Testing: fix unintended interactions between tests (#16003)

* For tests that use the real Spack package repository, the config
  needs to avoid using MPI providers that are not intended to be
  installed by Spack. Without this, it is possible that Spack tests
  which concretize the MPI virtual will end up trying to use an
  implementation that it shouldn't (e.g. one that is always
  provided externally). See #15666 for an example.
* The mutable_config test fixture was not initializing the scope
  roots to the right directories (so the resulting config was empty).
* The current_host fixture in the concretize.py tests was using the
  config fixture rather than mutable_config, and was polluting the
  config cache for other tests.
* One test in concretize.py was clearing a nonexistent cache
  (PackagePrefs._packages_config_cache). This reference has been
  removed.
* The test 'test_preferred_compilers' was was depending on cross
  test config pollution to succeed. The initial spec before
  concretization has been updated to updated to be explicit about
  the desired result.

lib/spack/spack/cmd/compiler.py

@@ -159,7 +159,19 @@ def compiler_list(args):
     tty.msg("Available compilers")
     index = index_by(spack.compilers.all_compilers(scope=args.scope),
                      lambda c: (c.spec.name, c.operating_system, c.target))
-    ordered_sections = sorted(index.items(), key=lambda item: item[0])
+
+    # For a container, take each element which does not evaluate to false and
+    # convert it to a string. For elements which evaluate to False (e.g. None)
+    # convert them to '' (in which case it still evaluates to False but is a
+    # string type). Tuples produced by this are guaranteed to be comparable in
+    # Python 3
+    convert_str = (
+        lambda tuple_container:
+        tuple(str(x) if x else '' for x in tuple_container))
+
+    index_str_keys = list(
+        (convert_str(x), y) for x, y in index.items())
+    ordered_sections = sorted(index_str_keys, key=lambda item: item[0])
     for i, (key, compilers) in enumerate(ordered_sections):
         if i >= 1:
             print()

lib/spack/spack/test/concretize.py

@@ -12,7 +12,6 @@
 
 from spack.concretize import find_spec, NoValidVersionError
 from spack.error import SpecError
-from spack.package_prefs import PackagePrefs
 from spack.spec import Spec, CompilerSpec, ConflictsInSpecError
 from spack.version import ver
 from spack.util.mock_package import MockPackageMultiRepo
@@ -103,8 +102,6 @@ def current_host(request, monkeypatch):
         monkeypatch.setattr(spack.platforms.test.Test, 'default', cpu)
         yield target
     else:
-        # There's a cache that needs to be cleared for unit tests
-        PackagePrefs._packages_config_cache = None
         with spack.config.override('packages:all', {'target': [cpu]}):
             yield target
 
@@ -112,7 +109,10 @@ def current_host(request, monkeypatch):
     spack.architecture.get_platform.cache.clear()
 
 
-@pytest.mark.usefixtures('config', 'mock_packages')
+# This must use the mutable_config fixture because the test
+# adjusting_default_target_based_on_compiler uses the current_host fixture,
+# which changes the config.
+@pytest.mark.usefixtures('mutable_config', 'mock_packages')
 class TestConcretize(object):
     def test_concretize(self, spec):
         check_concretize(spec)

lib/spack/spack/test/concretize_preferences.py

@@ -100,7 +100,7 @@ def test_preferred_compilers(self):
         # Try the last available compiler
         compiler = str(compiler_list[-1])
         update_packages('mpileaks', 'compiler', [compiler])
-        spec = concretize('mpileaks')
+        spec = concretize('mpileaks os=redhat6 target=x86')
         assert spec.compiler == spack.spec.CompilerSpec(compiler)
 
     def test_preferred_target(self, mutable_mock_repo):

lib/spack/spack/test/conftest.py

@@ -423,13 +423,13 @@ def config(mock_configuration):
 
 
 @pytest.fixture(scope='function')
-def mutable_config(tmpdir_factory, configuration_dir, monkeypatch):
+def mutable_config(tmpdir_factory, configuration_dir):
     """Like config, but tests can modify the configuration."""
     mutable_dir = tmpdir_factory.mktemp('mutable_config').join('tmp')
     configuration_dir.copy(mutable_dir)
 
     cfg = spack.config.Configuration(
-        *[spack.config.ConfigScope(name, str(mutable_dir))
+        *[spack.config.ConfigScope(name, str(mutable_dir.join(name)))
           for name in ['site', 'system', 'user']])
 
     with use_configuration(cfg):

lib/spack/spack/test/data/config/packages.yaml

@@ -1,4 +1,7 @@
 packages:
+  all:
+    providers:
+      mpi: [openmpi, mpich]
   externaltool:
     buildable: False
     paths: