Move compile-time conditional import of parallel modules to code-gen

To avoid the last deprecated compile-time `IF`, we instead separate
the two branches of the conditional into different files and choose
which one to include at build time.

.gitignore

@@ -5,6 +5,7 @@ dist/
 netCDF4/_netCDF4.c
 netCDF4/*.so
 include/constants.pyx
+include/parallel_support_imports.pxi
 netcdftime/_netcdftime.c
 venv/
 .eggs/

include/no_parallel_support_imports.pxi.in

@@ -0,0 +1,8 @@
+# Stubs for when parallel support is not enabled
+
+ctypedef int Comm
+ctypedef int Info
+cdef Comm MPI_COMM_WORLD
+cdef Info MPI_INFO_NULL
+MPI_COMM_WORLD = 0
+MPI_INFO_NULL = 0

include/parallel_support_imports.pxi.in

@@ -0,0 +1,16 @@
+# Imports and typedefs required at compile time for enabling parallel support
+
+cimport mpi4py.MPI as MPI
+from mpi4py.libmpi cimport (
+    MPI_Comm,
+    MPI_Info,
+    MPI_Comm_dup,
+    MPI_Info_dup,
+    MPI_Comm_free,
+    MPI_Info_free,
+    MPI_INFO_NULL,
+    MPI_COMM_WORLD,
+)
+
+ctypedef MPI.Comm Comm
+ctypedef MPI.Info Info

setup.py

@@ -487,7 +487,20 @@ def _populate_hdf5_info(dirstosearch, inc_dirs, libs, lib_dirs):
         inc_dirs.append(mpi4py.get_include())
         # mpi_incdir should not be needed if using nc-config
         # (should be included in nc-config --cflags)
-        if mpi_incdir is not None: inc_dirs.append(mpi_incdir)
+        if mpi_incdir is not None:
+            inc_dirs.append(mpi_incdir)
+
+        # Name of file containing imports required for parallel support
+        parallel_support_imports = "parallel_support_imports.pxi.in"
+    else:
+        parallel_support_imports = "no_parallel_support_imports.pxi.in"
+
+    # Copy the specific version of the file containing parallel
+    # support imports
+    shutil.copyfile(
+        osp.join("include", parallel_support_imports),
+        osp.join("include", "parallel_support_imports.pxi")
+    )
 
     ext_modules = [Extension("netCDF4._netCDF4",
                              [netcdf4_src_pyx],

src/netCDF4/_netCDF4.pyx

@@ -1244,7 +1244,7 @@ from numpy import ma
 from libc.string cimport memcpy, memset
 from libc.stdlib cimport malloc, free
 numpy.import_array()
-include "constants.pyx"
+include "parallel_support_imports.pxi"
 include "membuf.pyx"
 include "netCDF4.pxi"
 
@@ -1266,21 +1266,6 @@ __has_set_alignment__ = HAS_SET_ALIGNMENT
 __has_ncfilter__ = HAS_NCFILTER
 
 
-IF HAS_PARALLEL_SUPPORT:
-    cimport mpi4py.MPI as MPI
-    from mpi4py.libmpi cimport MPI_Comm, MPI_Info, MPI_Comm_dup, MPI_Info_dup, \
-                               MPI_Comm_free, MPI_Info_free, MPI_INFO_NULL,\
-                               MPI_COMM_WORLD
-    ctypedef MPI.Comm Comm
-    ctypedef MPI.Info Info
-ELSE:
-    ctypedef int Comm
-    ctypedef int Info
-    cdef Comm MPI_COMM_WORLD
-    cdef Info MPI_INFO_NULL
-    MPI_COMM_WORLD = 0
-    MPI_INFO_NULL = 0
-
 # set path to SSL certificates (issue #1246)
 # available starting in version 4.9.1
 if HAS_NCRCSET: