refactor: centralize checks for canonical courseware experience & URL (#26815)

Centralize the logic for choosing between
MFE and Legacy-frontend courseware within
three new functions:
* courseware_mfe_is_active
* courseware_mfe_is_visible
* courseware_legacy_is_visible

This allows us to create another new function:
* get_courseware_url
which can be called anywhere in LMS/Studio
to get the canonical URL to courseware
content (whether it be MFE or Legacy).

In future commits we we begin using
get_courseware_url throughout the platform.

TNL-7796

common/djangoapps/student/tests/test_models.py

@@ -40,6 +40,7 @@ from common.djangoapps.student.models import (
     PendingNameChange,
 )
 from common.djangoapps.student.tests.factories import AccountRecoveryFactory, CourseEnrollmentFactory, UserFactory
+from xmodule.modulestore import ModuleStoreEnum
 from xmodule.modulestore.tests.django_utils import SharedModuleStoreTestCase
 from xmodule.modulestore.tests.factories import CourseFactory
 
@@ -258,7 +259,7 @@ class UserCelebrationTests(SharedModuleStoreTestCase):
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
-        cls.course = CourseFactory()
+        cls.course = CourseFactory(default_store=ModuleStoreEnum.Type.split)
         cls.course_key = cls.course.id  # pylint: disable=no-member
 
     def setUp(self):

lms/djangoapps/courseware/tests/test_views.py

@@ -258,7 +258,7 @@ class TestJumpTo(ModuleStoreTestCase):
         )
         expected_url += "?{}".format(urlencode({'activate_block_id': six.text_type(staff_only_vertical.location)}))
 
-        assert expected_url == get_legacy_courseware_url(course_key, usage_key, request)
+        assert expected_url == get_legacy_courseware_url(usage_key, request)
 
 
 @ddt.ddt
@@ -548,9 +548,9 @@ class ViewsTestCase(BaseViewsTestCase):
 
     def test_get_redirect_url(self):
         # test the course location
-        assert u'/courses/{course_key}/courseware?{activate_block_id}'.format(course_key=text_type(self.course_key), activate_block_id=urlencode({'activate_block_id': text_type(self.course.location)})) == get_legacy_courseware_url(self.course_key, self.course.location)  # pylint: disable=line-too-long
+        assert u'/courses/{course_key}/courseware?{activate_block_id}'.format(course_key=text_type(self.course_key), activate_block_id=urlencode({'activate_block_id': text_type(self.course.location)})) == get_legacy_courseware_url(self.course.location)  # pylint: disable=line-too-long
         # test a section location
-        assert u'/courses/{course_key}/courseware/Chapter_1/Sequential_1/?{activate_block_id}'.format(course_key=text_type(self.course_key), activate_block_id=urlencode({'activate_block_id': text_type(self.section.location)})) == get_legacy_courseware_url(self.course_key, self.section.location)  # pylint: disable=line-too-long
+        assert u'/courses/{course_key}/courseware/Chapter_1/Sequential_1/?{activate_block_id}'.format(course_key=text_type(self.course_key), activate_block_id=urlencode({'activate_block_id': text_type(self.section.location)})) == get_legacy_courseware_url(self.section.location)  # pylint: disable=line-too-long
 
     def test_invalid_course_id(self):
         response = self.client.get('/courses/MITx/3.091X/')
@@ -3282,8 +3282,9 @@ class TestShowCoursewareMFE(TestCase):
                 assert show_courseware_mfe_link(global_staff_user, False, new_course_key)
                 assert show_courseware_mfe_link(regular_user, True, new_course_key)
 
-                # Regular users don't see the link.
-                assert not show_courseware_mfe_link(regular_user, False, new_course_key)
+                # (Regular users would see the link, but they can't see the Legacy
+                #  experience, so it doesn't matter.)
+
             with override_waffle_flag(REDIRECT_TO_COURSEWARE_MICROFRONTEND, active=False):
                 # (preview=on, redirect=off)
                 # Global and Course Staff can see the link.
@@ -3305,8 +3306,9 @@ class TestShowCoursewareMFE(TestCase):
                 # if preview=off.
                 assert show_courseware_mfe_link(regular_user, True, new_course_key)
 
