[jdk17u-dev] RFR: 8316304: (fs) Add support for BasicFileAttributes.creationTime() for Linux

[Severin Gehwolf]

On Thu, 25 Jan 2024 15:05:49 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> This is an unclean backport of the filesystem enhancement on Linux to use `statx` where available. The JDK 21u patch didn't apply cleanly because JDK 20+ has [JDK-8283335](https://bugs.openjdk.org/browse/JDK-8283335) which isn't needed for this backport. The relevant code has been adjusted to work without that enhancement being present (see different commits). Copyright dates needed manual adjusting and the test needed some trivial changes related to test lib and import so that it works (failed to compile on 17u).
> 
> As for the code changes in `UnixNativeDispatcher.c`, the JNI method `Java_sun_nio_fs_UnixNativeDispatcher_stat1()` returning the mode for a file/directory doesn't exist in JDK 21 (again [JDK-8283335](https://bugs.openjdk.org/browse/JDK-8283335)). Therefore, I've adjusted the code to also use `statx` where available for symmetry.
> 
> I'll follow up with the related build fix (for the test libs) in a dependent PR (JDK-8319922).
> 
> Please review!
> 
> Testing:
> - [ ] GHA (still running).
> - [x] Build test on various Linux (aarch64, Alpine x64, ppc64le, x64) and Unix OSes (AIX, Mac) so as to not break the build there.
> - [x] Manual testing for `BasicFileAttributes.creationTime()`. Looks good.

Thanks for the review!

> I wonder about backward compatibility - whether there is code that relies on creation time being the modification time. If so, its an error of course, but still may cause problems.

Users already get different behaviour on say, Mac OS, Windows and Linux (with statx support) today. The former two support `creationTime()`, Linux does not (prior this patch). The [javadoc](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/nio/file/attribute/BasicFileAttributes.html#creationTime()) from creationTime() mentions:


If the file system implementation does not support a time stamp to indicate the time when the file was
created then this method returns an implementation specific default value, typically the last-modified-time
or a FileTime representing the epoch (1970-01-01T00:00:00Z).


So there is no change there. Even on some Linux systems where `statx` isn't supported you'd get the old behaviour. This is according to spec.

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PR Comment: https://git.openjdk.org/jdk17u-dev/pull/2175#issuecomment-1910544331