[flang] Catch READ/WRITE on direct-access file without REC=

[klausler]

Summary

A data transfer statement must have REC= in its control list
if (and only if) the unit was opened with ACCESS='DIRECT'.
The runtime wasn't catching this error, but was just silently
advancing to the next record as if the access were sequential.

flang/runtime/unit.h

public:
 
   template <typename A, typename... X>
   IoStatementState &BeginIoStatement(X &&...xs) {
     lock_.Take(); // dropped in EndIoStatement()
     A &state{u_.emplace<A>(std::forward<X>(xs)...)};
     if constexpr (!std::is_same_v<A, OpenStatementState>) {
       state.mutableModes() = ConnectionState::modes;
     }
+    directAccessRecWasSet_ = false;
     io_.emplace(state);
     return *io_;
   }
 
   bool Emit(
       const char *, std::size_t, std::size_t elementBytes, IoErrorHandler &);
   bool Receive(char *, std::size_t, std::size_t elementBytes, IoErrorHandler &);
   std::size_t GetNextInputBytes(const char *&, IoErrorHandler &);
private:
   void BeginVariableFormattedInputRecord(IoErrorHandler &);
   void BackspaceFixedRecord(IoErrorHandler &);
   void BackspaceVariableUnformattedRecord(IoErrorHandler &);
   void BackspaceVariableFormattedRecord(IoErrorHandler &);
   bool SetVariableFormattedRecordLength();
   void DoImpliedEndfile(IoErrorHandler &);
   void DoEndfile(IoErrorHandler &);
   void CommitWrites();
+  bool CheckDirectAccess(IoErrorHandler &);
 
   int unitNumber_{-1};
   Direction direction_{Direction::Output};
   bool impliedEndfile_{false}; // sequential/stream output has taken place
   bool beganReadingRecord_{false};
+  bool directAccessRecWasSet_{false}; // REC= appeared
 
   Lock lock_;
 
   // When an I/O statement is in progress on this unit, holds its state.
   std::variant<std::monostate, OpenStatementState, CloseStatementState,
       ExternalFormattedIoStatementState<Direction::Output>,
       ExternalFormattedIoStatementState<Direction::Input>,
       ExternalListIoStatementState<Direction::Output>,

flang/runtime/unit.cpp

#endif
     // was a BACKSPACE or non advancing input statement.
     recordLength.reset();
     beganReadingRecord_ = false;
   }
   if (IsAfterEndfile()) {
     handler.SignalError(IostatWriteAfterEndfile);
     return false;
   }
+  CheckDirectAccess(handler);
   WriteFrame(frameOffsetInFile_, recordOffsetInFrame_ + furthestAfter, handler);
   if (positionInRecord > furthestPositionInRecord) {
     std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord, ' ',
         positionInRecord - furthestPositionInRecord);
   }
   char *to{Frame() + recordOffsetInFrame_ + positionInRecord};
   std::memcpy(to, data, bytes);
   if (swapEndianness_) {
void ExternalFileUnit::SetLeftTabLimit() {
   positionInRecord = furthestPositionInRecord;
 }
 
 bool ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
   RUNTIME_CHECK(handler, direction_ == Direction::Input);
   if (!beganReadingRecord_) {
     beganReadingRecord_ = true;
     if (access == Access::Direct) {
-      RUNTIME_CHECK(handler, openRecl);
+      CheckDirectAccess(handler);
       auto need{static_cast<std::size_t>(recordOffsetInFrame_ + *openRecl)};
       auto got{ReadFrame(frameOffsetInFile_, need, handler)};
       if (got >= need) {
         recordLength = openRecl;
       } else {
         recordLength.reset();
         handler.SignalEnd();
       }
if (access == Access::Direct) {
     currentRecordNumber = 1;
   }
 }
 
 void ExternalFileUnit::SetPosition(std::int64_t pos, IoErrorHandler &handler) {
   DoImpliedEndfile(handler);
   frameOffsetInFile_ = pos;
   recordOffsetInFrame_ = 0;
+  if (access == Access::Direct) {
+    directAccessRecWasSet_ = true;
+  }
   BeginRecord();
 }
 
 void ExternalFileUnit::EndIoStatement() {
   io_.reset();
   u_.emplace<std::monostate>();
   lock_.Drop();
 }

 
 void ExternalFileUnit::CommitWrites() {
   frameOffsetInFile_ +=
       recordOffsetInFrame_ + recordLength.value_or(furthestPositionInRecord);
   recordOffsetInFrame_ = 0;
   BeginRecord();
 }
 
+bool ExternalFileUnit::CheckDirectAccess(IoErrorHandler &handler) {
+  if (access == Access::Direct) {
+    RUNTIME_CHECK(handler, openRecl);
+    if (!directAccessRecWasSet_) {
+      handler.SignalError("No REC= was specified for a data transfer with ACCESS='DIRECT'");
+      return false;
+    }
+  }
+  return true;
+}
+
 ChildIo &ExternalFileUnit::PushChildIo(IoStatementState &parent) {
   OwningPtr<ChildIo> current{std::move(child_)};
   Terminator &terminator{parent.GetIoErrorHandler()};
   OwningPtr<ChildIo> next{New<ChildIo>{terminator}(parent, std::move(current))};
   child_.reset(next.release());
   return *child_;
 }