[clang] Enable making CompilerInstance VFS thread-safe (#135737)

The `llvm::vfs::FileSystem` interface makes no promises around
thread-safety. To enable making `CompilerInstance` thread-safe, this PR
allows passing an explicit VFS to `cloneForModuleCompile()`. This will
be used from the dependency scanner.

Author: jansvoboda11

clang/include/clang/Frontend/CompilerInstance.h

@@ -845,18 +845,25 @@ class CompilerInstance : public ModuleLoader {
   /// Creates a \c CompilerInstance for compiling a module.
   ///
   /// This expects a properly initialized \c FrontendInputFile.
+  ///
+  /// Explicitly-specified \c VFS takes precedence over the VFS of this instance
+  /// when creating the clone and also prevents \c FileManager sharing.
   std::unique_ptr<CompilerInstance> cloneForModuleCompileImpl(
       SourceLocation ImportLoc, StringRef ModuleName, FrontendInputFile Input,
-      StringRef OriginalModuleMapFile, StringRef ModuleFileName);
+      StringRef OriginalModuleMapFile, StringRef ModuleFileName,
+      IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS = nullptr);
 
 public:
   /// Creates a new \c CompilerInstance for compiling a module.
   ///
   /// This takes care of creating appropriate \c FrontendInputFile for
   /// public/private frameworks, inferred modules and such.
-  std::unique_ptr<CompilerInstance>
-  cloneForModuleCompile(SourceLocation ImportLoc, Module *Module,
-                        StringRef ModuleFileName);
+  ///
+  /// Explicitly-specified \c VFS takes precedence over the VFS of this instance
+  /// when creating the clone and also prevents \c FileManager sharing.
+  std::unique_ptr<CompilerInstance> cloneForModuleCompile(
+      SourceLocation ImportLoc, Module *Module, StringRef ModuleFileName,
+      IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS = nullptr);
 
   /// Compile a module file for the given module, using the options
   /// provided by the importing compiler instance. Returns true if the module

clang/lib/Frontend/CompilerInstance.cpp

@@ -1152,7 +1152,8 @@ static Language getLanguageFromOptions(const LangOptions &LangOpts) {
 
 std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompileImpl(
     SourceLocation ImportLoc, StringRef ModuleName, FrontendInputFile Input,
-    StringRef OriginalModuleMapFile, StringRef ModuleFileName) {
+    StringRef OriginalModuleMapFile, StringRef ModuleFileName,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
   // Construct a compiler invocation for creating this module.
   auto Invocation = std::make_shared<CompilerInvocation>(getInvocation());
 
@@ -1212,19 +1213,21 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompileImpl(
   auto &Inv = *Invocation;
   Instance.setInvocation(std::move(Invocation));
 
+  if (VFS) {
+    Instance.createFileManager(std::move(VFS));
+  } else if (FrontendOpts.ModulesShareFileManager) {
+    Instance.setFileManager(&getFileManager());
+  } else {
+    Instance.createFileManager(&getVirtualFileSystem());
+  }
+
   Instance.createDiagnostics(
-      getVirtualFileSystem(),
+      Instance.getVirtualFileSystem(),
       new ForwardingDiagnosticConsumer(getDiagnosticClient()),
       /*ShouldOwnClient=*/true);
-
   if (llvm::is_contained(DiagOpts.SystemHeaderWarningsModules, ModuleName))
     Instance.getDiagnostics().setSuppressSystemWarnings(false);
 
-  if (FrontendOpts.ModulesShareFileManager) {
-    Instance.setFileManager(&getFileManager());
-  } else {
-    Instance.createFileManager(&getVirtualFileSystem());
-  }
   Instance.createSourceManager(Instance.getFileManager());
   SourceManager &SourceMgr = Instance.getSourceManager();
 
@@ -1318,7 +1321,8 @@ static OptionalFileEntryRef getPublicModuleMap(FileEntryRef File,
 }
 
 std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompile(
-    SourceLocation ImportLoc, Module *Module, StringRef ModuleFileName) {
+    SourceLocation ImportLoc, Module *Module, StringRef ModuleFileName,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
   StringRef ModuleName = Module->getTopLevelModuleName();
 
   InputKind IK(getLanguageFromOptions(getLangOpts()), InputKind::ModuleMap);
@@ -1363,7 +1367,8 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompile(
     return cloneForModuleCompileImpl(
         ImportLoc, ModuleName,
         FrontendInputFile(ModuleMapFilePath, IK, IsSystem),
-        ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName);
+        ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName,
+        std::move(VFS));
   }
 
   // FIXME: We only need to fake up an input file here as a way of
@@ -1380,7 +1385,8 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompile(
   auto Instance = cloneForModuleCompileImpl(
       ImportLoc, ModuleName,
       FrontendInputFile(FakeModuleMapFile, IK, +Module->IsSystem),
-      ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName);
+      ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName,
+      std::move(VFS));
 
   std::unique_ptr<llvm::MemoryBuffer> ModuleMapBuffer =
       llvm::MemoryBuffer::getMemBufferCopy(InferredModuleMapContent);