[flang] Fix WRITE after BACKSPACE

[klausler]

Summary

A WRITE to an unformatted sequential variable-length unit after
a BACKSPACE needs to forget its previous knowledge of the length
of the record that's about to be overwritten, and a BACKSPACE
after an ENDFILE or at the start of the file needs to be a no-op.

flang/runtime/io-api.cpp

     if (unit.access == Access::Sequential && !unit.isFixedRecordLength) {
       // Create space for (sub)record header to be completed by
       // UnformattedIoStatementState<Direction::Output>::EndIoStatement()
+      unit.recordLength.reset(); // in case of prior BACKSPACE
       io.Emit("\0\0\0\0", 4); // placeholder for record length header
     }
   }

flang/runtime/unit.cpp

           static_cast<std::intmax_t>(*totalBytes));
     }
   }
+  endfileRecordNumber.reset();
+  currentRecordNumber = 1;
+  if (totalBytes && recordLength && *recordLength) {
+    endfileRecordNumber = 1 + (*totalBytes / *recordLength);
+  }
   if (position == Position::Append) {
-    if (totalBytes && recordLength && *recordLength) {
-      endfileRecordNumber = 1 + (*totalBytes / *recordLength);
-    } else {
+    if (!endfileRecordNumber) {
       // Fake it so that we can backspace relative from the end
-      endfileRecordNumber = std::numeric_limits<std::int64_t>::max() - 1;
+      endfileRecordNumber = std::numeric_limits<std::int64_t>::max() - 2;
     }
     currentRecordNumber = *endfileRecordNumber;
-  } else {
-    currentRecordNumber = 1;
   }
 }
 

 void ExternalFileUnit::FinishReadingRecord(IoErrorHandler &handler) {
   RUNTIME_CHECK(handler, direction_ == Direction::Input && beganReadingRecord_);
   beganReadingRecord_ = false;
-  if (access == Access::Sequential) {
+  if (handler.GetIoStat() != IostatOk) {
+    // avoid bogus crashes in END/ERR circumstances
+  } else if (access == Access::Sequential) {
     RUNTIME_CHECK(handler, recordLength.has_value());
     if (isFixedRecordLength) {
       frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;

     handler.SignalError(IostatBackspaceNonSequential,
         "BACKSPACE(UNIT=%d) on non-sequential file", unitNumber());
   } else {
-    DoImpliedEndfile(handler);
-    --currentRecordNumber;
-    BeginRecord();
-    if (isFixedRecordLength) {
-      BackspaceFixedRecord(handler);
-    } else if (isUnformatted) {
-      BackspaceVariableUnformattedRecord(handler);
+    if (endfileRecordNumber && currentRecordNumber > *endfileRecordNumber) {
+      // BACKSPACE after ENDFILE
     } else {
-      BackspaceVariableFormattedRecord(handler);
+      DoImpliedEndfile(handler);
+      if (frameOffsetInFile_ + recordOffsetInFrame_ > 0) {
+        --currentRecordNumber;
+        if (isFixedRecordLength) {
+          BackspaceFixedRecord(handler);
+        } else if (isUnformatted) {
+          BackspaceVariableUnformattedRecord(handler);
+        } else {
+          BackspaceVariableFormattedRecord(handler);
+        }
+      }
     }
+    BeginRecord();
   }
 }
 

   } else if (!mayWrite()) {
     handler.SignalError(IostatEndfileUnwritable,
         "ENDFILE(UNIT=%d) on read-only file", unitNumber());
+  } else if (endfileRecordNumber &&
+      currentRecordNumber > *endfileRecordNumber) {
+    // ENDFILE after ENDFILE
   } else {
     DoEndfile(handler);
+    ++currentRecordNumber;
   }
 }
 

     DoImpliedEndfile(handler);
     SetPosition(0);
     currentRecordNumber = 1;
-    // TODO: reset endfileRecordNumber?
   }
 }