Allow downstream software not to set mpi version

mache/deploy/run.py

@@ -545,7 +545,7 @@ def _resolve_login_pixi_env(
     prefix: str,
     pixi_cfg: dict[str, Any],
     runtime: dict[str, Any],
-    compute_mpi: str,
+    compute_mpi: str | None,
 ) -> dict[str, str] | None:
     runtime_pixi = runtime.get('pixi')
     if not isinstance(runtime_pixi, dict):
@@ -563,6 +563,8 @@ def _resolve_login_pixi_env(
     resolved_login_mpi, login_mpi_prefix = _get_mpi_settings(
         pixi_cfg={'mpi': login_mpi}
     )
+    if resolved_login_mpi is None or login_mpi_prefix is None:
+        raise ValueError('login_mpi must resolve to a non-empty MPI setting')
 
     if 'login_prefix' in runtime_pixi:
         login_prefix_raw = runtime_pixi.get('login_prefix')
@@ -885,15 +887,12 @@ def _resolve_pixi_mpi(
     *,
     pixi_cfg: dict[str, Any],
     runtime: dict[str, Any],
-) -> tuple[str, str]:
-    mpi_override = None
+) -> tuple[str | None, str | None]:
+    mpi_cfg = pixi_cfg
     runtime_pixi = runtime.get('pixi')
     if isinstance(runtime_pixi, dict):
-        mpi_override = runtime_pixi.get('mpi')
-    if mpi_override is not None:
-        mpi_cfg: dict[str, Any] = {'mpi': str(mpi_override)}
-    else:
-        mpi_cfg = pixi_cfg
+        if 'mpi' in runtime_pixi:
+            mpi_cfg = {'mpi': runtime_pixi.get('mpi')}
     return _get_mpi_settings(pixi_cfg=mpi_cfg)
 
 
@@ -1346,7 +1345,7 @@ def _write_load_script(
     software_version: str,
     runtime_version_cmd: str | None,
     machine: str | None,
-    compute_pixi_mpi: str,
+    compute_pixi_mpi: str | None,
     toolchain_compiler: str | None,
     toolchain_mpi: str | None,
     spack_library_view: str | None,
@@ -1434,7 +1433,7 @@ def _write_load_script(
         runtime_version_cmd_sh=shlex.quote(runtime_version_cmd or ''),
         machine=machine or '',
         load_script=os.path.abspath(script_path),
-        compute_pixi_mpi=compute_pixi_mpi,
+        compute_pixi_mpi=compute_pixi_mpi or '',
         login_prefix=login_prefix_abs,
         login_pixi_toml=login_pixi_toml,
         login_pixi_mpi=login_pixi_mpi,
@@ -1467,7 +1466,7 @@ def _write_load_scripts(
     software_version: str,
     runtime_version_cmd: str | None,
     machine: str | None,
-    compute_pixi_mpi: str,
+    compute_pixi_mpi: str | None,
     toolchain_pairs: list[tuple[str, str]],
     spack_results: Any,
     spack_software_env: Any,
@@ -1640,29 +1639,26 @@ def _pixi_install(
 
 def _get_mpi_settings(
     pixi_cfg: dict[str, Any],
-) -> tuple[str, str]:
+) -> tuple[str | None, str | None]:
     """Determine MPI-related template replacements.
 
     Returns
     -------
-    mpi : str
+    mpi : str or None
         The conda package name for MPI (e.g. "mpich", "openmpi", or "nompi").
-    mpi_prefix : str
+    mpi_prefix : str or None
         The conda-forge variant prefix used in build-string selectors
         (e.g. "nompi", "mpi_mpich", "mpi_openmpi").
     """
 
     if 'mpi' not in pixi_cfg:
-        raise ValueError(
-            "'mpi' not found in [pixi] section of deploy/config.yaml.j2"
-        )
+        return None, None
 
-    mpi_raw = pixi_cfg.get('mpi')
-    mpi = str(mpi_raw).strip().lower() if mpi_raw is not None else ''
-    if not mpi:
-        raise ValueError(
-            "'mpi' in [pixi] section of deploy/config.yaml.j2 is empty"
-        )
+    mpi = _normalize_optional_token(pixi_cfg.get('mpi'))
+    if mpi is None:
+        return None, None
+
+    mpi = mpi.lower()
     if any(ch.isspace() for ch in mpi):
         raise ValueError(
             "'mpi' in [pixi] section of deploy/config.yaml.j2 must not "

mache/deploy/templates/config.yaml.j2.j2

@@ -81,6 +81,7 @@ pixi:
 
   # MPI provider for conda packages.
   # Supported values in `mache deploy run`:
+  #   - null if MPI is not used in the target software
   #   - nompi
   #   - mpich
   #   - openmpi

tests/test_deploy_run.py

@@ -238,6 +238,30 @@ def test_resolve_pixi_omit_dependencies_prefers_runtime_override():
     assert omit_dependencies == ['git', 'ncview']
 
 
+def test_get_mpi_settings_allows_missing_mpi():
+    mpi, mpi_prefix = deploy_run._get_mpi_settings(pixi_cfg={})
+
+    assert mpi is None
+    assert mpi_prefix is None
+
+
+def test_get_mpi_settings_allows_null_mpi():
+    mpi, mpi_prefix = deploy_run._get_mpi_settings(pixi_cfg={'mpi': None})
+
+    assert mpi is None
+    assert mpi_prefix is None
+
+
+def test_resolve_pixi_mpi_runtime_override_can_disable_mpi():
+    mpi, mpi_prefix = deploy_run._resolve_pixi_mpi(
+        pixi_cfg={'mpi': 'openmpi'},
+        runtime={'pixi': {'mpi': None}},
+    )
+
+    assert mpi is None
+    assert mpi_prefix is None
+
+
 def test_resolve_login_pixi_env_uses_distinct_login_prefix():
     login_env = deploy_run._resolve_login_pixi_env(
         prefix='/tmp/compute-env',