[flang][runtime] Complete partial output records when positioning/closing after non-advancing output

[klausler]

Summary

Before positioning or closing a unit after a non-advancing output statement
has left a partial record in its buffer, complete the record by calling
AdvanceRecord(). Fixes https://github.com/llvm/llvm-project/issues/59761.

flang/runtime/unit.cpp

void ExternalFileUnit::BackspaceVariableFormattedRecord(
   }
   if (*recordLength > 0 &&
       Frame()[recordOffsetInFrame_ + *recordLength - 1] == '\r') {
     --*recordLength;
   }
 }
 
 void ExternalFileUnit::DoImpliedEndfile(IoErrorHandler &handler) {
+  if (!impliedEndfile_ && direction_ == Direction::Output && IsRecordFile() &&
+      access != Access::Direct && leftTabLimit) {
+    // Complete partial record after non-advancing write before
+    // positioning or closing the unit.  Usually sets impliedEndfile_.
+    AdvanceRecord(handler);
+  }
   if (impliedEndfile_) {
     impliedEndfile_ = false;
     if (access != Access::Direct && IsRecordFile() && mayPosition()) {
       DoEndfile(handler);
     }
   }
 }
 
 void ExternalFileUnit::DoEndfile(IoErrorHandler &handler) {
   if (IsRecordFile() && access != Access::Direct) {
     furthestPositionInRecord =
         std::max(positionInRecord, furthestPositionInRecord);
-    if (furthestPositionInRecord > 0) {
+    if (leftTabLimit) {
       // Last read/write was non-advancing, so AdvanceRecord() was not called.
       leftTabLimit.reset();
       ++currentRecordNumber;
     }
     endfileRecordNumber = currentRecordNumber;
   }
   frameOffsetInFile_ += recordOffsetInFrame_ + furthestPositionInRecord;
   recordOffsetInFrame_ = 0;