Refactor CTUIndex lazy initialization

clang/include/clang/CrossTU/CrossTranslationUnit.h

public:
   /// Get a name to identify a named decl.
   static std::string getLookupName(const NamedDecl *ND);
 
   /// Emit diagnostics for the user for potential configuration errors.
   void emitCrossTUDiagnostics(const IndexError &IE);
 
 private:
   bool checkThresholdReached() const;
+  llvm::Error lazyInitCTUIndex(StringRef CrossTUDir, StringRef IndexName);
   void lazyInitImporterSharedSt(TranslationUnitDecl *ToTU);
   ASTImporter &getOrCreateASTImporter(ASTContext &From);
   template <typename T>
   llvm::Expected<const T *> getCrossTUDefinitionImpl(const T *D,
                                                      StringRef CrossTUDir,
                                                      StringRef IndexName,
                                                      bool DisplayCTUProgress);
   template <typename T>

clang/lib/CrossTU/CrossTranslationUnit.cpp

 bool CrossTranslationUnitContext::checkThresholdReached() const {
   if (NumASTLoaded >= CTULoadThreshold) {
     ++NumASTLoadThresholdReached;
     return true;
   }
   return false;
 }
 
+llvm::Error CrossTranslationUnitContext::lazyInitCTUIndex(StringRef CrossTUDir,
+                                                          StringRef IndexName) {
+  // Dont initialize if the map is filled.
+  if (!NameFileMap.empty())
+    return llvm::Error::success();
+
+  // Get the absolute path to the index file.
+  SmallString<256> IndexFile = CrossTUDir;
+  if (llvm::sys::path::is_absolute(IndexName))
+    IndexFile = IndexName;
+  else
+    llvm::sys::path::append(IndexFile, IndexName);
+
+  if (auto IndexMapping = parseCrossTUIndex(IndexFile, CrossTUDir)) {
+    // Initialize member map.
+    NameFileMap = *IndexMapping;
+    return llvm::Error::success();
+  } else {
+    // Error while parsing CrossTU index file.
+    return IndexMapping.takeError();
+  };
+}
+
 llvm::Expected<ASTUnit *> CrossTranslationUnitContext::loadExternalAST(
     StringRef LookupName, StringRef CrossTUDir, StringRef IndexName,
     bool DisplayCTUProgress) {
   // FIXME: The current implementation only supports loading decls with
   //        a lookup name from a single translation unit. If multiple
   //        translation units contains decls with the same lookup name an
   //        error will be returned.
 
   // If import threshold is reached, don't import anything.
   if (checkThresholdReached())
     return llvm::make_error<IndexError>(
         index_error_code::load_threshold_reached);
 
   ASTUnit *Unit = nullptr;
   auto NameUnitCacheEntry = NameASTUnitMap.find(LookupName);
   if (NameUnitCacheEntry == NameASTUnitMap.end()) {
-    if (NameFileMap.empty()) {
-      SmallString<256> IndexFile = CrossTUDir;
-      if (llvm::sys::path::is_absolute(IndexName))
-        IndexFile = IndexName;
-      else
-        llvm::sys::path::append(IndexFile, IndexName);
-      llvm::Expected<llvm::StringMap<std::string>> IndexOrErr =
-          parseCrossTUIndex(IndexFile, CrossTUDir);
-      if (IndexOrErr)
-        NameFileMap = *IndexOrErr;
-      else
-        return IndexOrErr.takeError();
-    }
+    // Lazily initialize the mapping from function names to AST files.
+    if (llvm::Error InitFailed = lazyInitCTUIndex(CrossTUDir, IndexName))
+      return std::move(InitFailed);
 
     auto It = NameFileMap.find(LookupName);
     if (It == NameFileMap.end()) {
       ++NumNotInOtherTU;
       return llvm::make_error<IndexError>(index_error_code::missing_definition);
     }
     StringRef ASTFileName = It->second;
     auto ASTCacheEntry = FileASTUnitMap.find(ASTFileName);