[flang] Fix race condition in runtime

[klausler]

Summary

The code that initializes the default units 5 & 6 had
a race condition that would allow threads access to the
unit map before it had been populated.

Also add some missing calls to va_end() that will never
be called (they're in program abort situations) but might
elicit warnings if absent.

[kiranchandramohan]

There is a bug report which probably mentions the same issue that is addressed here. Adding the reporter to the review list.
https://bugs.llvm.org/show_bug.cgi?id=48987

[jeanPerier]

The fix looks in line with the issue reported by @rogfer01 and what he suggested.

flang/runtime/io-error.cpp

if (flags_ & (hasIoStat | hasErr)) {
       if (ioStat_ <= 0) {
         ioStat_ = iostatOrErrno; // priority over END=/EOR=
         if (msg && (flags_ & hasIoMsg)) {
           char buffer[256];
           va_list ap;
           va_start(ap, msg);
           std::vsnprintf(buffer, sizeof buffer, msg, ap);
           ioMsg_ = SaveDefaultCharacter(buffer, std::strlen(buffer) + 1, *this);
+          va_end(ap);
         }
       }
     } else if (msg) {
       va_list ap;
       va_start(ap, msg);
       CrashArgs(msg, ap);
+      va_end(ap);
     } else if (const char *errstr{IostatErrorString(iostatOrErrno)}) {
       Crash(errstr);
     } else {
       Crash("I/O error (errno=%d): %s", iostatOrErrno,
           std::strerror(iostatOrErrno));
     }
   }
 }

flang/runtime/terminator.cpp

 #include <cstdlib>
 
 namespace Fortran::runtime {
 
 [[noreturn]] void Terminator::Crash(const char *message, ...) const {
   va_list ap;
   va_start(ap, message);
   CrashArgs(message, ap);
+  va_end(ap);
 }
 
 static void (*crashHandler)(const char *, int, const char *, va_list &){
     nullptr};
 
 void Terminator::RegisterCrashHandler(
     void (*handler)(const char *, int, const char *, va_list &)) {
   crashHandler = handler;

flang/runtime/unit.cpp

if (unitMap) {
     return *unitMap;
   }
   CriticalSection critical{unitMapLock};
   if (unitMap) {
     return *unitMap;
   }
   Terminator terminator{__FILE__, __LINE__};
   IoErrorHandler handler{terminator};
-  unitMap = New<UnitMap>{terminator}().release();
-  ExternalFileUnit &out{ExternalFileUnit::CreateNew(6, terminator)};
+  UnitMap *newUnitMap{New<UnitMap>{terminator}().release()};
+  bool wasExtant{false};
+  ExternalFileUnit &out{newUnitMap->LookUpOrCreate(6, terminator, wasExtant)};
+  RUNTIME_CHECK(terminator, !wasExtant);
   out.Predefine(1);
   out.SetDirection(Direction::Output, handler);
   defaultOutput = &out;
-  ExternalFileUnit &in{ExternalFileUnit::CreateNew(5, terminator)};
+  ExternalFileUnit &in{newUnitMap->LookUpOrCreate(5, terminator, wasExtant)};
+  RUNTIME_CHECK(terminator, !wasExtant);
   in.Predefine(0);
   in.SetDirection(Direction::Input, handler);
   defaultInput = &in;
   // TODO: Set UTF-8 mode from the environment
+  unitMap = newUnitMap;
   return *unitMap;
 }
 
 void ExternalFileUnit::CloseAll(IoErrorHandler &handler) {
   CriticalSection critical{unitMapLock};
   if (unitMap) {
     unitMap->CloseAll(handler);
     FreeMemoryAndNullify(unitMap);