Added a context manager to swap architectures

This solves a few FIXMEs in conftest.py, where
we were manipulating globals and seeing side
effects prior to registering fixtures.

This commit solves the FIXMEs, but introduces
a performance regression on tests that may need
to be investigated

(cherry picked from commit 4558dc06e21e01ab07a43737b8cb99d1d69abb5d)

5 files changed, 89 insertions, 35 deletions

diff --git a/lib/spack/spack/architecture.py b/lib/spack/spack/architecture.py
index c0930fe2d0..ece9040a7d 100644
--- a/lib/spack/spack/architecture.py
+++ b/lib/spack/spack/architecture.py
@@ -56,6 +56,7 @@ set. The user can set the front-end and back-end operating setting by the class
 attributes front_os and back_os. The operating system as described earlier,
 will be responsible for compiler detection.
 """
+import contextlib
 import functools
 import inspect
 import warnings
@@ -67,6 +68,8 @@ import llnl.util.tty as tty
 from llnl.util.lang import memoized, list_modules, key_ordering
 
 import spack.compiler
+import spack.compilers
+import spack.config
 import spack.paths
 import spack.error as serr
 import spack.util.executable
@@ -491,7 +494,7 @@ def arch_for_spec(arch_spec):
 
 
 @memoized
-def all_platforms():
+def _all_platforms():
     classes = []
     mod_path = spack.paths.platform_path
     parent_module = "spack.platforms"
@@ -512,7 +515,7 @@ def all_platforms():
 
 
 @memoized
-def platform():
+def _platform():
     """Detects the platform for this machine.
 
     Gather a list of all available subclasses of platforms.
@@ -521,7 +524,7 @@ def platform():
     a file path (/opt/cray...)
     """
     # Try to create a Platform object using the config file FIRST
-    platform_list = all_platforms()
+    platform_list = _all_platforms()
     platform_list.sort(key=lambda a: a.priority)
 
     for platform_cls in platform_list:
@@ -529,6 +532,19 @@ def platform():
             return platform_cls()
 
 
+#: The "real" platform of the host running Spack. This should not be changed
+#: by any method and is here as a convenient way to refer to the host platform.
+real_platform = _platform
+
+#: The current platform used by Spack. May be swapped by the use_platform
+#: context manager.
+platform = _platform
+
+#: The list of all platform classes. May be swapped by the use_platform
+#: context manager.
+all_platforms = _all_platforms
+
+
 @memoized
 def default_arch():
     """Default ``Arch`` object for this machine.
@@ -563,3 +579,39 @@ def compatible_sys_types():
         arch = Arch(platform(), 'default_os', target)
         compatible_archs.append(str(arch))
     return compatible_archs
+
+
+class _PickleableCallable(object):
+    """Class used to pickle a callable that may substitute either
+    _platform or _all_platforms. Lambda or nested functions are
+    not pickleable.
+    """
+    def __init__(self, return_value):
+        self.return_value = return_value
+
+    def __call__(self):
+        return self.return_value
+
+
+@contextlib.contextmanager
+def use_platform(new_platform):
+    global platform, all_platforms
+
+    msg = '"{0}" must be an instance of Platform'
+    assert isinstance(new_platform, Platform), msg.format(new_platform)
+
+    original_platform_fn, original_all_platforms_fn = platform, all_platforms
+    platform = _PickleableCallable(new_platform)
+    all_platforms = _PickleableCallable([type(new_platform)])
+
+    # Clear configuration and compiler caches
+    spack.config.config.clear_caches()
+    spack.compilers._cache_config_files = []
+
+    yield new_platform
+
+    platform, all_platforms = original_platform_fn, original_all_platforms_fn
+
+    # Clear configuration and compiler caches
+    spack.config.config.clear_caches()
+    spack.compilers._cache_config_files = []
diff --git a/lib/spack/spack/test/architecture.py b/lib/spack/spack/test/architecture.py
index 66c87240a1..47e1e25fe8 100644
--- a/lib/spack/spack/test/architecture.py
+++ b/lib/spack/spack/test/architecture.py
@@ -6,6 +6,7 @@
 """ Test checks if the architecture class is created correctly and also that
     the functions are looking for the correct architecture name
 """
+import itertools
 import os
 import platform as py_platform
 
@@ -116,20 +117,26 @@ def test_user_defaults(config):
     assert default_target == default_spec.architecture.target
 
