Error if bx norm postprocessor doesn't execute on linear

framework/src/problems/EigenProblem.C

@@ -19,6 +19,7 @@
 #include "OutputWarehouse.h"
 #include "Function.h"
 #include "MooseVariableScalar.h"
+#include "UserObject.h"
 
 // libMesh includes
 #include "libmesh/system.h"
@@ -493,6 +494,16 @@ EigenProblem::checkProblemIntegrity()
 {
   FEProblemBase::checkProblemIntegrity();
   _nl_eigen->checkIntegrity();
+  if (_bx_norm_name)
+  {
+    if (!isNonlinearEigenvalueSolver())
+      paramWarning("bx_norm", "This parameter is only used for nonlinear solve types");
+    else if (auto & pp = getUserObjectBase(_bx_norm_name.value());
+             !pp.getExecuteOnEnum().contains(EXEC_LINEAR))
+      pp.paramError("execute_on",
+                    "If providing the Bx norm, this postprocessor must execute on linear e.g. "
+                    "during residual evaluations");
+  }
 }
 
 void

test/tests/problems/eigen_problem/jfnk_mo/tests

@@ -72,14 +72,23 @@
       slepc = true
       slepc_version = '>=3.11.0'
       detail = 'computed by a user provided postprocessor.'
+      prereq = 'ne_coupled_mo/l2_bx'
     []
   []
+  [missing_bx_norm_exec_linear]
+    type = RunException
+    issues = '#28993'
+    input = ne_coupled_mo.i
+    cli_args = "Problem/bx_norm=uint Postprocessors/uint/execute_on='initial nonlinear'"
+    requirement = 'The system shall error if the user wants the Bx norm provided by a postprocessor but the postprocessor is not executed during residual evaluations.'
+    expect_err = "If providing the Bx norm, this postprocessor must execute on linear"
+  []
 
   [ne_coupled_mo_full]
     type = 'CSVDiff'
     input = 'ne_coupled_mo.i'
     csvdiff = 'ne_coupled_eigenvalues_0001.csv'
-    prereq = 'ne_coupled_mo'
+    prereq = 'ne_coupled_mo/custom_bx'
     cli_args = 'Preconditioning/active='''
     slepc = true
     slepc_version = '>=3.11.0'