[ELF] Avoid data race in ObjectFile<ELFT>::getDILineInfo().

[ikudrin]

Summary

It is possible that this method is called simultaneously from several threads,
so the initialization of DwarfLine has to be protected.

In my case the call sequence was the following:
OutputSectionCommand::writeTo() -> parallelForEachN() ->
(two parallel threads) ->
InputSection::writeTo() -> InputSectionBase::relocate() ->
InputSectionBase::relocateAlloc() -> AArch64::relocateOne() ->
checkInt<28>() -> getErrorLocation() -> getErrorLoc() ->
InputSectionBase::getLocation() -> ObjectFile<ELFT>::getLineInfo() ->
ObjectFile<ELFT>::getDILineInfo() -> ObjectFile<ELFT>::initializeDwarfLine().

[ruiu]

LGTM

ELF/InputFiles.h

 #include "lld/Core/Reproduce.h"
 #include "llvm/ADT/CachedHashString.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/IR/Comdat.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/ELF.h"
 #include "llvm/Object/IRObjectFile.h"
+#include "llvm/Support/Threading.h"
 
 #include <map>
 
 namespace llvm {
 class DWARFDebugLine;
 class TarWriter;
 struct DILineInfo;
 namespace lto {
private:
   // .shstrtab contents.
   StringRef SectionStringTable;
 
   // Debugging information to retrieve source file and line for error
   // reporting. Linker may find reasonable number of errors in a
   // single object file, so we cache debugging information in order to
   // parse it only once for each object file we link.
   std::unique_ptr<llvm::DWARFDebugLine> DwarfLine;
+  llvm::once_flag InitDwarfLine;
 };
 
 // LazyObjectFile is analogous to ArchiveFile in the sense that
 // the file contains lazy symbols. The difference is that
 // LazyObjectFile wraps a single file instead of multiple files.
 //
 // This class is used for --start-lib and --end-lib options which
 // instruct the linker to link object files between them with the

ELF/InputFiles.cpp

template <class ELFT> void elf::ObjectFile<ELFT>::initializeDwarfLine() {
   DwarfLine->getOrParseLineTable(LineData, 0);
 }
 
 // Returns source line information for a given offset
 // using DWARF debug info.
 template <class ELFT>
 Optional<DILineInfo> elf::ObjectFile<ELFT>::getDILineInfo(InputSectionBase *S,
                                                           uint64_t Offset) {
-  if (!DwarfLine)
-    initializeDwarfLine();
+  llvm::call_once(InitDwarfLine, [this]() { initializeDwarfLine(); });
 
   // The offset to CU is 0.
   const DWARFDebugLine::LineTable *Tbl = DwarfLine->getLineTable(0);
   if (!Tbl)
     return None;
 
   // Use fake address calcuated by adding section file offset and offset in
   // section. See comments for ObjectInfo class.