-                # Regular users don't see the link.
-                assert not show_courseware_mfe_link(regular_user, False, new_course_key)
+                # (Regular users would see the link, but they can't see the Legacy
+                #  experience, so it doesn't matter.)
+
             with override_waffle_flag(REDIRECT_TO_COURSEWARE_MICROFRONTEND, active=False):
                 # (preview=off, redirect=off)
                 # Global staff see the link anyway

lms/djangoapps/courseware/testutils.py

@@ -175,7 +175,7 @@ class RenderXBlockTestMixin(six.with_metaclass(ABCMeta, object)):
             self.setup_user(admin=True, enroll=True, login=True)
 
             with check_mongo_calls(mongo_calls):
-                url = get_legacy_courseware_url(self.course.id, self.block_to_be_tested.location)
+                url = get_legacy_courseware_url(self.block_to_be_tested.location)
                 response = self.client.get(url)
                 expected_elements = self.block_specific_chrome_html_elements + self.COURSEWARE_CHROME_HTML_ELEMENTS
                 for chrome_element in expected_elements:

lms/djangoapps/courseware/toggles.py

@@ -3,8 +3,11 @@ Toggles for courseware in-course experience.
 """
 
 from edx_toggles.toggles import LegacyWaffleFlagNamespace
+from opaque_keys.edx.keys import CourseKey
+
 from openedx.core.djangoapps.waffle_utils import CourseWaffleFlag
 
+
 # Namespace for courseware waffle flags.
 WAFFLE_FLAG_NAMESPACE = LegacyWaffleFlagNamespace(name='courseware')
 
@@ -129,22 +132,79 @@ COURSEWARE_OPTIMIZED_RENDER_XBLOCK = CourseWaffleFlag(
 )
 
 
+def courseware_mfe_is_active(course_key: CourseKey) -> bool:
+    """
+    Should we serve the Learning MFE as the canonical courseware experience?
+    """
+    # NO: Old Mongo courses are always served in the Legacy frontend,
+    #     regardless of configuration.
+    if course_key.deprecated:
+        return False
+    # OTHERWISE: Defer to value of waffle flag for this course run and user.
+    return REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(course_key)
+
+
+def courseware_mfe_is_visible(
+        course_key: CourseKey,
+        is_global_staff=False,
+        is_course_staff=False,
+) -> bool:
+    """
+    Can we see a course run's content in the Learning MFE?
+    """
+    # DENY: Old Mongo courses don't work in the MFE.
+    if course_key.deprecated:
+        return False
+    # ALLOW: Where techincally possible, global staff may always see the MFE.
+    if is_global_staff:
+        return True
+    # ALLOW: If course team preview is enabled, then course staff may see their
+    #        course in the MFE.
+    if is_course_staff and COURSEWARE_MICROFRONTEND_COURSE_TEAM_PREVIEW.is_enabled(course_key):
+        return True
+    # OTHERWISE: The MFE is only visible if it's the active (ie canonical) experience.
+    return courseware_mfe_is_active(course_key)
+
+
+def courseware_legacy_is_visible(
+        course_key: CourseKey,
+        is_global_staff=False,
+        is_course_staff=False,
+) -> bool:
+    """
+    Can we see a course run's content in the Legacy frontend?
+
+    Note: This function will always return True for Old Mongo courses,
+    since `courseware_mfe_is_active` will always return False for them.
+    """
+    # ALLOW: Global staff may always see the Legacy experience.
+    if is_global_staff:
+        return True
+    # ALLOW: The course team may always see their course in the Legacy experience.
+    if is_course_staff:
+        return True
+    # OTHERWISE: Legacy is only visible if it's the active (ie canonical) experience.
+    #            Note that Old Mongo courses are never the active experience,
+    #            so we effectively always ALLOW them to be viewed in Legacy.
+    return not courseware_mfe_is_active(course_key)
+
+
 def course_exit_page_is_active(course_key):
     return (
-        REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(course_key) and
+        courseware_mfe_is_active(course_key) and
         COURSEWARE_MICROFRONTEND_COURSE_EXIT_PAGE.is_enabled(course_key)
     )
 
 
 def courseware_mfe_progress_milestones_are_active(course_key):
     return (
-        REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(course_key) and
+        courseware_mfe_is_active(course_key) and
         COURSEWARE_MICROFRONTEND_PROGRESS_MILESTONES.is_enabled(course_key)
     )
 
