[PATCH] D94811: [lldb] Fix fallthrough with strictly virtual working directory.

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith added a comment.

In D94811#2502504 <https://reviews.llvm.org/D94811#2502504>, @JDevlieghere wrote:

> Personally I'd just like to get rid of `ExternalFSValidWD` inside of `shouldUseExternalFS` and let the underlying FS take care of it, which I believe would better fit the abstraction but would be a change in behavior (as pointed out in https://reviews.llvm.org/D65677#inline-604694).

I'm not quite following why that would work; when the underlying FS fails to `cd`, then subsequent calls could fall through to the wrong place.

I just checked the code in `OverlayFileSystem` and it doesn't seem much better about this:

  std::error_code
  OverlayFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
    for (auto &FS : FSList)
      if (std::error_code EC = FS->setCurrentWorkingDirectory(Path))
        return EC;
    return {};
  }

That'll leave things horribly inconsistent as well.

The abstraction as I see it is that the caller sees the OverlayFileSystem / RedirectingFileSystem as the *only* filesystem, a single thing that can be `cd`'ed around in. A `cd` to any directory that exists in that virtual view should work as if that were true. Subsequent calls to `setWorkingDirectory()` or `getFile()` should behave as if that were the case.

I think one way to get that, at least for your use case, is to extract out RealFileSystem's handling for a custom working directory and reuse it.

- `getRealFileSystem()` still returns a `RealFileSystem`, but that class gets stripped down.
- `WorkingDirectoryFileSystem<BaseT>` gets that support, which can be layered on top of anything.
- `getPhysicalFileSystem()` returns a `WorkingDirectoryFileSystem<RealFileSystem>`.
- LLDB uses a `WorkingDirectoryFileSystem<ProxyFileSystem>` on top of the last VFS overlay.

Probably clang could take advantage of this abstraction as well, but that seems outside the scope of what you're trying to do.

WDYT?


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