[PATCH] D54826: [Support/FileSystem] Add sub-second precision for atime/mtime of sys::fs::file_status on unix platforms

Sun Nov 25 12:42:29 PST 2018

kristina added a comment.

>> In D54826#1307402 <https://reviews.llvm.org/D54826#1307402>, @aaron.ballman wrote:
>> 
>>> Is it going to be an issue that the Windows side of things has a more wild idea of file timestamp resolution? NTFS has a theoretical max precision of 100ns intervals, though according to MSDN, the access time on NTFS has a resolution of 1 hour, which is better than FAT file systems, where the resolution is 1 day. It seems odd to rely on ns resolution for access time, as that seems like it's going to be highly platform and filesystem dependent.
>> 
>> 
>> I can't speak for the Windows side of things but what you are pointing out is a general question about applications taking into account that file properties may differ across platforms. This is orthogonal to `file_status` itself reporting file properties accurately.
> 
> It is and it isn't. This is a cross-platform API and users may expect the values to be somewhat similarly reported across platforms, especially given that there is no documentation on `getLastAccessedTime()` or `getLastModificationTime()`.
> 
>> If current `file_status` has 1 hour resolution on NTFS for access time, this patch is not going to make it any better or worse, nor affect behavior of applications that are querying files from NTFS.
> 
> Agreed, and I wasn't suggesting that it would. I was more wondering whether you had thoughts on people using this across platforms where they will get very different behavior from the same API. Personally, I think it's reasonable to report the values with whatever precision we have available to us. I think that should be documented on the public API part with a mention that the resolution is expected to differ depending on the file system and OS. WDYT?

Yes, I can update the documentation for `LLVMSupport`, definitely worth mentioning it, I think it's worth dropping a comment there as well, as it seems that if someone does hit issues with it, it may be a very annoying thing to debug.

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