[PATCH] D37474: [PCH] Allow VFS to be used for tests that generate PCH files

[Ilya Biryukov via Phabricator via cfe-commits]

ilya-biryukov added a comment.

In https://reviews.llvm.org/D37474#861206, @cameron314 wrote:

> I suppose we could do the overlay manually in all the tests that need it; I guess it depends on what the goal of the VFS support is. To my mind, it doesn't make sense that a file is placed in the RealFS in the first place if a VFS is used, but this is quite entrenched in the current design and difficult to change. In which case, I would argue that parsing a file with a preamble and a VFS should not give a cryptic error without an explicit overlay, but rather "just work". But I agree my current patch is a little crude in that it allows access to the entire RealFS via the VFS.
>
> I could change the automatic overlay to *only* support access to the generated PCH file, similar to how the `FilteredFileSystem` you've pointed out works, but for a single file only. Would that be acceptable?


I totally agree, the current behavior is definitely confusing for clients using `vfs::FileSystem` without proper filesystem access on top of `ASTUnit`.
Having an overlay that provides **only** the PCH file sounds like a good middle ground.

Maybe we could even read the PCH contents using file APIs instead of `vfs::FileSystem` and add them into `PreprocessorOption::RemappedFileBuffers`? Do you think that would also work?
Since `ASTUnit` is responsible writes PCHs to disk, it also makes sense for it to read them from disk, but accesses to all other files would go through `vfs::FileSystem`.


https://reviews.llvm.org/D37474