Fix post_process non-MPI builds never reading input file

src/post_process/m_checker.fpp

@@ -24,16 +24,8 @@ contains
     !> Checks compatibility of parameters in the input file.
         !! Used by the post_process stage
     impure subroutine s_check_inputs
-
-        call s_check_inputs_output_format
-
     end subroutine s_check_inputs
 
-    !> Checks constraints on output format parameters
-    impure subroutine s_check_inputs_output_format
-        @:PROHIBIT(precision == 2 .and. wp == sp)
-    end subroutine s_check_inputs_output_format
-
     !> Checks constraints on fft_wrt
     impure subroutine s_check_inputs_fft
         integer :: num_procs_y, num_procs_z

src/post_process/m_start_up.fpp

@@ -1149,7 +1149,6 @@ contains
 
         num_dims = 1 + min(1, n) + min(1, p)
 
-#ifdef MFC_MPI
         ! Initialization of the MPI environment
         call s_mpi_initialize()
 
@@ -1167,14 +1166,12 @@ contains
 
         ! Broadcasting the user inputs to all of the processors and performing the
         ! parallel computational domain decomposition. Neither procedure has to be
-        ! carried out if the simulation is in fact not truly executed in parallel.
+        ! carried out if the post-process is in fact not truly executed in parallel.
         call s_mpi_bcast_user_inputs()
         call s_initialize_parallel_io()
         call s_mpi_decompose_computational_domain()
         call s_check_inputs_fft()
 
-#endif
-
     end subroutine s_initialize_mpi_domain
 
     !> @brief Destroy FFTW plans, free MPI communicators, and finalize all post-process sub-modules.

src/pre_process/m_checker.fpp

@@ -24,16 +24,6 @@ contains
     !> Checks compatibility of parameters in the input file.
         !! Used by the pre_process stage
     impure subroutine s_check_inputs
-
-        call s_check_parallel_io
-
     end subroutine s_check_inputs
 
-    !> Checks if mpi is enabled with parallel_io
-    impure subroutine s_check_parallel_io
-#ifndef MFC_MPI
-        @:PROHIBIT(parallel_io, "MFC built with --no-mpi requires parallel_io=F")
-#endif
-    end subroutine s_check_parallel_io
-
 end module m_checker

src/simulation/m_checker.fpp

@@ -36,7 +36,6 @@ contains
             else if (recon_type == MUSCL_TYPE) then
                 call s_check_inputs_muscl
             end if
-            call s_check_inputs_geometry_precision
         end if
 
         call s_check_inputs_time_stepping
@@ -76,14 +75,6 @@ contains
             "For 3D simulation, p must be greater than or equal to (num_stcls_min*muscl_order - 1), whose value is "//trim(numStr))
     end subroutine s_check_inputs_muscl
 
-    !> Checks constraints on geometry and precision
-    impure subroutine s_check_inputs_geometry_precision
-        ! Prevent spherical geometry in single precision
-#ifdef MFC_SINGLE_PRECISION
-        @:PROHIBIT(.not. (cyl_coord .neqv. .true. .or. (cyl_coord .and. p == 0)), "Fully 3D cylindrical grid (geometry = 3) is not supported in single precision.")
-#endif
-    end subroutine s_check_inputs_geometry_precision
-
     !> Checks constraints on time stepping parameters
     impure subroutine s_check_inputs_time_stepping
         if (.not. cfl_dt) then

toolchain/mfc/case_validator.py

@@ -19,6 +19,7 @@
 from functools import lru_cache
 from .common import MFCException
 from .params.definitions import CONSTRAINTS
+from .state import CFG
 
 
 # Physics documentation for check methods.
@@ -1794,6 +1795,10 @@ def check_output_format(self):
         if precision is not None:
             self.prohibit(precision not in [1, 2],
                          "precision must be 1 or 2")
+            self.prohibit(
+                precision == 2 and CFG().single,
+                "precision = 2 (double output) requires MFC built without --single"
+            )
 
     def check_vorticity(self):
         """Checks vorticity parameters (post-process)"""
@@ -2330,13 +2335,41 @@ def check_velocity_components(self):
                     self.prohibit(vel3 != 0,
                                  f"patch_icpp({i})%vel(3) = {vel3} but p = 0 (1D/2D simulation)")
 
+    # ===================================================================
+    # Build-Flag Compatibility Checks
+    # ===================================================================
+
+    def check_build_flags(self):
+        """Checks case parameters against the active build configuration.
+
+        These catch incompatibilities between case settings and how the
+        MFC binaries were compiled (--mpi/--no-mpi, --single, etc.)
+        before any binary is invoked.
+        """
+        parallel_io = self.get('parallel_io', 'F') == 'T'
+        self.prohibit(
+            parallel_io and not CFG().mpi,
+            "parallel_io = T requires MFC built with --mpi"
+        )
+
+    def check_geometry_precision_simulation(self):
+        """Checks that 3D cylindrical geometry is not used with --single builds."""
+        cyl_coord = self.get('cyl_coord', 'F') == 'T'
+        p = self.get('p', 0)
+        self.prohibit(
+            CFG().single and cyl_coord and p > 0,
+            "Fully 3D cylindrical geometry (cyl_coord = T, p > 0) is not supported "
+            "in single precision (--single)"
+        )
+
     # ===================================================================
     # Main Validation Entry Points
     # ===================================================================
 
     def validate_common(self):
         """Validate parameters common to all stages"""
         self.check_parameter_types()  # Type validation first
+        self.check_build_flags()
         self.check_simulation_domain()
         self.check_domain_bounds()
         self.check_model_eqns_and_num_fluids()
@@ -2359,6 +2392,7 @@ def validate_common(self):
     def validate_simulation(self):
         """Validate simulation-specific parameters"""
         self.validate_common()
+        self.check_geometry_precision_simulation()
         self.check_finite_difference()
         self.check_time_stepping()
         self.check_riemann_solver()

toolchain/mfc/lint_docs.py

@@ -423,6 +423,8 @@ def check_physics_docs_coverage(repo_root: Path) -> list[str]:
         "check_output_format",       # output format selection
         "check_restart",             # restart file logistics
         "check_parallel_io_pre_process",  # parallel I/O settings
+        "check_build_flags",              # build-flag compatibility (no physics meaning)
+        "check_geometry_precision_simulation",  # build-flag compatibility (no physics meaning)
         "check_misc_pre_process",    # miscellaneous pre-process flags
         "check_bc_patches",          # boundary patch geometry
         "check_grid_stretching",     # grid stretching parameters