Allow general field transfers to do siblings

refs idaholab#19451

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

@@ -64,7 +64,6 @@ The following features cannot be supported by general field transfers, as a limi
 the `MultiAppGeneralFieldTransfer` base class.
 
 - bi-directional transfer, a single transfer that send data to an app and from that same application
-- transfers between two sibling `MultiApps` with different numbers of child applications
 - reduction operations, sum/average/min/max, on data transferred from multiple child apps
 - transfers between vector variables
 
@@ -73,6 +72,13 @@ These features are currently unsupported, but could be enabled if necessary with
 
 - caching optimizations for when both the target and origin mesh are constant
 
+## Siblings transfer behavior
+
+This transfer supports sending data from a MultiApp to a MultiApp with an arbitrary number
+of source subapps in the source MultiApp and an arbitrary, possibly non-matching, number
+of target subapps in the target MultiApp. It is the user's responsibility to ensure the
+transfer is well defined, for example by avoiding overlaps between source multiapps which cause
+multiple valid values for a target point.
 
 ## Use of bounding boxes
 

framework/include/transfers/MultiAppGeneralFieldTransfer.h

@@ -52,6 +52,9 @@ class MultiAppGeneralFieldTransfer : public MultiAppConservativeTransfer
   VariableName getToVarName(unsigned int var_index);
 
 protected:
+  /// Siblings transfers fully supported
+  virtual void checkSiblingsTransferSupported() const override {}
+
   /*
    * Prepare evaluation of interpolation values
    * @param var_index index of the variable & component to prepare for. This routine is called once

framework/include/transfers/MultiAppTransfer.h

@@ -205,6 +205,13 @@ class MultiAppTransfer : public Transfer
   /// We use the fact that global app indexes are consecutive on a given rank
   unsigned int getLocalSourceAppIndex(unsigned int i_from) const;
 
+  /// Whether the transfer supports siblings transfer
+  virtual void checkSiblingsTransferSupported() const
+  {
+    mooseError("Siblings transfer not supported. You cannot transfer both from a multiapp to "
+               "another multiapp");
+  };
+
   /**
    * Get the target app point from a point in the reference frame
    * @param p the point in the reference frame

framework/src/transfers/MultiAppTransfer.C

@@ -140,12 +140,6 @@ MultiAppTransfer::MultiAppTransfer(const InputParameters & parameters)
     _current_direction = _directions[0];
   }
 
-  // Check for different number of subapps
-  if (_to_multi_app && _from_multi_app &&
-      _from_multi_app->numGlobalApps() != _to_multi_app->numGlobalApps())
-    mooseError(
-        "Between multiapp transfer is only supported with the same number of subapps per MultiApp");
-
   // Handle deprecated parameters
   if (parameters.isParamSetByUser("direction"))
   {
@@ -217,6 +211,10 @@ MultiAppTransfer::variableIntegrityCheck(const AuxVariableName & var_name) const
 void
 MultiAppTransfer::initialSetup()
 {
+  // Check for siblings transfer support
+  if (_to_multi_app && _from_multi_app)
+    checkSiblingsTransferSupported();
+
   getAppInfo();
 
   if (_from_multi_app)