[flang] Fix race condition in runtime

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.

Differential Revision: https://reviews.llvm.org/D101928

Author: klausler

flang/runtime/io-error.cpp

@@ -36,12 +36,14 @@ void IoErrorHandler::SignalError(int iostatOrErrno, const char *msg, ...) {
           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 {

flang/runtime/terminator.cpp

@@ -16,6 +16,7 @@ namespace Fortran::runtime {
   va_list ap;
   va_start(ap, message);
   CrashArgs(message, ap);
+  va_end(ap);
 }
 
 static void (*crashHandler)(const char *, int, const char *, va_list &){

flang/runtime/unit.cpp

@@ -205,16 +205,20 @@ UnitMap &ExternalFileUnit::GetUnitMap() {
   }
   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;
 }