chore(backend): init check and filter policies

Filter and check policies can be used in various parts of the code. These
policies are meant to trigger authorization based on the given provider.

Signed-off-by: Luca Zaninotto <luca.zaninotto@secomind.com>

Author: lusergit

backend/lib/edgehog/auth/policies/check.ex

@@ -0,0 +1,85 @@
+#
+# This file is part of Edgehog.
+#
+# Copyright 2026 SECO Mind Srl
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+defmodule Edgehog.Auth.Policies.Check do
+  @moduledoc """
+  An `Ash.Policy.SimpleCheck` check to query the provider for a specific `relation` with `object`
+
+  To use it, call it this way in a policy
+  ```elixir
+  {Edgehog.Auth.Policies.Check, id: :id_attribute, rel: :can_view, obj: "deployment"}
+  ```
+
+  The following opts are available:
+  - `rel` (required) :: the relationship you want to check for.
+  - `obj` (required) :: type object you want to check for.
+  - `obj_id`         :: the id of the resource. This is used to build the value `type:id` for the object in the tuple.
+
+  Example: for devices
+
+  ```elixir
+  alias Edgehog.Auth.Policies.Check
+
+  # This can be used to authorize reads on single devices
+  authorize_if {Check, rel: :can_view, obj: :device, obj_id: :device_id}
+  ```
+  """
+  use Ash.Policy.SimpleCheck
+
+  alias Edgehog.Auth.FGAService
+
+  require Logger
+
+  @subject_type "user"
+
+  @impl Ash.Policy.SimpleCheck
+  def match?(actor, %{data: data}, opts) do
+    obj_id = Keyword.get(opts, :obj_id, :id)
+
+    subj = actor.sub
+    rel = opts |> Keyword.fetch!(:rel) |> to_string()
+    obj = opts |> Keyword.fetch!(:obj) |> to_string()
+
+    obj_id =
+      data
+      |> Ash.load!(obj_id)
+      |> Map.get(obj_id, "*")
+      |> to_string()
+
+    Logger.debug("Authorizing a tuple",
+      subject: @subject_type,
+      subject_id: subj,
+      relationship: rel,
+      object: obj,
+      object_id: obj
+    )
+
+    FGAService.check("#{@subject_type}:#{subj}", rel, "#{obj}:#{obj_id}")
+  end
+
+  @impl Ash.Policy.Check
+  def describe(opts) do
+    rel = Keyword.fetch!(opts, :rel)
+    obj = Keyword.fetch!(opts, :obj)
+    obj_id = Keyword.get(opts, :obj_id, :id)
+
+    "Checking if the actor has relation #{inspect(rel)} on #{inspect(obj)} (using field #{inspect(obj_id)} for ID)"
+  end
+end

backend/lib/edgehog/auth/policies/filter.ex

@@ -0,0 +1,91 @@
+#
+# This file is part of Edgehog.
+#
+# Copyright 2026 SECO Mind Srl
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+defmodule Edgehog.Auth.Policies.Filter do
+  @moduledoc """
+  An `Ash.Policy.FilterCheck`, filtering the object of type `obj` available to the user `subj`.
+
+  To use it in a policy:
+  ```elixir
+  filter_by {Edgehog.Auth.Policies.Filter, rel: :can_view, obj: :device}
+  ```
+  """
+
+  use Ash.Policy.FilterCheck
+
+  alias Ash.Policy.FilterCheck
+  alias Edgehog.Auth.FGAService
+
+  require Logger
+
+  @impl FilterCheck
+  def filter(actor, _context, opts) do
+    subj = actor.sub
+
+    rel = opts |> Keyword.fetch!(:rel) |> to_string()
+    obj_type = opts |> Keyword.fetch!(:obj) |> to_string()
+    obj_id = Keyword.get(opts, :obj_id, :id)
+
+    log_context = [
+      subj: subj,
+      obj_type: obj_type,
+      obj_id: obj_id
+    ]
+
+    subj
+    |> FGAService.list_objects(rel, obj_type)
+    |> to_ash_expr(obj_id, log_context)
+  end
+
+  @impl FilterCheck
+  def reject(actor, _context, opts) do
+    subj = actor.sub
+
+    rel = opts |> Keyword.fetch!(:rel) |> to_string()
+    obj_type = opts |> Keyword.fetch!(:obj) |> to_string()
+    obj_id = Keyword.get(opts, :obj_id, :id)
+
+    log_context = [
+      subj: subj,
+      obj_type: obj_type,
+      obj_id: obj_id
+    ]
+
+    subj
+    |> FGAService.list_objects(rel, obj_type)
+    |> to_ash_expr(obj_id, log_context)
+  end
+
+  @impl Ash.Policy.Check
+  def describe(opts) do
+    rel = Keyword.fetch!(opts, :rel)
+    obj = Keyword.fetch!(opts, :obj)
+    "Filtering objects of type #{inspect(obj)} where actor has relation #{inspect(rel)}"
+  end
+
+  defp to_ash_expr({:ok, ids}, id_attribute, _log_context), do: expr(^ref(id_attribute) in ^ids)
+
+  defp to_ash_expr({:error, error}, _id_attribute, log_context) do
+    Logger.error("Error while filtering: #{inspect(error)}", log_context)
+
+    # We had an error while interrogating the provider. Poison everything.
+    expr(nil)
+  end
+end