Cygwin 3.7 wishlist: read/write NTFS alternate data streams via openat(O_XATTR)

[Jeffrey Altman]

On 3/24/2025 10:25 AM, Martin Wege via Cygwin wrote:
> Hello,
>
> another wishlist entry (old one):
> read/write NTFS alternate data streams via openat(O_XATTR)

 From my perspective as a filesystem developer, Extended Attributes and 
Alternate Data Streams are different concepts which I believe are 
inappropriate to mix.

Filesystems can implement one, the other, both or neither.

An Extended Attribute is a name:value pair that can be attached to an 
inode. The size of individual values and the sum of the value sizes are 
often quite constrained because they are stored in the inode metadata. 
Copying an inode is guaranteed to copy the Extended Attributes.

Alternate Data Streams are typically represented as independent inodes. 
The Alternate Data Stream namespace can be represented by a directory 
inode referenced from the file inode. Each ADS inode might have their 
own Extended Attributes to go along with the data stream. ACLs on ADS 
inodes can also be independent of the ACL on the primary inode depending 
upon the implementation.

Therefore, I am reluctant to see openat(O_XATTR) be used to access 
alternate data streams.

Jeffrey Altman

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