 
 def streak_celebration_is_active(course_key):
     return (
-        COURSEWARE_MICROFRONTEND_PROGRESS_MILESTONES.is_enabled(course_key) and
+        courseware_mfe_is_active(course_key) and
         COURSEWARE_MICROFRONTEND_PROGRESS_MILESTONES_STREAK_CELEBRATION.is_enabled(course_key)
     )

lms/djangoapps/courseware/views/index.py

@@ -65,7 +65,7 @@ from ..masquerade import check_content_start_date_for_masquerade_user, setup_mas
 from ..model_data import FieldDataCache
 from ..module_render import get_module_for_descriptor, toc_for_course
 from ..permissions import MASQUERADE_AS_STUDENT
-from ..toggles import COURSEWARE_MICROFRONTEND_COURSE_TEAM_PREVIEW, REDIRECT_TO_COURSEWARE_MICROFRONTEND
+from ..toggles import courseware_legacy_is_visible, courseware_mfe_is_visible
 from .views import CourseTabView
 
 log = logging.getLogger("edx.courseware.views.index")
@@ -170,21 +170,24 @@ class CoursewareIndex(View):
 
     def _redirect_to_learning_mfe(self):
         """
-        Redirect to the new courseware micro frontend,
-        unless this is a time limited exam.
+        Can the user access this sequence in Legacy courseware? If not, redirect to MFE.
+
+        We specifically allow users to stay in the Legacy frontend for special
+        (ie timed/proctored) exams since they're not yet supported by the MFE.
         """
-        # DENY: staff access
-        if self.is_staff:
+        # STAY: if the course run as a whole is visible in the Legacy experience.
+        if courseware_legacy_is_visible(
+                course_key=self.course_key,
+                is_global_staff=self.request.user.is_staff,
+                is_course_staff=self.is_staff,
+        ):
             return
-        # DENY: Old Mongo courses, until removed from platform
-        if self.course_key.deprecated:
-            return
-        # DENY: Timed Exams, until supported
+        # STAY: if we are in a special (ie proctored/timed) exam, which isn't yet
+        #       supported on the MFE.
         if getattr(self.section, 'is_time_limited', False):
             return
-        # ALLOW: when flag set for course
-        if REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(self.course_key):
-            raise Redirect(self.microfrontend_url)
+        # REDIRECT otherwise.
+        raise Redirect(self.microfrontend_url)
 
     @property
     def microfrontend_url(self):
@@ -622,24 +625,8 @@ def show_courseware_mfe_link(user, staff_access, course_key):
     """
     Return whether to display the button to switch to the Courseware MFE.
     """
-    # MFE does not work for Old Mongo courses.
-    if course_key.deprecated:
-        return False
-
-    # Global staff members always get to see the courseware MFE button if the
-    # platform and course are capable, regardless of rollout waffle flags.
-    if user.is_staff:
-        return True
-
-    # If you have course staff access, you can see this link if...
-    if staff_access:
-        # (a) we've turned on the redirect for your students, or...
-        mfe_enabled_for_course = REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(course_key)
-        if mfe_enabled_for_course:
-            return True
-        # (b) we've enabled the course team preview.
-        mfe_enabled_for_course_team = COURSEWARE_MICROFRONTEND_COURSE_TEAM_PREVIEW.is_enabled(course_key)
-        if mfe_enabled_for_course_team:
-            return True
-
-    return False
+    return courseware_mfe_is_visible(
+        course_key=course_key,
+        is_global_staff=user.is_staff,
+        is_course_staff=staff_access,
+    )

lms/djangoapps/courseware/views/views.py

@@ -396,7 +396,7 @@ def jump_to(_request, course_id, location):
     except InvalidKeyError:
         raise Http404(u"Invalid course_key or usage_key")  # lint-amnesty, pylint: disable=raise-missing-from
     try:
-        redirect_url = get_legacy_courseware_url(course_key, usage_key, _request)
+        redirect_url = get_legacy_courseware_url(usage_key, _request)
     except ItemNotFoundError:
         raise Http404(u"No data at this location: {0}".format(usage_key))  # lint-amnesty, pylint: disable=raise-missing-from
     except NoPathToItem:

