[PATCH] D122549: [VFS] RedirectingFileSystem only replace path if not already mapped

[Ben Barham via Phabricator via llvm-commits]

bnbarham created this revision.
bnbarham added reviewers: dexonsmith, keith, JDevlieghere, vsapsai, sammccall.
Herald added a subscriber: hiraditya.
Herald added a project: All.
bnbarham requested review of this revision.
Herald added projects: clang, LLVM.
Herald added subscribers: llvm-commits, cfe-commits.

If the `ExternalFS` has already remapped a path then the
`RedirectingFileSystem` should not change it to the originally provided
path. This fixes the original path always being used if multiple VFS
overlays were provided and the path wasn't found in the highest (ie.
first in the chain).

This also changes `IsVFSMapped` to mean the returned path is a *mapped*
path, rather than just from a virtual filesystem. But it was only being used
by a hack in `FileManager` for module searching, where `external-names: true`
is always used.

Resolves rdar://90578880 and llvm-project#53306.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122549

Files:
  clang/lib/Basic/FileManager.cpp
  clang/test/VFS/external-names-multi-overlay.c
  llvm/include/llvm/Support/VirtualFileSystem.h
  llvm/lib/Support/VirtualFileSystem.cpp
  llvm/unittests/Support/VirtualFileSystemTest.cpp

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