[flang][runtime] Fix WRITE after OPEN(.., POSITION="APPEND")

[klausler]

Summary

The initial size of the file was not being captured as the file position
on which the first output buffer should be framed.

[vdonaldson]

flang/runtime/unit.cpp

140

I don't see any other uses of IostatOpenBadAppend. Should this value be deleted?

[klausler]

Restore use of IostatOpenBadAppend error code instead of generic I/O error on a bad OPEN(POSITION='APPEND').

flang/runtime/file.cpp

if (fd_ < 0) {
       if (fd_ < 0) {
         handler.SignalErrno();
       }
     }
   }
   RUNTIME_CHECK(handler, action.has_value());
   pending_.reset();
   if (position == Position::Append && !RawSeekToEnd()) {
-    handler.SignalErrno();
+    handler.SignalError(IostatOpenBadAppend);
   }
   isTerminal_ = ::isatty(fd_) == 1;
   mayRead_ = *action != Action::Write;
   mayWrite_ = *action != Action::Read;
   if (status == OpenStatus::Old || status == OpenStatus::Unknown) {
     knownSize_.reset();
 #ifndef _WIN32
     struct stat buf;

flang/runtime/unit.cpp

if (!openRecl) {
       handler.SignalError(IostatOpenBadRecl,
           "OPEN(UNIT=%d,ACCESS='DIRECT'): record length is not known",
           unitNumber());
     } else if (*openRecl <= 0) {
       handler.SignalError(IostatOpenBadRecl,
           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is invalid",
           unitNumber(), static_cast<std::intmax_t>(*openRecl));
     } else if (totalBytes && (*totalBytes % *openRecl != 0)) {
-      handler.SignalError(IostatOpenBadAppend,
+      handler.SignalError(IostatOpenBadRecl,
           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is not an "
           "even divisor of the file size %jd",
           unitNumber(), static_cast<std::intmax_t>(*openRecl),
           static_cast<std::intmax_t>(*totalBytes));
     }
     recordLength = openRecl;
   }
   endfileRecordNumber.reset();
   currentRecordNumber = 1;
   if (totalBytes && access == Access::Direct && openRecl.value_or(0) > 0) {
     endfileRecordNumber = 1 + (*totalBytes / *openRecl);
   }
-  if (position == Position::Append && access != Access::Stream) {
+  if (position == Position::Append) {
+    if (totalBytes) {
+      frameOffsetInFile_ = *totalBytes;
+    }
+    if (access != Access::Stream) {
       if (!endfileRecordNumber) {
         // Fake it so that we can backspace relative from the end
         endfileRecordNumber = std::numeric_limits<std::int64_t>::max() - 2;
       }
       currentRecordNumber = *endfileRecordNumber;
     }
   }
+}
 
 void ExternalFileUnit::OpenAnonymousUnit(std::optional<OpenStatus> status,
     std::optional<Action> action, Position position, Convert convert,
     IoErrorHandler &handler) {
   // I/O to an unconnected unit reads/creates a local file, e.g. fort.7
   std::size_t pathMaxLen{32};
   auto path{SizedNew<char>{handler}(pathMaxLen)};
   std::snprintf(path.get(), pathMaxLen, "fort.%d", unitNumber_);