openedx/core/djangoapps/courseware_api/views.py

@@ -32,7 +32,7 @@ from lms.djangoapps.courseware.courses import check_course_access, get_course_by
 from lms.djangoapps.courseware.masquerade import setup_masquerade
 from lms.djangoapps.courseware.module_render import get_module_by_usage_id
 from lms.djangoapps.courseware.tabs import get_course_tab_list
-from lms.djangoapps.courseware.toggles import REDIRECT_TO_COURSEWARE_MICROFRONTEND, course_exit_page_is_active
+from lms.djangoapps.courseware.toggles import courseware_mfe_is_visible, course_exit_page_is_active
 from lms.djangoapps.courseware.views.views import get_cert_data
 from lms.djangoapps.grades.api import CourseGradeFactory
 from lms.djangoapps.verify_student.services import IDVerificationService
@@ -69,6 +69,7 @@ class CoursewareMeta:
             course_key,
         )
         self.original_user_is_staff = has_access(self.request.user, 'staff', self.overview).has_access
+        self.original_user_is_global_staff = self.request.user.is_staff
         self.course_key = course_key
         self.course_masquerade, self.effective_user = setup_masquerade(
             self.request,
@@ -85,25 +86,13 @@ class CoursewareMeta:
 
     def is_microfrontend_enabled_for_user(self):
         """
-        This method is the "opposite" of _redirect_to_learning_mfe in
-        lms/djangoapps/courseware/views/index.py. But not exactly...
-
-        1. It needs to redirect for old Mongo courses.
-        2. It does NOT need to worry about exams - the MFE will handle
-           those on its own. As of this writing, it will redirect back to
-           the LMS experience, but that may change soon.
-        3. Finally, it needs to redirect users who are bucketed out of
-           the MFE experience, but who aren't staff. Staff are allowed to
-           stay.
+        Can this user see the MFE for this course?
         """
-        # REDIRECT: Old Mongo courses, until removed from platform
-        if self.course_key.deprecated:
-            return False
-        # REDIRECT: If the user isn't staff, redirect if they're bucketed into the old LMS experience.
-        if not self.original_user_is_staff and not REDIRECT_TO_COURSEWARE_MICROFRONTEND.is_enabled(self.course_key):
-            return False
-        # STAY: If the user has made it past all the above, they're good to stay!
-        return True
+        return courseware_mfe_is_visible(
+            course_key=self.course_key,
+            is_global_staff=self.original_user_is_global_staff,
+            is_course_staff=self.original_user_is_staff
+        )
 
     @property
     def enrollment(self):

