  fs.writeFileSync(file, 'abcdef');\n\n  const fd = fs.openSync(file, 'r');\n  const buffer = Buffer.alloc(2);\n\n  try {\n    const bytesRead = fs.readSync(fd, buffer, 0, buffer.length, null);\n    console.log('bytesRead:', bytesRead);\n    console.log('buffer:', buffer.toString());\n  } finally {\n    fs.closeSync(fd);\n  }\n} finally {\n  mounted.unmount();\n  fs.rmSync(mountPoint, { recursive: true, force: true });\n}\n```\n\n### How often does it reproduce? Is there a required condition?\n\nAlways\n\n### What is the expected behavior? Why is that the expected behavior?\n\n```console\nbytesRead: 2\nbuffer: ab\n```\n\nIn `fs.readSync`, `position: null` means read from and advance the descriptor’s current file position, so VFS should translate the internal sentinel back to “no explicit position.”\n\n\n### What do you see instead?\n\n```console\nnode:buffer:253\n      throw new ERR_OUT_OF_RANGE('sourceStart', `\u003e= 0 \u0026\u0026 \u003c= ${source.byteLength}`, sourceStart);\n            ^\n\nRangeError [ERR_OUT_OF_RANGE]: The value of \"sourceStart\" is out of range. It must be \u003e= 0 \u0026\u0026 \u003c= 6. Received -1\n    at copyImpl (node:buffer:253:13)\n    at Buffer.copy (node:buffer:894:12)\n    at MemoryFileHandle.readSync (node:internal/vfs/file_handle:479:13)\n    at Object.readSync (node:internal/vfs/setup:360:33)\n    at Object.readSync (node:fs:892:22)\n    at file:///Users/trivikram/workspace/test-repro/repro.js:20:26\n    at ModuleJob.run (node:internal/modules/esm/module_job:447:25)\n    at async node:internal/modules/esm/loader:646:26\n    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:101:5) {\n  code: 'ERR_OUT_OF_RANGE'\n}\n```\n\nVFS receives `-1` as the position and treats it as an explicit negative offset.\n\n### Additional information\n\nNative repro\n\n```js\nimport fs from 'node:fs';\nimport os from 'node:os';\nimport path from 'node:path';\n\nconst dir = fs.mkdtempSync(path.join(os.tmpdir(), 'native-readsync-'));\n\ntry {\n  const file = path.join(dir, 'file.txt');\n  fs.writeFileSync(file, 'abcdef');\n\n  const fd = fs.openSync(file, 'r');\n  const buffer = Buffer.alloc(2);\n\n  try {\n    const bytesRead = fs.readSync(fd, buffer, 0, buffer.length, null);\n    console.log('bytesRead:', bytesRead);\n    console.log('buffer:', buffer.toString());\n  } finally {\n    fs.closeSync(fd);\n  }\n} finally {\n  fs.rmSync(dir, { recursive: true, force: true });\n}\n```","author":{"url":"https://github.com/trivikr","@type":"Person","name":"trivikr"},"datePublished":"2026-06-27T06:17:25.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/64162/node/issues/64162"}