fix: correct permission check logic for readonly files in copyFile operations

[Copilot]

The copyFile and copyFileSync operations were incorrectly failing when trying to copy readonly files (e.g., files with mode 0o400). This behavior was inconsistent with Node.js's native filesystem, which allows copying readonly files since copying only requires read access to the source file.

Root Cause

The issue was in the permission check logic in src/core/Superblock.ts at line 452. The condition !(flagsNum & O_RDONLY) was always evaluating to true because O_RDONLY = 0. This caused write permission checks to be performed even when opening files for read-only access.

// Before (incorrect)
if (!(flagsNum & O_RDONLY)) {  // Always true since O_RDONLY = 0
  if (!node.canWrite()) {
    throw createError(ERROR_CODE.EACCES, 'open', link.getPath());
  }
}

Solution

Fixed the permission check logic to properly distinguish between read and write access:

// After (correct)
// For read access: check if flags are O_RDONLY or O_RDWR (i.e., not only O_WRONLY)
if ((flagsNum & (O_RDONLY | O_RDWR | O_WRONLY)) !== O_WRONLY) {
  if (!node.canRead()) {
    throw createError(ERROR_CODE.EACCES, 'open', link.getPath());
  }
}
// For write access: check if flags are O_WRONLY or O_RDWR
if (flagsNum & (O_WRONLY | O_RDWR)) {
  if (!node.canWrite()) {
    throw createError(ERROR_CODE.EACCES, 'open', link.getPath());
  }
}

Testing

Added comprehensive test coverage for both synchronous and asynchronous copyFile operations on readonly files:

// This now works correctly (previously threw EACCES)
const vol = create({ '/foo': 'hello world' });
vol.chmodSync('/foo', 0o400); // read-only for owner
vol.copyFileSync('/foo', '/bar'); // ✅ succeeds

The fix maintains backward compatibility and doesn't affect existing permission checks for truly inaccessible files (e.g., files with mode 0o333 that lack read permissions).

Fixes #1175.

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[github-actions]

🎉 This PR is included in version 4.38.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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