openedx/features/course_experience/tests/test_url_helpers.py

@@ -0,0 +1,257 @@
+"""
+Test some of the functions in url_helpers
+"""
+from unittest import mock
+
+import ddt
+
+from xmodule.modulestore import ModuleStoreEnum
+from xmodule.modulestore.tests.django_utils import SharedModuleStoreTestCase
+from xmodule.modulestore.tests.factories import CourseFactory, ItemFactory
+
+from .. import url_helpers
+
+
+def _patch_courseware_mfe_is_active(ret_val):
+    return mock.patch.object(
+        url_helpers,
+        'courseware_mfe_is_active',
+        return_value=ret_val,
+    )
+
+
+@ddt.ddt
+class GetCoursewareUrlTests(SharedModuleStoreTestCase):
+    """
+    Test get_courseware_url.
+
+    Mock out `courseware_mfe_is_active`; that is tested elseware.
+    """
+
+    @classmethod
+    def setUpClass(cls):
+        """
+        Set up data used across test functions.
+        """
+        # pylint: disable=super-method-not-called
+        with super().setUpClassAndTestData():
+            cls.items = cls.create_test_courses()
+
+    @classmethod
+    def create_test_courses(cls):
+        """
+        We build two simple course structures (one using Split, the other Old Mongo).
+        Each course structure is a non-branching tree from the root Course block down
+        to the Component-level problem block; that is, we make one item for each course
+        hierarchy level.
+
+        For easy access in the test functions, we return them in a dict like this:
+        {
+            "split": {
+                "course_run": <course block for Split Mongo course>,
+                "section": <chapter block in course run>
+                "subsection": <sequence block in section>
+                "unit": <vertical block in subsection>
+                "component": <problem block in unit>
+            },
+            "mongo": {
+                "course_run": <course block for (deprecated) Old Mongo course>,
+                ... etc ...
+            }
+        }
+        """
+
+        # Make Split Mongo course.
+        with cls.store.default_store(ModuleStoreEnum.Type.split):
+            course_run = CourseFactory.create(
+                org='TestX',
+                number='UrlHelpers',
+                run='split',
+                display_name='URL Helpers Test Course',
+            )
+            with cls.store.bulk_operations(course_run.id):
+                section = ItemFactory.create(
+                    parent_location=course_run.location,
+                    category='chapter',
+                    display_name="Generated Section",
+                )
+                subsection = ItemFactory.create(
+                    parent_location=section.location,
+                    category='sequential',
+                    display_name="Generated Subsection",
+                )
+                unit = ItemFactory.create(
+                    parent_location=subsection.location,
+                    category='vertical',
+                    display_name="Generated Unit",
+                )
+                component = ItemFactory.create(
+                    parent_location=unit.location,
+                    category='problem',
+                    display_name="Generated Problem Component",
+                )
+
+        # Make (deprecated) Old Mongo course.
+        with cls.store.default_store(ModuleStoreEnum.Type.mongo):
+            deprecated_course_run = CourseFactory.create(
+                org='TestX',
+                number='UrlHelpers',
+                run='mongo',
+                display_name='URL Helpers Test Course (Deprecated)',
+            )
+            with cls.store.bulk_operations(deprecated_course_run.id):
+                deprecated_section = ItemFactory.create(
+                    parent_location=deprecated_course_run.location,
+                    category='chapter',
+                    display_name="Generated Section",
+                )
+                deprecated_subsection = ItemFactory.create(
+                    parent_location=deprecated_section.location,
+                    category='sequential',
+                    display_name="Generated Subsection",
+                )
+                deprecated_unit = ItemFactory.create(
+                    parent_location=deprecated_subsection.location,
+                    category='vertical',
+                    display_name="Generated Unit",
+                )
+                deprecated_component = ItemFactory.create(
+                    parent_location=deprecated_unit.location,
+                    category='problem',
+                    display_name="Generated Problem Component",
+                )
+
+        return {
+            ModuleStoreEnum.Type.split: {
+                'course_run': course_run,
+                'section': section,
+                'subsection': subsection,
+                'unit': unit,
+                'component': component,
+            },
+            ModuleStoreEnum.Type.mongo: {
+                'course_run': deprecated_course_run,
+                'section': deprecated_section,
+                'subsection': deprecated_subsection,
+                'unit': deprecated_unit,
+                'component': deprecated_component,
+            }
+        }
+
+    @ddt.data(
+        (
+            ModuleStoreEnum.Type.split,
+            'mfe',
+            'course_run',
+            'http://learning-mfe/course/course-v1:TestX+UrlHelpers+split'
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'mfe',
+            'section',
+            (
+                'http://learning-mfe/course/course-v1:TestX+UrlHelpers+split' +
+                '/block-v1:TestX+UrlHelpers+split+type@chapter+block@Generated_Section'
+            ),
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'mfe',
+            'subsection',
+            (
+                'http://learning-mfe/course/course-v1:TestX+UrlHelpers+split' +
+                '/block-v1:TestX+UrlHelpers+split+type@sequential+block@Generated_Subsection'
+            ),
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'mfe',
+            'unit',
+            (
+                'http://learning-mfe/course/course-v1:TestX+UrlHelpers+split' +
+                '/block-v1:TestX+UrlHelpers+split+type@sequential+block@Generated_Subsection' +
+                '/block-v1:TestX+UrlHelpers+split+type@vertical+block@Generated_Unit'
+            ),
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'mfe',
+            'component',
+            (
+                'http://learning-mfe/course/course-v1:TestX+UrlHelpers+split' +
+                '/block-v1:TestX+UrlHelpers+split+type@sequential+block@Generated_Subsection' +
+                '/block-v1:TestX+UrlHelpers+split+type@vertical+block@Generated_Unit'
+            ),
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'legacy',
+            'course_run',
+            '/courses/course-v1:TestX+UrlHelpers+split/courseware',
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'legacy',
+            'subsection',
+            '/courses/course-v1:TestX+UrlHelpers+split/courseware/Generated_Section/Generated_Subsection/',
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'legacy',
+            'unit',
+            '/courses/course-v1:TestX+UrlHelpers+split/courseware/Generated_Section/Generated_Subsection/1',
+        ),
+        (
+            ModuleStoreEnum.Type.split,
+            'legacy',
+            'component',
+            '/courses/course-v1:TestX+UrlHelpers+split/courseware/Generated_Section/Generated_Subsection/1',
+        ),
+        (
+            ModuleStoreEnum.Type.mongo,
+            'legacy',
+            'course_run',
+            '/courses/TestX/UrlHelpers/mongo/courseware',
+        ),
+        (
+            ModuleStoreEnum.Type.mongo,
+            'legacy',
+            'subsection',
+            '/courses/TestX/UrlHelpers/mongo/courseware/Generated_Section/Generated_Subsection/',
+        ),
+        (
+            ModuleStoreEnum.Type.mongo,
+            'legacy',
+            'unit',
+            '/courses/TestX/UrlHelpers/mongo/courseware/Generated_Section/Generated_Subsection/1',
+        ),
+        (
+            ModuleStoreEnum.Type.mongo,
+            'legacy',
+            'component',
+            '/courses/TestX/UrlHelpers/mongo/courseware/Generated_Section/Generated_Subsection/1',
+        ),
+    )
+    @ddt.unpack
+    def test_get_courseware_url(
+        self,
+        store_type,
+        active_experience,
+        structure_level,
+        expected_path,
+    ):
+        """
+        Given:
+        * a `store_type` ('split' or [old] 'mongo'),
+        * an `active_experience` ('mfe' or 'legacy'),
+        * and a `structure_level` ('course_run', 'section', 'subsection', 'unit', or 'component'),
+
+        check that the expected path (URL without querystring) is returned by `get_courseware_url`.
+        """
+        block = self.items[store_type][structure_level]
+        with _patch_courseware_mfe_is_active(active_experience == 'mfe') as mock_mfe_is_active:
+            url = url_helpers.get_courseware_url(block.location)
+        path = url.split('?')[0]
+        assert path == expected_path
+        course_run = self.items[store_type]['course_run']
+        mock_mfe_is_active.assert_called_once_with(course_run.id)

