[clang][modules] Fix filesystem races in ModuleManager

[jansvoboda11]

The ModuleManager uses FileEntry objects to uniquely identify module files. This requires first consulting the FileManager (and therefore the file system) when loading PCM files. This is problematic, as this might load a different PCM file to what's already in the InMemoryModuleCache and fail the size and mtime checks. (This is a bit artificial, but that could happen when using implicit modules without PCM signatures, with FileManager sharing disabled, and without a caching VFS.)

This PR changes things so that module files are identified by their file system path. This removes the need of knowing the file entry at the start and allows us to fix the race. The downside is that we no longer get the FileManager inode-based uniquing, meaning symlinks in the module cache no longer work. I think this is fine, since Clang itself never creates symlinks in that directory. Moreover, we already had to work around filesystems recycling inode numbers, so this actually seems like a win to me (and resolves a long-standing FIXME-like comment).

Note that this change also requires ModuleManager to use consistent paths to refer to module files. This might be problematic if there are concurrent Clang processes operating on the same module cache directory but with different spellings of its path - the IMPORT records in PCM files will be valid and consistent within single process, but may break uniquing in another process. We currently only do path-based canonicalization instead of more robust solution of something like real paths.

[hahnjo]

Thanks for the PR, testing for the use case described in #130795 it seems to fix the issue!

tl;dr I'm building LLVM+Clang+libcxx with

-DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind"

and then use the built clang with libc++ to configure LLVM with LLVM_ENABLE_MODULES=ON:

-DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=/path/to/clang -DCMAKE_CXX_COMPILER=/path/to/clang++ -DCMAKE_CXX_FLAGS="-Xclang -fno-implicit-modules-use-lock" -DLLVM_ENABLE_LIBCXX=ON -DLLVM_ENABLE_MODULES=ON

Using -Xclang -fno-implicit-modules-use-lock as described in #130795 (comment) instead of my manual changes still fails (almost) instantly with main, but passed two times the complete build with this PR 👏

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[jansvoboda11]

Superseded by #185765 that doesn't rely on path normalization.