feat(__future__): Delete a flag's segment override

api/api/urls/future.py

@@ -7,7 +7,7 @@
 
 from django.urls import path
 
-from features.future.views import FlagAPIView
+from features.future.views import FlagAPIView, SegmentOverrideAPIView
 
 app_name = "future"
 
@@ -17,4 +17,10 @@
         FlagAPIView.as_view(),
         name="flag",
     ),
+    path(
+        "environments/<str:environment_key>/features/<int:feature_id>"
+        "/segment-overrides/<int:segment_id>/",
+        SegmentOverrideAPIView.as_view(),
+        name="segment-override",
+    ),
 ]

api/features/future/exceptions.py

@@ -1,7 +1,7 @@
 """https://docs.flagsmith.com/managing-flags/updating-flags"""
 
 from rest_framework import status
-from rest_framework.exceptions import APIException
+from rest_framework.exceptions import APIException, NotFound
 
 
 class ChangeRequestsEnabledError(APIException):
@@ -23,3 +23,9 @@ class DuplicatePriorityError(APIException):
 
     status_code = status.HTTP_400_BAD_REQUEST
     default_detail = "Segment overrides must not share a priority."
+
+
+class SegmentOverrideNotFoundError(NotFound):
+    """Raised where a flag serves a segment nothing of its own."""
+
+    default_detail = "Segment override not found."

api/features/future/permissions.py

@@ -48,3 +48,10 @@ def check_update_permissions(
         raise NotFound()
     if any(property_name in properties for property_name in denied):
         raise PermissionDenied()
+
+
+def check_segment_overrides_permissions(
+    user: UserABC, environment: Environment
+) -> None:
+    """Authorise a caller to write a flag's segment overrides."""
+    check_update_permissions(user, environment, {"segment_overrides": None})

api/features/future/services.py

@@ -9,7 +9,10 @@
 
 from api_keys.user import APIKeyUser
 from environments.models import Environment
-from features.future.exceptions import DuplicatePriorityError
+from features.future.exceptions import (
+    DuplicatePriorityError,
+    SegmentOverrideNotFoundError,
+)
 from features.future.mappers import (
     map_environment_default,
     map_segment_override,
@@ -70,6 +73,27 @@ def _get_feature_states_to_write(
     )
 
 
+def _publish_version(
+    version: EnvironmentFeatureVersion, author: FFAdminUser | APIKeyUser
+) -> None:
+    # `UserABC.__subclasshook__` matches any user against `APIKeyUser`
+    published_by = author if isinstance(author, FFAdminUser) else None
+    version.publish(
+        published_by=published_by,
+        published_by_api_key=None if published_by else author.key,
+    )
+
+
+def _get_overrides_by_segment_id(
+    feature_states: Sequence[FeatureState],
+) -> dict[int, FeatureState]:
+    return {
+        feature_segment.segment_id: feature_state
+        for feature_state in feature_states
+        if (feature_segment := feature_state.feature_segment) is not None
+    }
+
+
 def _write_variants(feature_state: FeatureState, variants: Sequence[Variant]) -> None:
     weighted = {
         multivariate_value.multivariate_feature_option_id: multivariate_value
@@ -299,22 +323,13 @@ def update_flag(
                 feature=feature,
                 version=version,
                 environment_default=environment_default,
-                overrides={
-                    feature_segment.segment_id: feature_state
-                    for feature_state in feature_states
-                    if (feature_segment := feature_state.feature_segment) is not None
-                },
+                overrides=_get_overrides_by_segment_id(feature_states),
                 changes=override_changes,
                 replace=replace,
             )
 
         if version is not None:
-            # `UserABC.__subclasshook__` matches any user against `APIKeyUser`
-            published_by = author if isinstance(author, FFAdminUser) else None
-            version.publish(
-                published_by=published_by,
-                published_by_api_key=None if published_by else author.key,
-            )
+            _publish_version(version, author)
 
     logger.info(
         "flag.updated",
@@ -330,6 +345,45 @@ def update_flag(
     return get_flag(environment=environment, feature=feature)
 
