build/python: Add a sanity check phase.

Add a new phase validating the usability of installed Python packages.

* gnu/packages/aux-files/python/sanity-check.py: New file.
* Makefile.am (AUX_FILES): Register it.
* guix/build-system/python.scm (sanity-check.py): New variable.
(lower): Add the script as an implicit input.
* guix/build/python-build-system.scm: Remove trailing #t.
(sanity-check): New phase.
(%standard-phases): Use it.
* tests/builders.scm: (make-python-dummy)
(dummy-ok, dummy-dummy-nosetuptools, dummy-fail-requirements)
(dummy-fail-import, dummy-fail-console-script): New variables.
("python-build-system: dummy-ok")
("python-build-system: dummy-dummy-nosetuptools")
("python-build-system: dummy-fail-requirements")
("python-build-system: dummy-fail-import")
("python-build-system: dummy-fail-console-script"): Add tests.

gnu/packages/aux-files/python/sanity-check.py

@@ -0,0 +1,91 @@
+# -*- coding: utf-8 -*-
+# GNU Guix --- Functional package management for GNU
+# Copyright © 2021 Lars-Dominik Braun <lars@6xq.net>
+#
+# This file is part of GNU Guix.
+#
+# GNU Guix is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or (at
+# your option) any later version.
+#
+# GNU Guix is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+from __future__ import print_function  # Python 2 support.
+import importlib
+import pkg_resources
+import sys
+import traceback
+
+try:
+    from importlib.machinery import PathFinder
+except ImportError:
+    PathFinder = None
+
+ret = 0
+
+# Only check site-packages installed by this package, but not dependencies
+# (which pkg_resources.working_set would include). Path supplied via argv.
+ws = pkg_resources.find_distributions(sys.argv[1])
+
+for dist in ws:
+    print('validating', repr(dist.project_name), dist.location)
+    try:
+        print('...checking requirements: ', end='')
+        req = str(dist.as_requirement())
+        # dist.activate() is not enough to actually check requirements, we
+        # have to .require() it.
+        pkg_resources.require(req)
+        print('OK')
+    except Exception as e:
+        print('ERROR:', req, e)
+        ret = 1
+        continue
+
+    # Try to load top level modules. This should not have any side-effects.
+    try:
+        metalines = dist.get_metadata_lines('top_level.txt')
+    except KeyError:
+        # distutils (i.e. #:use-setuptools? #f) will not install any metadata.
+        print('WARNING: cannot determine top-level modules')
+        continue
+    for name in metalines:
+        # Only available on Python 3.
+        if PathFinder and PathFinder.find_spec(name) is None:
+            # Ignore unavailable modules, often C modules, which were not
+            # installed at the top-level. Cannot use ModuleNotFoundError,
+            # because it is raised by failed imports too.
+            continue
+        try:
+            print('...trying to load module', name, end=': ')
+            importlib.import_module(name)
+            print('OK')
+        except Exception:
+            print('ERROR:')
+            traceback.print_exc(file=sys.stdout)
+            ret = 1
+            continue
+
+    # Try to load entry points of console scripts too, making sure they
+    # work. They should be removed if they don't. Other groups may not be
+    # safe, as they can depend on optional packages.
+    for group, v in dist.get_entry_map().items():
+        if group not in {'console_scripts', 'gui_scripts'}:
+            continue
+        for name, ep in v.items():
+            try:
+                print('...trying to load endpoint', group, name, end=': ')
+                ep.load()
+                print('OK')
+            except Exception:
+                print('ERROR:')
+                traceback.print_exc(file=sys.stdout)
+                ret = 1
+
+sys.exit(ret)