✨(backend) add authenticated recording file access method

Implement secure recording file access through authentication instead of
exposing S3 bucket or using temporary signed links with loose permissions.
Inspired by docs and @spaccoud's implementation, with comprehensive
viewset checks to prevent unauthorized recording downloads.

The ingress reserved to media intercept the original request, and thanks to
Nginx annotations, check with the backend if the user is allowed to donwload
this recording file. This might introduce a dependency to Nginx in the project
by the way.

Note: Tests are integration-based rather than unit tests, requiring minio in
the compose stack and CI environment. Implementation includes known botocore
deprecation warnings that per GitHub issues won't be resolved for months.

src/backend/core/api/viewsets.py

@@ -2,6 +2,7 @@
 
 import uuid
 from logging import getLogger
+from urllib.parse import urlparse
 
 from django.conf import settings
 from django.db.models import Q
@@ -20,7 +21,8 @@ from rest_framework import (
     status as drf_status,
 )
 
-from core import models, utils
+from core import enums, models, utils
+from core.recording.enums import FileExtension
 from core.recording.event.authentication import StorageEventAuthentication
 from core.recording.event.exceptions import (
     InvalidBucketError,
@@ -618,3 +620,85 @@ class RecordingViewSet(
         return drf_response.Response(
             {"message": "Event processed."},
         )
+
+    def _auth_get_original_url(self, request):
+        """
+        Extracts and parses the original URL from the "HTTP_X_ORIGINAL_URL" header.
+        Raises PermissionDenied if the header is missing.
+        The original url is passed by nginx in the "HTTP_X_ORIGINAL_URL" header.
+        See corresponding ingress configuration in Helm chart and read about the
+        nginx.ingress.kubernetes.io/auth-url annotation to understand how the Nginx ingress
+        is configured to do this.
+        Based on the original url and the logged-in user, we must decide if we authorize Nginx
+        to let this request go through (by returning a 200 code) or if we block it (by returning
+        a 403 error). Note that we return 403 errors without any further details for security
+        reasons.
+        """
+        # Extract the original URL from the request header
+        original_url = request.META.get("HTTP_X_ORIGINAL_URL")
+        if not original_url:
+            logger.debug("Missing HTTP_X_ORIGINAL_URL header in subrequest")
+            raise drf_exceptions.PermissionDenied()
+
+        logger.debug("Original url: '%s'", original_url)
+        return urlparse(original_url)
+
+    def _auth_get_url_params(self, pattern, fragment):
+        """
+        Extracts URL parameters from the given fragment using the specified regex pattern.
+        Raises PermissionDenied if parameters cannot be extracted.
+        """
+
+        match = pattern.search(fragment)
+
+        try:
+            return match.groupdict()
+        except (ValueError, AttributeError) as exc:
+            logger.debug("Failed to extract parameters from subrequest URL: %s", exc)
+            raise drf_exceptions.PermissionDenied() from exc
+
+    @decorators.action(detail=False, methods=["get"], url_path="media-auth")
+    def media_auth(self, request, *args, **kwargs):
+        """
+        This view is used by an Nginx subrequest to control access to a recording's
+        media file.
+        When we let the request go through, we compute authorization headers that will be added to
+        the request going through thanks to the nginx.ingress.kubernetes.io/auth-response-headers
+        annotation. The request will then be proxied to the object storage backend who will
+        respond with the file after checking the signature included in headers.
+        """
+
+        parsed_url = self._auth_get_original_url(request)
+
+        url_params = self._auth_get_url_params(
+            enums.RECORDING_STORAGE_URL_PATTERN, parsed_url.path
+        )
+
+        user = request.user
+        recording_id = url_params["recording_id"]
+
+        extension = url_params["extension"]
+        if extension not in [item.value for item in FileExtension]:
+            raise drf_exceptions.ValidationError({"detail": "Unsupported extension."})
+
+        try:
+            recording = models.Recording.objects.get(id=recording_id)
+        except models.Recording.DoesNotExist as e:
+            raise drf_exceptions.NotFound("No recording found for this event.") from e
+
+        if extension != recording.extension:
+            raise drf_exceptions.NotFound("No recording found with this extension.")
+
+        abilities = recording.get_abilities(user)
+
+        if not abilities["retrieve"]:
+            logger.debug("User '%s' lacks permission for attachment", user.id)
+            raise drf_exceptions.PermissionDenied()
+
+        if not recording.is_saved:
+            logger.debug("Recording '%s' has not been saved", recording)
+            raise drf_exceptions.PermissionDenied()
+
+        request = utils.generate_s3_authorization_headers(recording.key)
+
+        return drf_response.Response("authorized", headers=request.headers, status=200)