 
+def delete_segment_override(
+    *,
+    environment: Environment,
+    feature: Feature,
+    segment_id: int,
+    author: FFAdminUser | APIKeyUser,
+) -> UpdateFlagResponse:
+    """Remove a flag's override for one segment, leaving the rest of the flag alone."""
+    with transaction.atomic():
+        version = _create_draft_version(environment, feature)
+        feature_states = _get_feature_states_to_write(environment, feature, version)
+
+        if segment_id not in _get_overrides_by_segment_id(feature_states):
+            raise SegmentOverrideNotFoundError()
+
+        _delete_segment_overrides(
+            environment=environment,
+            feature=feature,
+            version=version,
+            segment_ids=[segment_id],
+        )
+
+        if version is not None:
+            _publish_version(version, author)
+
+    logger.info(
+        "flag.updated",
+        organisation__id=environment.project.organisation_id,
+        project__id=environment.project_id,
+        environment__id=environment.id,
+        feature__id=feature.id,
+        segment_overrides__created__segment__ids=[],
+        segment_overrides__updated__segment__ids=[],
+        segment_overrides__deleted__segment__ids=[segment_id],
+    )
+
+    return get_flag(environment=environment, feature=feature)
+
+
 def get_flag(*, environment: Environment, feature: Feature) -> UpdateFlagResponse:
     """Read what a flag serves in an environment."""
     feature_states = _get_feature_states(environment, feature)

api/features/future/views.py

@@ -15,10 +15,11 @@
 from features.future.exceptions import ChangeRequestsEnabledError
 from features.future.permissions import (
     check_read_permissions,
+    check_segment_overrides_permissions,
     check_update_permissions,
 )
 from features.future.serializers import UpdateFlagSerializer
-from features.future.services import get_flag, update_flag
+from features.future.services import delete_segment_override, get_flag, update_flag
 from features.future.types import UpdateFlagRequest, UpdateFlagResponse
 from features.models import Feature
 
@@ -41,6 +42,22 @@ def _get_feature(environment: Environment, feature_id: int) -> Feature:
         raise NotFound() from None
 
 
+def _check_change_requests_disabled(environment: Environment, feature: Feature) -> None:
+    """Refuse to write a flag that can only be changed by a change request."""
+    if not environment.is_workflow_enabled:
+        return
+    api_error = ChangeRequestsEnabledError()
+    logger.warning(
+        "flag.update_rejected",
+        organisation__id=environment.project.organisation_id,
+        project__id=environment.project_id,
+        environment__id=environment.id,
+        feature__id=feature.id,
+        reason=api_error.default_code,
+    )
+    raise api_error
+
+
 class FlagAPIView(APIView):
     """Read or update what a flag serves in an environment."""
 
@@ -96,18 +113,7 @@ def _update_flag(
         environment = _get_environment(environment_key)
         check_update_permissions(request.user, environment, request.data)
         feature = _get_feature(environment, feature_id)
-
-        if environment.is_workflow_enabled:
-            api_error = ChangeRequestsEnabledError()
-            logger.warning(
-                "flag.update_rejected",
-                organisation__id=environment.project.organisation_id,
-                project__id=environment.project_id,
-                environment__id=environment.id,
-                feature__id=feature.id,
-                reason=api_error.default_code,
-            )
-            raise api_error
+        _check_change_requests_disabled(environment, feature)
 
         serializer = UpdateFlagSerializer(
             data=request.data,
@@ -124,3 +130,37 @@ def _update_flag(
                 author=request.user,
             )
         )
+
+
+class SegmentOverrideAPIView(APIView):
+    """Remove what a flag serves to a segment in an environment."""
+
+    permission_classes = [IsAuthenticated]
+
+    @extend_schema(
+        # Responds with the flag, like the other methods, rather than no content.
+        responses={200: UpdateFlagResponse},
+        tags=["experimental"],
+        description=(
+            "Remove the flag's override for the segment, "
+            "leaving the rest of the flag as it is."
+        ),
+    )
+    def delete(
+        self, request: Request, environment_key: str, feature_id: int, segment_id: int
+    ) -> Response:
+        assert not isinstance(request.user, AnonymousUser)
+
+        environment = _get_environment(environment_key)
+        check_segment_overrides_permissions(request.user, environment)
+        feature = _get_feature(environment, feature_id)
+        _check_change_requests_disabled(environment, feature)
+
+        return Response(
+            delete_segment_override(
+                environment=environment,
+                feature=feature,
+                segment_id=segment_id,
+                author=request.user,
+            )
+        )