openedx/features/course_experience/url_helpers.py

@@ -4,30 +4,108 @@ Helper functions for logic related to learning (courseare & course home) URLs.
 Centralizdd in openedx/features/course_experience instead of lms/djangoapps/courseware
 because the Studio course outline may need these utilities.
 """
+from typing import Optional, Tuple
+
 import six
 from django.conf import settings
+from django.contrib.auth import get_user_model
+from django.http import HttpRequest
 from django.urls import reverse
+from opaque_keys.edx.keys import CourseKey, UsageKey
 from six.moves.urllib.parse import urlencode
 
+from lms.djangoapps.courseware.toggles import courseware_mfe_is_active
 from xmodule.modulestore.django import modulestore
 from xmodule.modulestore.search import navigation_index, path_to_location
 
+User = get_user_model()
 
-def get_legacy_courseware_url(course_key, usage_key, request=None):
+
+def get_courseware_url(
+        usage_key: UsageKey,
+        request: Optional[HttpRequest] = None,
+) -> str:
     """
-    Return a str with the URL for the specified legacy (LMS-rendered) coursweare content.
+    Return the URL to the canonical learning experience for a given block.
 
-    Args:
-        course_id(str): Course Id string
-        usage_key(str): The location id of course component
+    We choose between either the Legacy frontend or Learning MFE depending on the
+    course that the block is in, the requesting user, and the state of
+    the 'courseware' waffle flags.
+
+    If you know that you want a Learning MFE URL, regardless of configuration,
+    then it is more performant to call `get_learning_mfe_courseware_url` directly.
 
     Raises:
-        ItemNotFoundError if no data at the location or NoPathToItem if location not in any class
-
-    Returns:
-        Redirect url string
+        * ItemNotFoundError if no data at the `usage_key`.
+        * NoPathToItem if we cannot build a path to the `usage_key`.
     """
+    course_key = usage_key.course_key.replace(version_guid=None, branch=None)
+    if courseware_mfe_is_active(course_key):
+        sequence_key, unit_key = _get_sequence_and_unit_keys(
+            usage_key=usage_key,
+            request=request,
+        )
+        return get_learning_mfe_courseware_url(
+            course_key=course_key,
+            sequence_key=sequence_key,
+            unit_key=unit_key,
+        )
+    else:
+        return get_legacy_courseware_url(
+            usage_key=usage_key,
+            request=request
+        )
 
