environment modifications for externals (#41723)

* allow externals to configure environment modifications

* docs for external env modification

---------

Co-authored-by: becker33 <becker33@users.noreply.github.com>

lib/spack/docs/packages_yaml.rst

@@ -97,6 +97,35 @@ Each package version and compiler listed in an external should
 have entries in Spack's packages and compiler configuration, even
 though the package and compiler may not ever be built.
 
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Extra attributes for external packages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sometimes external packages require additional attributes to be used
+effectively. This information can be defined on a per-package basis
+and stored in the ``extra_attributes`` section of the external package
+configuration. In addition to per-package information, this section
+can be used to define environment modifications to be performed
+whenever the package is used. For example, if an external package is
+built without ``rpath`` support, it may require ``LD_LIBRARY_PATH``
+settings to find its dependencies. This could be configured as
+follows:
+
+.. code-block:: yaml
+
+   packages:
+     mpich:
+       externals:
+       - spec: mpich@3.3 %clang@12.0.0 +hwloc
+         prefix: /path/to/mpich
+         extra_attributes:
+           environment:
+             prepend_path:
+               LD_LIBRARY_PATH: /path/to/hwloc/lib64
+
+See :ref:`configuration_environment_variables` for more information on
+how to configure environment modifications in Spack config files.
+
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Prevent packages from being built from sources
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

lib/spack/spack/build_environment.py

@@ -1032,6 +1032,11 @@ def get_env_modifications(self) -> EnvironmentModifications:
                     if id(spec) in self.nodes_in_subdag:
                         pkg.setup_dependent_run_environment(run_env_mods, spec)
                 pkg.setup_run_environment(run_env_mods)
+
+                external_env = (dspec.extra_attributes or {}).get("environment", {})
+                if external_env:
+                    run_env_mods.extend(spack.schema.environment.parse(external_env))
+
                 if self.context == Context.BUILD:
                     # Don't let the runtime environment of comiler like dependencies leak into the
                     # build env

lib/spack/spack/schema/packages.py

@@ -7,6 +7,7 @@
 .. literalinclude:: _spack_root/lib/spack/spack/schema/packages.py
    :lines: 13-
 """
+import spack.schema.environment
 
 permissions = {
     "type": "object",
@@ -155,7 +156,13 @@
                                 "spec": {"type": "string"},
                                 "prefix": {"type": "string"},
                                 "modules": {"type": "array", "items": {"type": "string"}},
-                                "extra_attributes": {"type": "object"},
+                                "extra_attributes": {
+                                    "type": "object",
+                                    "additionalProperties": True,
+                                    "properties": {
+                                        "environment": spack.schema.environment.definition
+                                    },
+                                },
                             },
                             "additionalProperties": True,
                             "required": ["spec"],

lib/spack/spack/test/build_environment.py

@@ -285,6 +285,24 @@ def platform_pathsep(pathlist):
         assert name not in os.environ
 
 
+def test_external_config_env(mock_packages, mutable_config, working_env):
+    cmake_config = {
+        "externals": [
+            {
+                "spec": "cmake@1.0",
+                "prefix": "/fake/path",
+                "extra_attributes": {"environment": {"set": {"TEST_ENV_VAR_SET": "yes it's set"}}},
+            }
+        ]
+    }
+    spack.config.set("packages:cmake", cmake_config)
+
+    cmake_client = spack.spec.Spec("cmake-client").concretized()
+    spack.build_environment.setup_package(cmake_client.package, False)
+
+    assert os.environ["TEST_ENV_VAR_SET"] == "yes it's set"
+
+
 @pytest.mark.regression("9107")
 def test_spack_paths_before_module_paths(config, mock_packages, monkeypatch, working_env):
     s = spack.spec.Spec("cmake")