[flang] Fix variable unformatted I/O bug with output after input (#92828)

When reading variable-length unformatted records, the external I/O
library frames the input buffer so that the footer of the previous
record remains in frame. This is done so that a BACKSPACE doesn't have
to do an extra read to get the length of the previous record before
repositioning over it.

When switching from input to output to overwrite or append new records
after reading any, it is necessary to undo this framing of the last word
in the previous record, since the new output isn't going to define it in
the buffer and it'll be overwritten in the filesystem with garbage.

Author: klausler

flang/runtime/external-unit.cpp

@@ -214,6 +214,13 @@ Iostat ExternalFileUnit::SetDirection(Direction direction) {
     }
   } else {
     if (mayWrite()) {
+      if (direction_ == Direction::Input) {
+        // Don't retain any input data from previous record, like a
+        // variable-length unformatted record footer, in the frame,
+        // since we're going start writing frames.
+        frameOffsetInFile_ += recordOffsetInFrame_;
+        recordOffsetInFrame_ = 0;
+      }
       direction_ = Direction::Output;
       return IostatOk;
     } else {
@@ -332,5 +339,4 @@ bool ExternalFileUnit::Wait(int id) {
 }
 
 } // namespace Fortran::runtime::io
-
 #endif // !defined(RT_USE_PSEUDO_FILE_UNIT)

flang/runtime/unit.cpp

@@ -265,6 +265,7 @@ void ExternalFileUnit::FinishReadingRecord(IoErrorHandler &handler) {
     furthestPositionInRecord =
         std::max(furthestPositionInRecord, positionInRecord);
     frameOffsetInFile_ += recordOffsetInFrame_ + furthestPositionInRecord;
+    recordOffsetInFrame_ = 0;
   }
   BeginRecord();
 }