 
-@pytest.mark.parametrize('operating_system', [
-    x for x in spack.architecture.platform().operating_sys
-] + ["fe", "be", "frontend", "backend"])
-@pytest.mark.parametrize('target', [
-    x for x in spack.architecture.platform().targets
-] + ["fe", "be", "frontend", "backend"])
-def test_user_input_combination(config, operating_system, target):
-    platform = spack.architecture.platform()
-    spec = Spec("libelf os=%s target=%s" % (operating_system, target))
-    spec.concretize()
-    assert spec.architecture.os == str(
-        platform.operating_system(operating_system)
-    )
-    assert spec.architecture.target == platform.target(target)
+def test_user_input_combination(config):
+    valid_keywords = ["fe", "be", "frontend", "backend"]
+
+    possible_targets = ([x for x in spack.architecture.platform().targets]
+                        + valid_keywords)
+
+    possible_os = ([x for x in spack.architecture.platform().operating_sys]
+                   + valid_keywords)
+
+    for target, operating_system in itertools.product(
+        possible_targets, possible_os
+    ):
+        platform = spack.architecture.platform()
+        spec_str = "libelf os={0} target={1}".format(operating_system, target)
+        spec = Spec(spec_str)
+        spec.concretize()
+        assert spec.architecture.os == str(
+            platform.operating_system(operating_system)
+        )
+        assert spec.architecture.target == platform.target(target)
 
 
 def test_operating_system_conversion_to_dict():
diff --git a/lib/spack/spack/test/cmd/extensions.py b/lib/spack/spack/test/cmd/extensions.py
index 7fc56593eb..ad993244ae 100644
--- a/lib/spack/spack/test/cmd/extensions.py
+++ b/lib/spack/spack/test/cmd/extensions.py
@@ -27,7 +27,7 @@ def python_database(mock_packages, mutable_database):
 
 
 @pytest.mark.db
-def test_extensions(mock_packages, python_database, capsys):
+def test_extensions(mock_packages, python_database, config, capsys):
     ext2   = Spec("py-extension2").concretized()
 
     def check_output(ni, na):
diff --git a/lib/spack/spack/test/cmd/undevelop.py b/lib/spack/spack/test/cmd/undevelop.py
index d44330fe76..493bb99228 100644
--- a/lib/spack/spack/test/cmd/undevelop.py
+++ b/lib/spack/spack/test/cmd/undevelop.py
@@ -12,7 +12,7 @@ env = SpackCommand('env')
 concretize = SpackCommand('concretize')
 
 
-def test_undevelop(tmpdir, mock_packages, mutable_mock_env_path):
+def test_undevelop(tmpdir, config, mock_packages, mutable_mock_env_path):
     # setup environment
     envdir = tmpdir.mkdir('env')
     with envdir.as_cwd():
@@ -39,7 +39,7 @@ env:
     assert not after.satisfies('dev_path=*')
 
 
-def test_undevelop_nonexistent(tmpdir, mock_packages, mutable_mock_env_path):
+def test_undevelop_nonexistent(tmpdir, config, mock_packages, mutable_mock_env_path):
     # setup environment
     envdir = tmpdir.mkdir('env')
     with envdir.as_cwd():
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 18a7924c9e..297ae0d4f2 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -1,4 +1,4 @@
-# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
@@ -315,24 +315,18 @@ def _skip_if_missing_executables(request):
             pytest.skip(msg.format(', '.join(missing_execs)))
 
 
-# FIXME: The lines below should better be added to a fixture with
-# FIXME: session-scope. Anyhow doing it is not easy, as it seems
-# FIXME: there's some weird interaction with compilers during concretization.
-spack.architecture.real_platform = spack.architecture.platform
-
-
+@pytest.fixture(scope='session')
 def test_platform():
     return spack.platforms.test.Test()
 
 
-spack.architecture.platform = test_platform
-
-
-# FIXME: Since we change the architecture above, we have to (re)initialize
-# FIXME: the config singleton. If it gets initialized too early with the
-# FIXME: actual architecture, tests will fail.
-spack.config.config = spack.config._config()
-
+@pytest.fixture(autouse=True, scope='session')
+def _use_test_platform(test_platform):
+    # This is the only context manager used at session scope (see note
+    # below for more insight) since we want to use the test platform as
+    # a default during tests.
+    with spack.architecture.use_platform(test_platform):
+        yield
 
 #
 # Note on context managers used by fixtures
@@ -356,6 +350,7 @@ spack.config.config = spack.config._config()
 # *USE*, or things can get really confusing.
 #
 
+
 #
 # Test-specific fixtures
 #