[PATCH] D144638: [lit] Detect Inconsistent File Access Times

[Sam Elliott via Phabricator via cfe-commits]

lenary added a comment.

In D144638#4155217 <https://reviews.llvm.org/D144638#4155217>, @michaelplatings wrote:

> I see some of these tests previously had `UNSUPPORTED: system-netbsd` but not `UNSUPPORTED: system-windows` - do you know why?

I'm not entirely sure why, but I have some ideas:

- rGa680ea2c720751a3d724fd8282515924acb3bd32 <https://reviews.llvm.org/rGa680ea2c720751a3d724fd8282515924acb3bd32> disabled the llvm-objdump test - I don't quite know why it didn't disable the ThinLTO/x86/cache.ll test (which did exist at the time).
- rG929ce27cc77dd1fbf909ef4d42614eee9d11880e <https://reviews.llvm.org/rG929ce27cc77dd1fbf909ef4d42614eee9d11880e>, rG29935acdb10679f27f028cf9dff1971c839dc51d <https://reviews.llvm.org/rG29935acdb10679f27f028cf9dff1971c839dc51d>, and rG92dc7dce4a6f117a497ced1650bc48e5b658f0ea <https://reviews.llvm.org/rG92dc7dce4a6f117a497ced1650bc48e5b658f0ea> are the netbsd disables, which were done all approximately at the same time, across clang, llvm and lld.

It looks like the pruning logic for both LTO and Clang modules is the same, using `llvm::sys::fs::file_status::getLastAccessedTime()` (but, this of course has to use the underlying system information), which is:

- On windows, atime will be zero if unsupported: If the underlying file system does not support last access time, this member is zero. https://learn.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-win32_find_dataw which should be a long time in the past.
- The mac, netbsd and linux docs for `noatime` just say something to the effect of "the access time is never updated", not the value that it reads as if you have noatime.

I've actually just done a more thorough search, and there are other tests that use `touch -a` (a few in clang/test/PCH, some others in clang/test/Modules). I presume this comes down to logic as to whether the code uses `getLastAccessedTime` which seems to be used:

- in the modules cache prune logic (pruneModuleCache in clang/lib/Frontend/CompilerInstance.cpp)
- in the LTO cache pruning logic (llvm::pruneCache)
- in FileCollector, as used by Modules (llvm/include/llvm/Support/FileCollector.h)
- in FilePermissionsApplier, as used by llvm-dwarfutil and llvm-objcopy (declared in llvm/include/llvm/Support/FileUtilities.h)

I cannot really see any correlation between the logic under test, and the use of `touch -a` or `touch -a -m` vs just `touch`. I'm not sure what the right "fix" should be any more. I guess I'm just happy for everyone who is not running into this issue.


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