+
+def _get_sequence_and_unit_keys(
+        usage_key: UsageKey,
+        request: Optional[HttpRequest] = None,
+) -> Tuple[Optional[UsageKey], Optional[UsageKey]]:
+    """
+    Find the sequence and unit containg a block within a course run.
+
+    Performance consideration: Currently, this function incurs a modulestore query.
+
+    Raises:
+        * ItemNotFoundError if no data at the `usage_key`.
+        * NoPathToItem if we cannot build a path to the `usage_key`.
+
+    Returns: (sequence_key|None, unit_key|None)
+    * sequence_key points to a Section (ie chapter) or Subsection (ie sequential).
+    * unit_key points to  Unit (ie vertical).
+    Either of these may be None if we are above that level in the course hierarchy.
+    For example, if `usage_key` points to a Subsection, then unit_key will be None.
+    """
+    path = path_to_location(modulestore(), usage_key, request, full_path=True)
+    if len(path) <= 1:
+        # Course-run-level block:
+        # We have no Sequence or Unit to return.
+        return None, None
+    elif len(path) == 2:
+        # Section-level (ie chapter) block:
+        # The Section is the Sequence. We have no Unit to return.
+        return path[1], None
+    elif len(path) == 3:
+        # Subsection-level block:
+        # The Subsection is the Sequence. We still have no Unit to return.
+        return path[2], None
+    else:
+        # Unit-level (or lower) block:
+        # The Subsection is the Sequence, and the next level down is the Unit.
+        return path[2], path[3]
+
+
+def get_legacy_courseware_url(
+        usage_key: UsageKey,
+        request: Optional[HttpRequest] = None,
+) -> str:
+    """
+    Return the URL to Legacy (LMS-rendered) courseware content.
+
+    Raises:
+        * ItemNotFoundError if no data at the usage_key.
+        * NoPathToItem if location not in any class.
+    """
     (
         course_key, chapter, section, vertical_unused,
         position, final_target_id
@@ -57,18 +135,27 @@ def get_legacy_courseware_url(course_key, usage_key, request=None):
     return redirect_url
 
 
-def get_learning_mfe_courseware_url(course_key, sequence_key=None, unit_key=None):
+def get_learning_mfe_courseware_url(
+        course_key: CourseKey,
+        sequence_key: Optional[UsageKey] = None,
+        unit_key: Optional[UsageKey] = None,
+) -> str:
     """
-    Return a str with the URL for the specified coursweare content in the Learning MFE.
+    Return a str with the URL for the specified courseware content in the Learning MFE.
 
     The micro-frontend determines the user's position in the vertical via
-    a separate API call, so all we need here is the course_key, section, and
+    a separate API call, so all we need here is the course_key, sequence, and
     vertical IDs to format it's URL. For simplicity and performance reasons,
     this method does not inspect the modulestore to try to figure out what
     Unit/Vertical a sequence is in. If you try to pass in a unit_key without
     a sequence_key, the value will just be ignored and you'll get a URL pointing
     to just the course_key.
 
+    Note that `sequence_key` may either point to a Section (ie chapter) or
+    Subsection (ie sequential), as those are both abstractly understood as
+    "sequences". If you pass in a Section-level `sequence_key`, then the MFE
+    will replace it with key of the first Subsection in that Section.
+
     It is also capable of determining our section and vertical if they're not
     present.  Fully specifying it all is preferable, though, as the
     micro-frontend can save itself some work, resulting in a better user
@@ -92,7 +179,9 @@ def get_learning_mfe_courseware_url(course_key, sequence_key=None, unit_key=None
     return mfe_link
 
 
-def get_learning_mfe_home_url(course_key, view_name=None):
+def get_learning_mfe_home_url(
+        course_key: CourseKey, view_name: Optional[str] = None
+) -> str:
     """
     Given a course run key and view name, return the appropriate course home (MFE) URL.
 
@@ -111,7 +200,7 @@ def get_learning_mfe_home_url(course_key, view_name=None):
     return mfe_link
 
 
-def is_request_from_learning_mfe(request):
+def is_request_from_learning_mfe(request: HttpRequest):
     """
     Returns whether the given request was made by the frontend-app-learning MFE.
     """