core: fs.openSync in fingerprint.js fails with EINVAL on Windows under Node.js >= 26.8.0 (O_SYNC | O_DSYNC combination) #38692

Describe the bug

On Windows, asset fingerprinting throws EINVAL: invalid argument, open for every file it hashes when running on Node.js 26.8.0 or newer. The failure originates in aws-cdk-lib/core/lib/fs/fingerprint.js, in contentFingerprintMiss(), which opens files with the flag combination fs.constants.O_DSYNC | fs.constants.O_RDONLY | fs.constants.O_SYNC.

Before Node.js 26.8.0, O_SYNC and O_DSYNC were undefined on Windows, so this expression coerced to 0 (effectively O_RDONLY) and the open succeeded. Node.js 26.8.0 (nodejs/node#64707) defined these constants on Windows as real, non-zero bits. The unchanged CDK line now passes both O_SYNC and O_DSYNC set simultaneously to fs.openSync. libuv's Windows fs__open() explicitly rejects that combination and returns UV_EINVAL / Win32 ERROR_INVALID_PARAMETER.

This breaks CDK synth / asset staging on Windows with Node.js >= 26.8.0.

Regression Issue

• Select this option if this issue appears to be a regression.

This is a regression triggered by the Node.js runtime version (26.7.0 works, 26.8.0/26.8.1 fail on the identical aws-cdk-lib code).

Last Known Working CDK Library Version

aws-cdk-lib 2.244.0 works on Node.js <= 26.7.0. The aws-cdk-lib code is unchanged across the break — the regression is gated purely on the Node.js runtime version.

Expected Behavior

Asset fingerprinting / synth should open files for read-hashing successfully on Windows regardless of Node.js patch version. A read-only content hash has no need for O_SYNC/O_DSYNC (write-through) semantics, so the open should not fail with EINVAL.

Current Behavior

On Windows + Node.js >= 26.8.0, every file hashed during asset staging throws:

Error: EINVAL: invalid argument, open 'D:\...\index.js'
    at Object.openSync (node:fs:779:18)
    at contentFingerprintMiss (node_modules/aws-cdk-lib/core/lib/fs/fingerprint.js)
    at contentFingerprint (node_modules/aws-cdk-lib/core/lib/fs/fingerprint.js)
    at _processFileOrDirectory (node_modules/aws-cdk-lib/core/lib/fs/fingerprint.js)
    at FileSystem.fingerprint (node_modules/aws-cdk-lib/core/lib/fs/index.js)
    at AssetStaging.calculateHash (node_modules/aws-cdk-lib/core/lib/asset-staging.js)
  { errno: -4071, code: 'EINVAL', syscall: 'open', path: '...' }

The offending call in fingerprint.js contentFingerprintMiss():

const fd = fs.openSync(file, fs.constants.O_DSYNC | fs.constants.O_RDONLY | fs.constants.O_SYNC);

Reproduction Steps

On a Windows machine with Node.js >= 26.8.0, this self-contained snippet reproduces the exact failure against the same code path:

// repro.js — run with `node repro.js` on Windows under Node >= 26.8.0
const fs = require('fs');
const os = require('os');
const path = require('path');

const file = path.join(os.tmpdir(), 'cdk-einval-repro.txt');
fs.writeFileSync(file, 'hello');

// Exactly what aws-cdk-lib/core/lib/fs/fingerprint.js contentFingerprintMiss() does:
const flags = fs.constants.O_DSYNC | fs.constants.O_RDONLY | fs.constants.O_SYNC;
const fd = fs.openSync(file, flags); // throws EINVAL on Windows + Node >= 26.8.0
fs.closeSync(fd);
console.log('opened OK');

Alternatively, cdk synth of any stack containing a Lambda/asset on Windows + Node.js 26.8.0 triggers it via AssetStaging.calculateHash.

Possible Solution

In core/lib/fs/fingerprint.js contentFingerprintMiss(), drop the O_SYNC/O_DSYNC flags — they request write-through/sync semantics that are meaningless for a read-only hash — and open read-only:

// before
const fd = fs.openSync(file, fs.constants.O_DSYNC | fs.constants.O_RDONLY | fs.constants.O_SYNC);
// after
const fd = fs.openSync(file, fs.constants.O_RDONLY);

This is platform-agnostic and removes flags that never served a purpose for hashing. (Separately, libuv/Node arguably should treat O_SYNC+O_DSYNC as idempotent on Windows like POSIX; a companion nodejs/node issue against #64707 may be warranted, but the CDK-side change fully resolves the user-facing break.)

Additional Information/Context

Verified root-cause chain:

1. Node.js <= 26.7.0: fs.constants.O_SYNC/O_DSYNC are undefined on Windows → undefined | 0 | undefined coerces to 0 → benign O_RDONLY open. Works.
2. src: define O_SYNC, O_DSYNC and O_DIRECT on Windows nodejs/node#64707 ("src: define O_SYNC, O_DSYNC and O_DIRECT on Windows", shipped in v26.8.0) defines them on Windows as real bits: O_SYNC=0x08000000, O_DSYNC=0x04000000.
3. CDK's unchanged line now computes flags = 0x0C000000 (both sync bits set) and passes them to fs.openSync.
4. libuv Windows fs__open() (src/win/fs.c): switch (flags & (UV_FS_O_DSYNC | UV_FS_O_SYNC)) handles 0 / O_DSYNC-alone / O_SYNC-alone, but BOTH bits set falls through to default: goto einval; → UV_EINVAL / ERROR_INVALID_PARAMETER.
5. Surfaces as EINVAL … errno -4071 … syscall 'open'.
6. POSIX kernels treat both-sync-bits as harmless, so Linux/macOS are unaffected.

AWS CDK Library version (aws-cdk-lib)

2.244.0

AWS CDK CLI version

2.1113.0

Node.js Version

Fails on 26.8.0 and 26.8.1; last known working 26.7.0

OS

Windows (GitHub Actions windows-2025 runner; expected on Windows generally)

Language

TypeScript

Other information

Suspected trigger: nodejs/node#64707 (companion #64775 exposes further Windows fs flags). aws-cdk-lib is unchanged across the regression — the break is gated purely on the Node.js runtime version.