Document and add check on reporter transfer doing N-N siblings transf…

…er, refs idaholab#19451

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

@@ -10,6 +10,13 @@ This MultiAppReporterTransfer provides a method to transfer a reporter value (se
 
 When transferring data from the main application the data from the main is copied to each sub-application. If the [!param](/Transfers/MultiAppReporterTransfer/subapp_index) is used then data is only transferred to the specified sub-application. When transferring data to the main application the [!param](/Transfers/MultiAppReporterTransfer/subapp_index) must be supplied if there is more than one sub-application.
 
+## 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 Input File Syntax
 
 !alert! tip

framework/include/transfers/MultiAppReporterTransfer.h

@@ -35,4 +35,7 @@ class MultiAppReporterTransfer : public MultiAppTransfer, public ReporterTransfe
 
   /// If set, indicates a particular subapp to transfer the reporter to/from
   const unsigned int & _subapp_index;
+
+private:
+  virtual void checkSiblingsTransferSupported() const override;
 };

framework/src/transfers/MultiAppReporterTransfer.C

@@ -154,3 +154,19 @@ MultiAppReporterTransfer::execute()
   else
     executeToMultiapp();
 }
+
+void
+MultiAppReporterTransfer::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 reporter transfer");
+  }
+  else
+    mooseError("Number of source and target child apps must match for siblings transfer");
+}