fix/integration test for pants_requirement() (pantsbuild#5457)

### Problem

It's not possible to use `pants_requirement()` to stand in for `python_requirement_library(requirements=[python_requirement('pantsbuild.pants==1.5.0.dev3')])` in a separate repo. This defeats the purpose of `pants_requirement()`, which expands into an `==` requirement on the pants version denoted in the repo's `pants.ini`. The error is:
```
u"There is no type registered for alias <class 'pants.backend.python.targets.python_requirement_library.PythonRequirementLibrary'>"
```

### Solution

`ParseContext#create_object()` accepts an alias as the first argument, not the class itself. [Changing `PythonRequirementLibrary` to `'python_requirement_library'` in `src/python/pants/backend/python/pants_requirement.py`](https://github.com/cosmicexplorer/pants/blob/1fc4ba0310d3fd20d26384e5e1c356e8f99b9eea/src/python/pants/backend/python/pants_requirement.py#L37) allows `pants_requirement()` to be used as documented. Integration tests were added in `tests/python/pants_test/backend/python{test_pants_requirement_integration,/tasks/test_setup_py_integration}.py`.

src/python/pants/backend/python/pants_requirement.py

@@ -8,7 +8,6 @@
 import os
 
 from pants.backend.python.python_requirement import PythonRequirement
-from pants.backend.python.targets.python_requirement_library import PythonRequirementLibrary
 from pants.base.build_environment import pants_version
 
 
@@ -34,5 +33,6 @@ def __call__(self, name=None):
     """
     name = name or os.path.basename(self._parse_context.rel_path)
     requirement = PythonRequirement(requirement='pantsbuild.pants=={}'.format(pants_version()))
-    self._parse_context.create_object(PythonRequirementLibrary, name=name,
+    self._parse_context.create_object('python_requirement_library',
+                                      name=name,
                                       requirements=[requirement])

testprojects/pants-plugins/3rdparty/python/pants/BUILD

@@ -0,0 +1 @@
+pants_requirement()

testprojects/pants-plugins/src/python/test_pants_plugin/BUILD

@@ -0,0 +1,15 @@
+python_library(
+  dependencies=[
+    'testprojects/pants-plugins/3rdparty/python/pants',
+    'testprojects/pants-plugins/src/python/test_pants_plugin/subsystems',
+  ],
+  provides=setup_py(
+    name='test_pants_plugin',
+    description='A test pants plugin',
+    version='0.0.1',
+    namespace_packages=[
+      'test_pants_plugin',
+      'test_pants_plugin.subsystems',
+    ],
+  )
+)

testprojects/pants-plugins/src/python/test_pants_plugin/__init__.py

@@ -0,0 +1 @@
+__import__('pkg_resources').declare_namespace(__name__)

testprojects/pants-plugins/src/python/test_pants_plugin/pants_infra_tests.py

@@ -0,0 +1,22 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from test_pants_plugin.subsystems.pants_test_infra import PantsTestInfra
+
+from pants.backend.python.targets.python_tests import PythonTests
+
+
+class PantsInfraTests(object):
+
+  def __init__(self, parse_context):
+    self._parse_context = parse_context
+    self._pants_test_infra = PantsTestInfra.global_instance()
+
+  def __call__(self, dependencies=[], **kwargs):
+    dependencies.extend(self._pants_test_infra.dependent_target_addrs())
+    self._parse_context.create_object(
+      PythonTests.alias(), dependencies=dependencies, **kwargs)

testprojects/pants-plugins/src/python/test_pants_plugin/register.py

@@ -0,0 +1,22 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from test_pants_plugin.pants_infra_tests import PantsInfraTests
+from test_pants_plugin.subsystems.pants_test_infra import PantsTestInfra
+
+from pants.build_graph.build_file_aliases import BuildFileAliases
+
+
+def build_file_aliases():
+  return BuildFileAliases(
+    context_aware_object_factories={
+      'pants_infra_tests': PantsInfraTests,
+    }
+  )
+
+def global_subsystems():
+  return (PantsTestInfra,)

testprojects/pants-plugins/src/python/test_pants_plugin/subsystems/BUILD

@@ -0,0 +1,5 @@
+python_library(
+  dependencies=[
+    'testprojects/pants-plugins/3rdparty/python/pants',
+  ],
+)

testprojects/pants-plugins/src/python/test_pants_plugin/subsystems/pants_test_infra.py

@@ -0,0 +1,31 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from pants.backend.python.targets.python_target import PythonTarget
+from pants.build_graph.target import Target
+from pants.subsystem.subsystem import Subsystem
+
+
+class PantsTestInfra(Subsystem):
+  options_scope = 'pants-test-infra'
+
+  @classmethod
+  def register_options(cls, register):
+    super(PantsTestInfra, cls).register_options(register)
+    register('--pants-requirement-target', advanced=True, fingerprint=True,
+             help='Address for a python target providing the pants sdist.',
+             type=str, default=None)
+    register('--pants-test-infra-target', advanced=True, fingerprint=True,
+             help='Address for a python target providing the '
+                  'pants test infra sdist.',
+             type=str, default=None)
+
+  def dependent_target_addrs(self):
+    return [
+      self.get_options().pants_requirement_target,
+      self.get_options().pants_test_infra_target,
+    ]

testprojects/pants-plugins/tests/python/test_pants_plugin/TEST_BUILD

@@ -0,0 +1 @@
+pants_infra_tests()

testprojects/pants-plugins/tests/python/test_pants_plugin/test_pants_plugin_pants_requirement.py

