[clang] Avoid reparsing VFS overlay files for module dep collector (#158372)

This PR uses the new-ish `llvm::vfs::FileSystem::visit()` interface to
collect VFS overlay entries from an existing `FileSystem` instance
rather than parsing the VFS YAML file anew. This prevents duplicate
diagnostics as observed by `clang/test/VFS/broken-vfs-module-dep.c`.

Author: jansvoboda11

clang/lib/Frontend/CompilerInstance.cpp

@@ -271,19 +271,12 @@ static void collectIncludePCH(CompilerInstance &CI,
 
 static void collectVFSEntries(CompilerInstance &CI,
                               std::shared_ptr<ModuleDependencyCollector> MDC) {
-  if (CI.getHeaderSearchOpts().VFSOverlayFiles.empty())
-    return;
-
   // Collect all VFS found.
   SmallVector<llvm::vfs::YAMLVFSEntry, 16> VFSEntries;
-  for (const std::string &VFSFile : CI.getHeaderSearchOpts().VFSOverlayFiles) {
-    llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
-        llvm::MemoryBuffer::getFile(VFSFile);
-    if (!Buffer)
-      return;
-    llvm::vfs::collectVFSFromYAML(std::move(Buffer.get()),
-                                  /*DiagHandler*/ nullptr, VFSFile, VFSEntries);
-  }
+  CI.getVirtualFileSystem().visit([&](llvm::vfs::FileSystem &VFS) {
+    if (auto *RedirectingVFS = dyn_cast<llvm::vfs::RedirectingFileSystem>(&VFS))
+      llvm::vfs::collectVFSEntries(*RedirectingVFS, VFSEntries);
+  });
 
   for (auto &E : VFSEntries)
     MDC->addFile(E.VPath, E.RPath);

clang/test/VFS/broken-vfs-module-dep.c

@@ -2,6 +2,5 @@
 // RUN: mkdir -p %t
 // RUN: not %clang_cc1 -module-dependency-dir %t -ivfsoverlay %S/Inputs/invalid-yaml.yaml %s 2>&1 | FileCheck %s
 
-// CHECK: error: Unexpected token
 // CHECK: error: Unexpected token
 // CHECK: 1 error generated

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -1114,14 +1114,10 @@ class LLVM_ABI RedirectingFileSystem
 };
 
 /// Collect all pairs of <virtual path, real path> entries from the
-/// \p YAMLFilePath. This is used by the module dependency collector to forward
+/// \p VFS. This is used by the module dependency collector to forward
 /// the entries into the reproducer output VFS YAML file.
-LLVM_ABI void collectVFSFromYAML(
-    std::unique_ptr<llvm::MemoryBuffer> Buffer,
-    llvm::SourceMgr::DiagHandlerTy DiagHandler, StringRef YAMLFilePath,
-    SmallVectorImpl<YAMLVFSEntry> &CollectedEntries,
-    void *DiagContext = nullptr,
-    IntrusiveRefCntPtr<FileSystem> ExternalFS = getRealFileSystem());
+void collectVFSEntries(RedirectingFileSystem &VFS,
+                       SmallVectorImpl<YAMLVFSEntry> &CollectedEntries);
 
 class YAMLVFSWriter {
   std::vector<YAMLVFSEntry> Mappings;

llvm/lib/Support/VirtualFileSystem.cpp

@@ -2707,19 +2707,9 @@ static void getVFSEntries(RedirectingFileSystem::Entry *SrcE,
   Entries.push_back(YAMLVFSEntry(VPath.c_str(), FE->getExternalContentsPath()));
 }
 
-void vfs::collectVFSFromYAML(std::unique_ptr<MemoryBuffer> Buffer,
-                             SourceMgr::DiagHandlerTy DiagHandler,
-                             StringRef YAMLFilePath,
-                             SmallVectorImpl<YAMLVFSEntry> &CollectedEntries,
-                             void *DiagContext,
-                             IntrusiveRefCntPtr<FileSystem> ExternalFS) {
-  std::unique_ptr<RedirectingFileSystem> VFS = RedirectingFileSystem::create(
-      std::move(Buffer), DiagHandler, YAMLFilePath, DiagContext,
-      std::move(ExternalFS));
-  if (!VFS)
-    return;
-  ErrorOr<RedirectingFileSystem::LookupResult> RootResult =
-      VFS->lookupPath("/");
+void vfs::collectVFSEntries(RedirectingFileSystem &VFS,
+                            SmallVectorImpl<YAMLVFSEntry> &CollectedEntries) {
+  ErrorOr<RedirectingFileSystem::LookupResult> RootResult = VFS.lookupPath("/");
   if (!RootResult)
     return;
   SmallVector<StringRef, 8> Components;