[flang][runtime] Keep frame buffer in sync with file when truncating

[klausler]

Summary

When the I/O runtime is truncating an external file due to an
implied ENDFILE or explicit ENDFILE, ensure that the unit's frame
buffer for the file discards any data that have become obsolete.

This bug caused trouble with ACCESS='STREAM' I/O using POS= on
a WRITE, but it may have not been limited to that scenario.

flang/runtime/buffer.h

if (dirty_) {
         }
       }
       if (length_ == 0) {
         Reset(fileOffset_);
       }
     }
   }
 
+  void TruncateFrame(std::int64_t at, IoErrorHandler &handler) {
+    RUNTIME_CHECK(handler, !dirty_);
+    if (at <= fileOffset_) {
+      Reset(at);
+    } else if (at < fileOffset_ + length_) {
+      length_ = at - fileOffset_;
+    }
+  }
+
 private:
   STORE &Store() { return static_cast<STORE &>(*this); }
 
   void Reallocate(std::int64_t bytes, const Terminator &terminator) {
     if (bytes > size_) {
       char *old{buffer_};
       auto oldSize{size_};
       size_ = std::max<std::int64_t>(bytes, size_ + minBuffer);

flang/runtime/unit.cpp

if (furthestPositionInRecord > 0) {
       // Last read/write was non-advancing, so AdvanceRecord() was not called.
       leftTabLimit.reset();
       ++currentRecordNumber;
     }
     endfileRecordNumber = currentRecordNumber;
   }
   frameOffsetInFile_ += recordOffsetInFrame_ + furthestPositionInRecord;
   recordOffsetInFrame_ = 0;
-  // Flush (if dirty) and reset the frame (even if reading)
-  WriteFrame(frameOffsetInFile_, 0, handler);
+  FlushOutput(handler);
   Truncate(frameOffsetInFile_, handler);
+  TruncateFrame(frameOffsetInFile_, handler);
   BeginRecord();
   impliedEndfile_ = false;
 }
 
 void ExternalFileUnit::CommitWrites() {
   frameOffsetInFile_ +=
       recordOffsetInFrame_ + recordLength.value_or(furthestPositionInRecord);
   recordOffsetInFrame_ = 0;