SWIFT crashes when writing snapshots after a restart

@matthieu: I opened a new issue here to gather debugging output/data so we don't do things twice.

Current master branch, code configured with

--disable-mpi --disable-doxygen-doc --enable-cell-graph --disable-hand-vec --with-rt=none --with-hydro=anarchy-du --enable-debug --enable-debugging-checks --enable-task-debugging

compiled with gcc 10.2.0.

Running the examples/HydroTests/UniformBox_3D example and restarting from step 12 consistently gives me an error:

[00003.7] tools.c:safe_checkdir():1090: Failed to create directory  (No such file or directory)

I tracked the issue down to single_io.c:write_output_single(), where the call to output_options_get_basename(...) gives different results.

I added more messages to output_options_get_basename():

void output_options_get_basename(const struct output_options* output_options,
                                 const char* selection_name,
                                 const char* default_subdirname,
                                 const char* default_basename,
                                 char subdir_name[FILENAME_BUFFER_SIZE],
                                 char basename[FILENAME_BUFFER_SIZE]) {

  /* Get the ID of the output selection in the structure */
  int selection_id =
      parser_get_section_id(output_options->select_output, selection_name);

  message("check 1.0 - selection_id=%d", selection_id);

  /* Special treatment for absent `Default` section */
  if (selection_id < 0) {
    selection_id = output_options->select_output->sectionCount;
    message("check 1.1 - selection_id=%d", selection_id);
  }

  /* If the default keyword is found, we use the name provided
   * in the param file (not the select output!), aka. the argument
   * of the function. */
  message("check 1.1.1 - output_options->basenames[]='%s'; select_output_default_basename='%s'", 
            output_options->basenames[selection_id], select_output_default_basename);
  if (strcmp(output_options->basenames[selection_id],
             select_output_default_basename) == 0) {
    sprintf(basename, "%s", default_basename);
    message("check 1.2 - basename '%s'; '%s';", basename, default_basename);
  } else {
    sprintf(basename, "%s", output_options->basenames[selection_id]);
    message("check 1.3 - basename '%s'; '%s';", basename, default_basename);
  }

  /* If the default keyword is found, we use the subdir name provided
   * in the param file (not the select output!), aka. the argument
   * of the function. */
  if (strcmp(output_options->subdir_names[selection_id],
             select_output_default_subdir_name) == 0) {
    sprintf(subdir_name, "%s", default_subdirname);
    message("check 1.4 - subdir_name '%s'; '%s';", subdir_name, default_subdirname);
  } else {
    sprintf(subdir_name, "%s", output_options->subdir_names[selection_id]);
    message("check 1.5 - subdir_name '%s'; '%s';", subdir_name, default_subdirname);
  }
}

The output when starting the simulation from scratch:

[00028.1] output_options_get_basename: check 1.0 - selection_id=0
[00028.1] output_options_get_basename: check 1.1.1 - output_options->basenames[]='Standard'; select_output_default_basename='Standard'
[00028.1] output_options_get_basename: check 1.2 - basename 'uniformBox'; 'uniformBox';
[00028.1] output_options_get_basename: check 1.4 - subdir_name '.'; '.';

The output when restarting from step 12:

[00003.5] output_options_get_basename: check 1.0 - selection_id=0
[00003.5] output_options_get_basename: check 1.1.1 - output_options->basenames[]=''; select_output_default_basename='Standard'
[00003.5] output_options_get_basename: check 1.3 - basename ''; 'uniformBox';
[00003.5] output_options_get_basename: check 1.5 - subdir_name ''; '.';

So somehow the output_options->basenames[0] values are changed when the code restarts.