Document and add check on copy transfer doing N-N siblings transfer, …

…refs idaholab#19451

framework/doc/content/source/transfers/MultiAppCopyTransfer.md

@@ -5,6 +5,13 @@ and [auxiliary](/AuxVariables/index.md)) between [MultiApps](/MultiApps/index.md
 of variables, including higher order, elemental, and nodal are supported. The only limitation is that
 the meshes in the parent and sub application must be identical.
 
+## Siblings transfer behavior
+
+This transfer supports sending data from a MultiApp to a MultiApp if and only if the number of subapps
+in the source MultiApp matches the number of subapps in the target MultiApp, and they are distributed
+the same way on the parallel processes. Each source app is then matched to the target app with the same
+subapp index.
+
 ## Example Syntax
 
 !listing test/tests/transfers/multiapp_copy_transfer/linear_lagrange_to_sub/parent.i block=Transfers

framework/include/transfers/MultiAppCopyTransfer.h

@@ -47,4 +47,7 @@ class MultiAppCopyTransfer : public MultiAppDofCopyTransfer
   VariableName _from_var_name;
   /// Name of variables transferring to
   AuxVariableName _to_var_name;
+
+private:
+  virtual void checkSiblingsTransferSupported() const override;
 };

framework/src/transfers/MultiAppCopyTransfer.C

@@ -82,3 +82,19 @@ MultiAppCopyTransfer::execute()
                  "per multiapp involved on each rank");
   }
 }
+
+void
+MultiAppCopyTransfer::checkSiblingsTransferSupported() const
+{
+  // Check that we are in the supported configuration: same number of source and target apps
+  // The allocation of the child apps on the processors must be the same
+  if (getFromMultiApp()->numGlobalApps() == getToMultiApp()->numGlobalApps())
+  {
+    for (const auto i : make_range(getToMultiApp()->numGlobalApps()))
+      if (getFromMultiApp()->hasLocalApp(i) + getToMultiApp()->hasLocalApp(i) == 1)
+        mooseError("Child application allocation on parallel processes must be the same to support "
+                   "siblings variable field copy transfer");
+  }
+  else
+    mooseError("Number of source and target child apps must match for siblings transfer");
+}