RFR: JDK-8030048: (fs) Support UserDefinedFileAttributeView/extended attributes on OS X / HFS+

[Alan Bateman]

On Sun, 10 Jan 2021 17:17:16 GMT, Sebastian Stenzel <github.com+1204330+overheadhunter at openjdk.org> wrote:

> This adds support for UserDefinedFileAttributeView on macOS 10.4 and newer.
> 
> While the main audience for this will probably be macOS users, the relevant changes take place in (mostly existing) classes carrying BSD in their names as none of this is really macOS-specific.
> 
> Code is mostly copied from the Linux implementation except for three differences:
> 1. max length for attribute names is 127 bytes
> 2. we know for sure that APFS and HFS+ support extended attributes, therefore we [simply return `true`](https://github.com/skymatic/jdk/blob/7a3619df12853bd3a44e4f49c98f35e15bceb652/src/java.base/macosx/classes/sun/nio/fs/BsdFileStore.java#L114-L118) from `supportsFileAttributeView(UserDefinedFileAttributeView.class)` for these two FS types, while for all other types support is determined experimentally in [`isExtendedAttributesEnabled(...)`](https://github.com/skymatic/jdk/blob/7a3619df12853bd3a44e4f49c98f35e15bceb652/src/java.base/macosx/classes/sun/nio/fs/BsdFileStore.java#L83-L100)
> 3. For the aforementioned check the new `UnixConstants.ENOATTR` is added, which [seems to be macOS and BSD-specific](https://mail-index.netbsd.org/tech-kern/2012/04/30/msg013090.html).
> 
> As discussed in the mailing list, for ease of tracking changes it is not a goal of this patch to modernize the existing Linux implementation, nor to deduplicate anything.

src/java.base/macosx/classes/sun/nio/fs/BsdFileStore.java line 104:

> 102:     @Override
> 103:     public boolean supportsFileAttributeView(Class<? extends FileAttributeView> type) {
> 104:         // support DosFileAttributeView and UserDefinedAttributeView if extended

A minor point here is that comment carrier over from the Linux implementation isn't quite right here as the BSD version doesn't support the "dos" attribute view.

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PR: https://git.openjdk.java.net/jdk/pull/2017