A feature for shareable emoji packs, use it in download_from & tests

lib/pleroma/web/emoji_api/emoji_api_controller.ex

@@ -153,6 +153,15 @@ def download_shared(conn, %{"name" => name}) do
   from that instance, otherwise it will be downloaded from the fallback source, if there is one.
   """
   def download_from(conn, %{"instance_address" => address, "pack_name" => name} = data) do
+    shareable_packs_available =
+      "#{address}/nodeinfo/2.1.json"
+      |> Tesla.get!()
+      |> Map.get(:body)
+      |> Jason.decode!()
+      |> Map.get("features")
+      |> Enum.member?("shareable_emoji_packs")
+
+    if shareable_packs_available do
       full_pack =
         "#{address}/api/pleroma/emoji/packs/list"
         |> Tesla.get!()
@@ -178,7 +187,8 @@ def download_from(conn, %{"instance_address" => address, "pack_name" => name} =
              }}
 
           _ ->
-          {:error, "The pack was not set as shared and there is no fallback src to download from"}
+            {:error,
+             "The pack was not set as shared and there is no fallback src to download from"}
         end
 
       with {:ok, %{sha: sha, uri: uri} = pinfo} <- pack_info_res,
@@ -212,6 +222,11 @@ def download_from(conn, %{"instance_address" => address, "pack_name" => name} =
           |> put_status(:internal_server_error)
           |> json(%{error: "SHA256 for the pack doesn't match the one sent by the server"})
       end
+    else
+      conn
+      |> put_status(:internal_server_error)
+      |> json(%{error: "The requested instance does not support sharing emoji packs"})
+    end
   end
 
   @doc """

lib/pleroma/web/nodeinfo/nodeinfo_controller.ex

@@ -57,6 +57,7 @@ def raw_nodeinfo do
         "mastodon_api_streaming",
         "polls",
         "pleroma_explicit_addressing",
+        "shareable_emoji_packs",
         if Config.get([:media_proxy, :enabled]) do
           "media_proxy"
         end,

test/web/emoji_api_controller_test.exs

@@ -54,6 +54,12 @@ test "downloading shared & unshared packs from another instance via download_fro
     end)
 
     mock(fn
+      %{method: :get, url: "https://old-instance/nodeinfo/2.1.json"} ->
+        json(%{features: []})
+
+      %{method: :get, url: "https://example.com/nodeinfo/2.1.json"} ->
+        json(%{features: ["shareable_emoji_packs"]})
+
       %{
         method: :get,
         url: "https://example.com/api/pleroma/emoji/packs/list"
@@ -87,6 +93,22 @@ test "downloading shared & unshared packs from another instance via download_fro
 
     conn = build_conn() |> assign(:user, admin)
 
+    assert (conn
+            |> put_req_header("content-type", "application/json")
+            |> post(
+              emoji_api_path(
+                conn,
+                :download_from
+              ),
+              %{
+                instance_address: "https://old-instance",
+                pack_name: "test_pack",
+                as: "test_pack2"
+              }
+              |> Jason.encode!()
+            )
+            |> json_response(500))["error"] =~ "does not support"
+
     assert conn
            |> put_req_header("content-type", "application/json")
            |> post(