✨(backend) add file upload feature & tests

For the coming features we will need to store files on the meet side.
(for instance user backgrounds).

This commits adds a new Model to manage files, and the associated
serializers & viewsets. All are tested.

This work was heavily inspired by the work done by our friends at
https://github.com/suitenumerique/drive
It build on the same architecture design (upload directly to S3 but
download goes through our proxy), but model is much much simplier
(no folders, no file sharing, etc.).

src/backend/core/api/serializers.py

@@ -1,10 +1,15 @@
 """Client serializers for the Meet core app."""
 
 # pylint: disable=abstract-method,no-name-in-module
+import logging
+from os.path import splitext
 from typing import Literal
+from urllib.parse import quote
 
 from django.conf import settings
 from django.core.exceptions import SuspiciousOperation
+
+# pylint: disable=abstract-method,no-name-in-module
 from django.utils.translation import gettext_lazy as _
 
 from django_pydantic_field.rest_framework import SchemaField
@@ -15,6 +20,8 @@ from timezone_field.rest_framework import TimeZoneSerializerField
 
 from core import models, utils
 
+logger = logging.getLogger(__name__)
+
 
 class UserSerializer(serializers.ModelSerializer):
     """Serialize users."""
@@ -27,6 +34,15 @@ class UserSerializer(serializers.ModelSerializer):
         read_only_fields = ["id", "email", "full_name", "short_name"]
 
 
+class UserLightSerializer(serializers.ModelSerializer):
+    """Serialize users with limited fields."""
+
+    class Meta:
+        model = models.User
+        fields = ["id", "full_name", "short_name"]
+        read_only_fields = ["id", "full_name", "short_name"]
+
+
 class ResourceAccessSerializerMixin:
     """
     A serializer mixin to share controlling that the logged-in user submitting a room access object
@@ -377,3 +393,143 @@ class UpdateParticipantSerializer(BaseParticipantsManagementSerializer):
             )
 
         return attrs
+
+
+class ListFileSerializer(serializers.ModelSerializer):
+    """Serialize File model for the API."""
+
+    url = serializers.SerializerMethodField(read_only=True)
+    creator = UserLightSerializer(read_only=True)
+    abilities = serializers.SerializerMethodField(read_only=True)
+
+    class Meta:
+        model = models.File
+        fields = [
+            "id",
+            "created_at",
+            "updated_at",
+            "title",
+            "type",
+            "creator",
+            "deleted_at",
+            "hard_deleted_at",
+            "filename",
+            "upload_state",
+            "mimetype",
+            "size",
+            "description",
+            "url",
+            "abilities",
+        ]
+        read_only_fields = [
+            "id",
+            "created_at",
+            "updated_at",
+            "creator",
+            "deleted_at",
+            "hard_deleted_at",
+            "filename",
+            "upload_state",
+            "mimetype",
+            "size",
+            "url",
+            "abilities",
+        ]
+
+    def get_url(self, obj):
+        """Return the URL of the file."""
+        if obj.is_pending_upload:
+            return None
+
+        return f"{settings.MEDIA_BASE_URL}{settings.MEDIA_URL}{quote(obj.file_key)}"
+
+    def get_abilities(self, file) -> dict:
+        """Return abilities of the logged-in user on the instance."""
+        request = self.context.get("request")
+        if not request:
+            return {}
+
+        return file.get_abilities(request.user)
+
+
+class FileSerializer(ListFileSerializer):
+    """Default serializer File model for the API."""
+
+    def create(self, validated_data):
+        raise NotImplementedError("Create method can not be used.")
+
+
+class CreateFileSerializer(ListFileSerializer):
+    """Serializer used to create a new file"""
+
+    title = serializers.CharField(max_length=255, required=False)
+    policy = serializers.SerializerMethodField()
+
+    class Meta:
+        model = models.File
+        fields = [*ListFileSerializer.Meta.fields, "policy"]
+        read_only_fields = [
+            *(
+                field
+                for field in ListFileSerializer.Meta.read_only_fields
+                if field != "filename"
+            ),
+            "policy",
+        ]
+
+    def get_fields(self):
+        """Force the id field to be writable."""
+        fields = super().get_fields()
+        fields["id"].read_only = False
+
+        return fields
+
+    def validate_id(self, value):
+        """Ensure the provided ID does not already exist when creating a new file."""
+        request = self.context.get("request")
+
+        # Only check this on POST (creation)
+        if request and models.File.objects.filter(id=value).exists():
+            raise serializers.ValidationError(
+                "A file with this ID already exists. You cannot override it.",
+                code="file_create_existing_id",
+            )
+
+        return value
+
+    def validate(self, attrs):
+        """Validate extension and fill title."""
+        # we run the default validation first to make sure the base data in attrs is ok
+        attrs = super().validate(attrs)
+
+        filename_root, ext = splitext(attrs["filename"])
+
+        if settings.FILE_UPLOAD_APPLY_RESTRICTIONS:
+            config_for_file_type = settings.FILE_UPLOAD_RESTRICTIONS[attrs["type"]]
+            if ext.lower() not in config_for_file_type["allowed_extensions"]:
+                logger.info(
+                    "create_item: file extension not allowed %s for filename %s",
+                    ext,
+                    attrs["filename"],
+                )
+                raise serializers.ValidationError(
+                    {"filename": _("This file extension is not allowed.")},
+                    code="item_create_file_extension_not_allowed",
+                )
+
+        # The title will be the filename if not provided
+        if not attrs.get("title", None):
+            attrs["title"] = filename_root
+
+        return attrs
+
+    def get_policy(self, file):
+        """Return the policy to use if the item is a file."""
+
+        if file.upload_state == models.FileUploadStateChoices.READY:
+            return None
+
+        return utils.generate_upload_policy(file)
+
+    def update(self, instance, validated_data):
+        raise NotImplementedError("Update method can not be used.")