Merge pull request #2559 from GEOS-ESM/bugfix/wjiang/add_0size_message

add message to oserver-root even it is 0-sized

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- added 0-size message to o-server root processes
+
 ### Removed
 
 ### Deprecated

pfio/MultiGroupServer.F90

@@ -367,7 +367,7 @@ subroutine receive_output_data(this, rc)
               call handle%wait()
               words = word_size(q%type_kind)
               local_size = product(q%count)*words
-              if (local_size > 0) then
+              if (local_size > 0 .or. this%I_am_front_root) then
                  collection_counter = this%stage_offset%at(i_to_string(q%collection_id))
                  call c_f_pointer(handle%data_reference%base_address, i_ptr, shape=[local_size])
                  call f_d_ms(collection_counter)%add_data_message(q, i_ptr)