fix(templates): Use instance-suffixed MASKTABLE for AUTO_MASKTABLE runs

cime_config/testdefs/testlist_mom.xml

@@ -298,4 +298,22 @@
       <option name="wallclock">00:30:00</option>
     </options>
   </test>
+  <!-- Multi-Instance with Auto Masktable Enabled -->
+  <test name="SMS_C2" grid="T62_t232" compset="G">
+    <machines>
+      <machine name="derecho" compiler="intel" category="aux_mom"/>
+    </machines>
+    <options>
+      <option name="wallclock">00:30:00</option>
+    </options>
+  </test>
+  <!-- Multi-Instance with Auto Masktable Disabled -->
+  <test name="SMS_C2" grid="T62_t232" compset="G" testmods="mom/tx10deg">
+    <machines>
+      <machine name="derecho" compiler="intel" category="aux_mom"/>
+    </machines>
+    <options>
+      <option name="wallclock">00:30:00</option>
+    </options>
+  </test>
 </testlist>

param_templates/MOM_input.yaml

@@ -3940,6 +3940,13 @@ Global:
             $OCN_GRID in ["tx2_3v2", "tx2_3v3"] and $TEST: = - ( - $NTASKS_OCN // 256)
             $OCN_GRID == "tx2_0v1" and $TEST: = - ( - $NTASKS_OCN // 256)
             $OCN_GRID == "tx0.25v1": = - ( - $NTASKS_OCN // 128)
+    MASKTABLE:
+        description: |
+            A text file to specify n_mask, layout and mask_list.
+        datatype: string
+        value:
+            $AUTO_MASKTABLE: = f'MOM_auto_mask_table{$INST_SUFFIX}'
+            else: MOM_mask_table
     GEOM_FILE:
         description: |
             default = ocean_geometry.nc

param_templates/json/MOM_input.json

@@ -3255,6 +3255,14 @@
             "$OCN_GRID == \"tx0.25v1\"": "= - ( - $NTASKS_OCN // 128)"
          }
       },
+      "MASKTABLE": {
+         "description": "A text file to specify n_mask, layout and mask_list.\n",
+         "datatype": "string",
+         "value": {
+            "$AUTO_MASKTABLE": "= f'MOM_auto_mask_table{$INST_SUFFIX}'",
+            "else": "MOM_mask_table"
+         }
+      },
       "GEOM_FILE": {
          "description": "default = ocean_geometry.nc\nThe file into which to write the ocean geometry.\n",
          "datatype": "string",

tests/check_buildnml.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python
+
+"""
+Unit test for cime_config/buildnml and MOM parameter expansion.
+Verifies MASKTABLE expansion under single-instance and multi-instance modes with AUTO_MASKTABLE True/False.
+"""
+
+import os
+import sys
+
+# Add cime_config/MOM_RPS to path
+sys.path.append(os.path.join(os.path.dirname(__file__), "..", "cime_config", "MOM_RPS"))
+from FType_MOM_params import FType_MOM_params
+
+
+def get_param_val(mom_params, varname):
+    for module in mom_params._data:
+        if varname in mom_params._data[module]:
+            return mom_params._data[module][varname]["value"]
+    return None
+
+
+def test_masktable_expansion():
+    json_path = os.path.join(
+        os.path.dirname(__file__), "..", "param_templates", "json", "MOM_input.json"
+    )
+
+    # Provide default CIME environment mock values (booleans, floats, strings) to satisfy
+    # CIME.ParamGen formulas/guards across all MOM_input.json parameters and prevent
+    # Python eval() type errors (e.g., float / str or bool vs str comparisons).
+    default_env = {
+        "OCN_GRID": "tx2_3v2",
+        "TEST": False,
+        "NTASKS_OCN": 64,
+        "DIN_LOC_ROOT": "/tmp/inputdata",
+        "CASEROOT": "/tmp/caseroot",
+        "CASEBUILD": "/tmp/casebuild",
+        "CASE": "case",
+        "SRCROOT": "/tmp/srcroot",
+        "RUNDIR": "/tmp/rundir",
+        "COMP_ROOT_DIR_OCN": "/tmp/mom",
+        "RUN_TYPE": "startup",
+        "CONTINUE_RUN": False,
+        "GET_REFCASE": False,
+        "RUN_REFCASE": "refcase",
+        "RUN_REFDATE": "0001-01-01",
+        "RUN_REFTOD": "00000",
+        "CPL_I2O_PER_CAT": False,
+        "ICE_NCAT": 5,
+        "MARBL_DIAG_MODE": "none",
+        "MOM6_INFRA_API": "FMS2",
+        "MOM6_VERTICAL_GRID": "zstar_65L",
+        "COMP_ATM": "cam",
+        "COMP_ICE": "cice",
+        "COMP_WAVE": "ww3",
+        "RESTINT": 1.0,
+        "REST_OPTION": "nmonths",
+        "REST_N": 1,
+        "REST_FREQ": "monthly",
+        "STOP_OPTION": "nmonths",
+        "STOP_N": 1,
+        "STOP_FREQ": "monthly",
+        "ATM_CO2_OPT": "constant",
+        "ATM_ALT_CO2_OPT": "constant",
+        "ATM_CO2_CONST": 280.0,
+        "ATM_ALT_CO2_CONST": 280.0,
+        "USE_MARBL_TRACERS": False,
+        "NCPL_BASE_PERIOD": "day",
+        "OCN_NCPL": 24,
+        "DT": 3600.0,
+    }
+
+    def run_reduction(auto_masktable, inst_suffix):
+        mom_params = FType_MOM_params.from_json(json_path)
+        dyn_env = {
+            "AUTO_MASKTABLE": auto_masktable,
+            "INST_SUFFIX": inst_suffix,
+        }
+        mom_params.reduce(lambda var: dyn_env.get(var, default_env.get(var, "")))
+        return get_param_val(mom_params, "MASKTABLE")
+
+    # 1. Test Single-Instance, AUTO_MASKTABLE=True
+    val1 = run_reduction(auto_masktable=True, inst_suffix="")
+    print(f"Single-instance AUTO_MASKTABLE=True MASKTABLE: {val1}")
+    assert (
+        val1 == "MOM_auto_mask_table"
+    ), f"Expected 'MOM_auto_mask_table', got '{val1}'"
+
+    # 2. Test Multi-Instance, AUTO_MASKTABLE=True (_0001)
+    val2 = run_reduction(auto_masktable=True, inst_suffix="_0001")
+    print(f"Multi-instance _0001 AUTO_MASKTABLE=True MASKTABLE: {val2}")
+    assert (
+        val2 == "MOM_auto_mask_table_0001"
+    ), f"Expected 'MOM_auto_mask_table_0001', got '{val2}'"
+
+    # 3. Test Single-Instance, AUTO_MASKTABLE=False
+    val3 = run_reduction(auto_masktable=False, inst_suffix="")
+    print(f"Single-instance AUTO_MASKTABLE=False MASKTABLE: {val3}")
+    assert val3 == "MOM_mask_table", f"Expected 'MOM_mask_table', got '{val3}'"
+
+    # 4. Test Multi-Instance, AUTO_MASKTABLE=False (_0001)
+    val4 = run_reduction(auto_masktable=False, inst_suffix="_0001")
+    print(f"Multi-instance _0001 AUTO_MASKTABLE=False MASKTABLE: {val4}")
+    assert val4 == "MOM_mask_table", f"Expected 'MOM_mask_table', got '{val4}'"
+
+    print("ALL MASKTABLE TESTS PASSED")
+
+
+if __name__ == "__main__":
+    test_masktable_expansion()