[flang][runtime] BACKSPACE after non-advancing I/O

[klausler]

Summary

A BACKSPACE statement on a unit after a READ or WRITE with ADVANCE="NO"
must reset the position to the beginning of the record, not to the
beginning of the previous one.

flang/runtime/io-stmt.cpp

 
 template <Direction DIR>
 void ExternalIoStatementState<DIR>::CompleteOperation() {
   if (completedOperation()) {
     return;
   }
   if constexpr (DIR == Direction::Input) {
     BeginReadingRecord(); // in case there were no I/O items
+    if (mutableModes().nonAdvancing) {
+      unit().leftTabLimit = unit().furthestPositionInRecord;
+    }
     if (!mutableModes().nonAdvancing || GetIoStat() == IostatEor) {
       FinishReadingRecord();
     }
   } else { // output
     if (mutableModes().nonAdvancing) {
       // Make effects of positioning past the last Emit() visible with blanks.
       std::int64_t n{unit().positionInRecord - unit().furthestPositionInRecord};
       unit().positionInRecord = unit().furthestPositionInRecord;

flang/runtime/unit.cpp

void ExternalFileUnit::BackspaceRecord(IoErrorHandler &handler) {
   if (access == Access::Direct || !IsRecordFile()) {
     handler.SignalError(IostatBackspaceNonSequential,
         "BACKSPACE(UNIT=%d) on direct-access file or unformatted stream",
         unitNumber());
   } else {
     if (IsAfterEndfile()) {
       // BACKSPACE after explicit ENDFILE
       currentRecordNumber = *endfileRecordNumber;
+    } else if (leftTabLimit) {
+      // BACKSPACE after non-advancing I/O
+      leftTabLimit.reset();
     } else {
       DoImpliedEndfile(handler);
       if (frameOffsetInFile_ + recordOffsetInFrame_ > 0) {
         --currentRecordNumber;
         if (openRecl && access == Access::Direct) {
           BackspaceFixedRecord(handler);
         } else {
           RUNTIME_CHECK(handler, isUnformatted.has_value());