Add retrieve functionality

closes pulp#1010

In particular we are adding the new behaviour for:

- ReleaseArchitecture
- ReleaseComponent

but deliberately not for PackageReleaseComponent, which should only be
created indirectly using package actions.

CHANGES/1010.feature

@@ -0,0 +1 @@
+Added retrieve functionality for ReleaseArchitecture and ReleaseComponent content.

CHANGES/1010.removal

@@ -0,0 +1,2 @@
+The API endpoints for ReleaseArchitecture and ReleaseComponent creation will no longer return a 400 ``non_field_errors`` if the content to be created already exists.
+Instead a task is triggered that will list the existing content in its ``created_resources`` field.

pulp_deb/app/serializers/content_serializers.py

@@ -750,6 +750,21 @@ class ReleaseArchitectureSerializer(NoArtifactContentSerializer):
     architecture = CharField(help_text="Name of the architecture.")
     distribution = CharField(help_text="Name of the distribution.")
 
+    def get_unique_together_validators(self):
+        """
+        We do not want UniqueTogetherValidator since we have retrieve logic!
+        """
+        return []
+
+    def retrieve(self, validated_data):
+        """
+        If the ReleaseArchitecture already exists, retrieve it!
+        """
+        return ReleaseComponent.objects.filter(
+            architecture=validated_data["architecture"],
+            distribution=validated_data["distribution"],
+        ).first()
+
     class Meta(NoArtifactContentSerializer.Meta):
         model = ReleaseArchitecture
         fields = NoArtifactContentSerializer.Meta.fields + (
@@ -763,6 +778,21 @@ class ReleaseComponentSerializer(NoArtifactContentSerializer):
     A Serializer for ReleaseComponent.
     """
 
+    def get_unique_together_validators(self):
+        """
+        We do not want UniqueTogetherValidator since we have retrieve logic!
+        """
+        return []
+
+    def retrieve(self, validated_data):
+        """
+        If the ReleaseComponent already exists, retrieve it!
+        """
+        return ReleaseComponent.objects.filter(
+            distribution=validated_data["distribution"],
+            component=validated_data["component"],
+        ).first()
+
     component = CharField(help_text="Name of the component.")
     distribution = CharField(help_text="Name of the distribution.")
 

pulp_deb/app/viewsets/content.py

@@ -1,19 +1,54 @@
 from gettext import gettext as _  # noqa
 
 from django_filters import Filter
+from drf_spectacular.utils import extend_schema
+from pulpcore.plugin.tasking import general_create
 from pulpcore.plugin.models import Repository, RepositoryVersion
+from pulpcore.plugin.serializers import AsyncOperationResponseSerializer
 from pulpcore.plugin.serializers.content import ValidationError
+from pulpcore.plugin.tasking import dispatch
 from pulpcore.plugin.viewsets import (
     NAME_FILTER_OPTIONS,
     ContentFilter,
     ContentViewSet,
     NamedModelViewSet,
+    OperationPostponedResponse,
     SingleArtifactContentUploadViewSet,
 )
+from pulpcore.plugin.viewsets.content import DefaultDeferredContextMixin
 
 from pulp_deb.app import models, serializers
 
 
+class NoArtifactContentViewSet(DefaultDeferredContextMixin, ContentViewSet):
+    """A ViewSet for content creation that does not require a file to be uploaded."""
+
+    @extend_schema(
+        description="Trigger an asynchronous task to create content,"
+        "optionally create new repository version.",
+        responses={202: AsyncOperationResponseSerializer},
+    )
+    def create(self, request):
+        """Create a content unit."""
+        serializer = self.get_serializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
+        exclusive_resources = [
+            item for item in (serializer.validated_data.get(key) for key in ("repository",)) if item
+        ]
+
+        task = dispatch(
+            general_create,
+            exclusive_resources=exclusive_resources,
+            args=(self.queryset.model._meta.app_label, serializer.__class__.__name__),
+            kwargs={
+                "data": {k: v for k, v in request.data.items()},
+                "context": self.get_deferred_context(request),
+            },
+        )
+        return OperationPostponedResponse(task, request)
+
+
 class GenericContentFilter(ContentFilter):
     """
     FilterSet for GenericContent.
@@ -440,7 +475,7 @@ class Meta:
         fields = ["architecture", "distribution"]
 
 
-class ReleaseArchitectureViewSet(ContentViewSet):
+class ReleaseArchitectureViewSet(NoArtifactContentViewSet):
     # The doc string is a top level element of the user facing REST API documentation:
     """
     A ReleaseArchitecture represents a single dpkg architecture string.
@@ -479,7 +514,7 @@ class Meta:
         fields = ["component", "distribution"]
 
 
-class ReleaseComponentViewSet(ContentViewSet):
+class ReleaseComponentViewSet(NoArtifactContentViewSet):
     # The doc string is a top level element of the user facing REST API documentation:
     """
     A ReleaseComponent represents a single APT repository component.