Issue idaholab#17471 test

framework/doc/content/source/meshgenerators/FileMeshGenerator.md

@@ -27,7 +27,13 @@ supports reading and writing a large number of formats and could be extended to
 | .cpr        | Checkpoint file |
 
 When reading a mesh file in Sandia's ExodusII format, users can use parameter `exodus_extra_element_integers` to load elemental variables for setting extra element integers of the mesh.
-The names of the extra element integers will be the same as the names of the element variables in the mesh file.
+The names of the extra element integers will be the same as the names of the
+element variables in the mesh file. This generator can also be used for
+restarting variables from the Exodus file format. In order to indicate that the
+mesh file can be used to restart variables, simply set the parameter
+`use_for_exodus_restart = true`. The `initial_from_file_var` parameter must also
+be set in the variables sub-block as described in [MooseVariableBase.md#restart]
+in order to perform variable restart.
 
 ## Further FileMeshGenerator Documentation
 

framework/doc/content/source/variables/MooseVariableBase.md

@@ -73,6 +73,15 @@ similar methods of `MooseVariableFE<Real>`.
 There are a myriad of ways to access Moose variables from user interfaces. We'll
 outline a few below.
 
+## Restart
+
+Variables can be restarted/initialized from variable values in a file on disk by
+setting the parameter `initial_from_file_var = source_var_name` in the variable
+sub-block, where `source_var_name` is the name of the source variable in the
+file. Note that the user will also have to set parameters in the `[Mesh]` block
+in order for this to work, as described for example in the documentation of the
+[FileMeshGenerator.md].
+
 ### SystemBase
 
 It's common for interface objects (`Kernel` objects for example) to have a `_sys`

test/tests/restart/scalar-var/gold/part2_out.e

@@ -0,0 +1 @@
+part1_out.e

test/tests/restart/scalar-var/part1.i

@@ -0,0 +1,50 @@
+[Mesh]
+  type = GeneratedMesh
+  dim = 1
+  nx = 21
+[]
+
+[Variables]
+  [v]
+    family = MONOMIAL
+    order = CONSTANT
+    fv = true
+  []
+  [lambda]
+    family = SCALAR
+    order = FIRST
+  []
+[]
+
+[FVKernels]
+  [advection]
+    type = FVElementalAdvection
+    variable = v
+    velocity = '1 0 0'
+  []
+  [lambda]
+    type = FVScalarLagrangeMultiplier
+    variable = v
+    lambda = lambda
+    phi0 = 1
+  []
+[]
+
+[Problem]
+  kernel_coverage_check = off
+[]
+
+[Executioner]
+  type = Steady
+  petsc_options_iname = '-pc_type -pc_factor_shift_type'
+  petsc_options_value = 'lu       NONZERO'
+  nl_rel_tol = 1e-12
+  solve_type = NEWTON
+[]
+
+[Outputs]
+  [out]
+    type = Exodus
+    execute_on = 'final'
+  []
+[]

test/tests/restart/scalar-var/part2.i

@@ -0,0 +1,53 @@
+[Mesh]
+  [fmg]
+    type = FileMeshGenerator
+    file = part1_out.e
+    use_for_exodus_restart = true
+  []
+[]
+
+[Variables]
+  [v]
+    family = MONOMIAL
+    order = CONSTANT
+    fv = true
+    initial_from_file_var = v
+  []
+  [lambda]
+    family = SCALAR
+    order = FIRST
+    initial_from_file_var = lambda
+  []
+[]
+
+[FVKernels]
+  [advection]
+    type = FVElementalAdvection
+    variable = v
+    velocity = '1 0 0'
+  []
+  [lambda]
+    type = FVScalarLagrangeMultiplier
+    variable = v
+    lambda = lambda
+    phi0 = 1
+  []
+[]
+
+[Problem]
+  kernel_coverage_check = off
+[]
+
+[Executioner]
+  type = Steady
+  petsc_options_iname = '-snes_max_it'
+  petsc_options_value = '0'
+  nl_abs_tol = 1e-10
+[]
+
+[Outputs]
+  [out]
+    type = Exodus
+    execute_on = 'final'
+  []
+[]

test/tests/restart/scalar-var/tests

@@ -0,0 +1,19 @@
+[Tests]
+  design = 'MooseVariableBase.md'
+  issues = '#17471'
+  [restarting]
+    requirement = 'The system shall be able to restart a scalar variable in the presence of other variables with the standard sequence of:'
+    [part1]
+      type = Exodiff
+      input = part1.i
+      exodiff = part1_out.e
+      detail = 'run initial input'
+    []
+    [part2]
+      type = Exodiff
+      input = part2.i
+      exodiff = part2_out.e
+      detail = 'run restart input'
+    []
+  []
+[]