federation/in: improve reply on requests from blocked domains

Previously all such requests led to '401 Unauthorized'
whih might have triggered retries.
Now, to not leak any MRF info, we just indicate an
accept for POST requests without actually processing the object
and indiscriminately return "not found" for GET requests.

Notably this change also now causes all signed fetch requests from
blocked domains to be rejected even if authorized_fetch isn’t enabled.

Fixes: #929

lib/pleroma/web/plugs/http_signature_plug.ex

@@ -95,6 +95,20 @@ defp maybe_halt(conn, :gone) do
     end
   end
 
+  defp maybe_halt(conn, {:reject, _}) do
+    cond do
+      conn.method == "POST" ->
+        conn
+        |> resp(202, "Accepted")
+        |> halt()
+
+      true ->
+        conn
+        |> resp(404, "Not found")
+        |> halt()
+    end
+  end
+
   defp maybe_halt(conn, _), do: conn
 
   defp assign_valid_signature(%{assigns: %{valid_signature: true}} = conn, _),

test/pleroma/web/plugs/http_signature_plug_test.exs

@@ -163,19 +163,27 @@ test "fails on gone key for non-Delete" do
     assert conn.assigns.signature_user == nil
   end
 
-  test "fails on rejected keys", %{user: user} do
-    conn =
-      build_conn(:post, "/inbox", %{"type" => "Note"})
-      |> put_format("activity+json")
-      |> assign(:rejected_key_id, true)
-      |> put_req_header(
-        "signature",
-        "keyId=\"#{user.signing_key.key_id}\""
-      )
-      |> HTTPSignaturePlug.call(%{})
+  test "fakes accept for POST on rejected keys", %{user: user} do
+    build_conn(:post, "/inbox", %{"type" => "Note"})
+    |> put_format("activity+json")
+    |> assign(:rejected_key_id, true)
+    |> put_req_header(
+      "signature",
+      "keyId=\"#{user.signing_key.key_id}\""
+    )
+    |> HTTPSignaturePlug.call(%{})
+    |> response(202)
+  end
 
-    refute conn.halted
-    assert conn.assigns.valid_signature == false
-    assert conn.assigns.signature_user == nil
+  test "fakes not found for GET on rejected keys", %{user: user} do
+    build_conn(:get, "/doesntmattter", %{"user" => user.ap_id})
+    |> put_format("activity+json")
+    |> assign(:rejected_key_id, true)
+    |> put_req_header(
+      "signature",
+      "keyId=\"#{user.signing_key.key_id}\""
+    )
+    |> HTTPSignaturePlug.call(%{})
+    |> response(404)
   end
 end