@@ -0,0 +1,15 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from pants.base.build_environment import pants_version
+from pants.version import VERSION as _VERSION
+from pants_test.base_test import BaseTest
+
+
+class PantsPluginPantsRequirementTest(BaseTest):
+  def test_version(self):
+    self.assertEqual(pants_version(), _VERSION)

tests/python/pants_test/backend/python/BUILD

@@ -65,3 +65,13 @@ python_tests(
   ],
   timeout=1200
 )
+
+python_tests(
+  name='integration',
+  sources=globs('*_integration.py'),
+  dependencies=[
+    'src/python/pants/base:build_environment',
+    'tests/python/pants_test:int-test',
+  ],
+  tags={'integration'},
+)

tests/python/pants_test/backend/python/tasks/test_setup_py_integration.py

@@ -132,3 +132,29 @@ def test_setup_py_with_codegen_unexported_deps(self):
                        'org/pantsbuild/example/precipitation/__init__.py',
                        'org/pantsbuild/example/precipitation/constants.py',
                        'org/pantsbuild/example/precipitation/ttypes.py'])
+
+  def test_setup_py_unregistered_pants_plugin(self):
+    """setup-py should succeed on a pants plugin target that:
+
+    1. uses a pants_requirement() instead of linking directly to targets in the
+       pants codebase
+    2. is not on the pythonpath nor registered as a backend package."""
+
+    self.maxDiff = None
+
+    command = [
+      'setup-py',
+      'testprojects/pants-plugins/src/python/test_pants_plugin',
+    ]
+    pants_run = self.run_pants(command=command)
+    self.assert_success(pants_run)
+
+    self.assert_sdist(pants_run, 'test_pants_plugin', [
+      'test_pants_plugin/',
+      'test_pants_plugin/__init__.py',
+      'test_pants_plugin/pants_infra_tests.py',
+      'test_pants_plugin/register.py',
+      'test_pants_plugin/subsystems/',
+      'test_pants_plugin/subsystems/__init__.py',
+      'test_pants_plugin/subsystems/pants_test_infra.py',
+    ])

tests/python/pants_test/backend/python/test_pants_requirement_integration.py

@@ -0,0 +1,47 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+import os
+
+from pants.base.build_environment import get_buildroot
+from pants_test.pants_run_integration_test import PantsRunIntegrationTest
+
+
+class PantsRequirementIntegrationTest(PantsRunIntegrationTest):
+  """A pants plugin should be able to depend on a pants_requirement() alone to
+  declare its dependencies on pants modules. This plugin, when added to the
+  pythonpath and backend_packages, should be able to declare new BUILD file
+  objects."""
+
+  def run_with_testproject_backend_pkgs(self, cmd):
+    testproject_backend_src_dir = os.path.join(
+      get_buildroot(), 'testprojects/pants-plugins/src/python')
+    testproject_backend_pkg_name = 'test_pants_plugin'
+    pants_req_addr = 'testprojects/pants-plugins/3rdparty/python/pants'
+    pants_test_infra_addr = 'tests/python/pants_test:test_infra'
+    pre_cmd_args = [
+      "--pythonpath=+['{}']".format(testproject_backend_src_dir),
+      "--backend-packages=+['{}']".format(testproject_backend_pkg_name),
+      "--pants-test-infra-pants-requirement-target={}".format(pants_req_addr),
+      "--pants-test-infra-pants-test-infra-target={}".format(pants_test_infra_addr),
+    ]
+    command = pre_cmd_args + cmd
+    return self.run_pants(command=command)
+
+  def test_pants_requirement(self):
+    self.maxDiff = None
+
+    tests_dir = 'testprojects/pants-plugins/tests/python/test_pants_plugin'
+    command = [
+      'test',
+      tests_dir,
+    ]
+
+    tests_dir_absolute = os.path.join(get_buildroot(), tests_dir)
+    with self.file_renamed(tests_dir_absolute, 'TEST_BUILD', 'BUILD'):
+      pants_run = self.run_with_testproject_backend_pkgs(command)
+    self.assert_success(pants_run)

tests/python/pants_test/engine/test_isolated_process.py

@@ -11,8 +11,8 @@
 
 from pants.engine.fs import PathGlobs, Snapshot, create_fs_rules
 from pants.engine.isolated_process import (Binary, ExecuteProcess, ExecuteProcessRequest,
-  ExecuteProcessResult, SnapshottedProcess,
-  SnapshottedProcessRequest, create_process_rules)
+                                           ExecuteProcessResult, SnapshottedProcess,
+                                           SnapshottedProcessRequest, create_process_rules)
 from pants.engine.nodes import Return, Throw
 from pants.engine.rules import SingletonRule
 from pants.engine.selectors import Select