Expose expires_at datetime in mastoAPI only for the activity actor

In the "pleroma" section of the MastoAPI for status activities you can
see an expires_at item that states when the activity will expire, or
nothing if the activity will not expire.

The expires_at date is only visible to the person who posted the
activity. This is the conservative approach in case some attacker
decides to write a logger for expiring posts. However, in the future of
OCAP, signed requests, and all that stuff, this attack might not be that
likely. Some other pleroma dev should remove the restriction in the code
at that time, if they're satisfied with the security implications of
doing so.

docs/api/differences_in_mastoapi_responses.md

@@ -25,6 +25,7 @@ Has these additional fields under the `pleroma` object:
 - `in_reply_to_account_acct`: the `acct` property of User entity for replied user (if any)
 - `content`: a map consisting of alternate representations of the `content` property with the key being it's mimetype. Currently the only alternate representation supported is `text/plain`
 - `spoiler_text`: a map consisting of alternate representations of the `spoiler_text` property with the key being it's mimetype. Currently the only alternate representation supported is `text/plain`
+- `expires_on`: a datetime (iso8601) that states when the post will expire (be deleted automatically), or empty if the post won't expire
 
 ## Attachments
 

lib/pleroma/web/mastodon_api/views/status_view.ex

@@ -6,6 +6,7 @@ defmodule Pleroma.Web.MastodonAPI.StatusView do
   use Pleroma.Web, :view
 
   alias Pleroma.Activity
+  alias Pleroma.ActivityExpiration
   alias Pleroma.HTML
   alias Pleroma.Object
   alias Pleroma.Repo
@@ -165,6 +166,15 @@ def render("status.json", %{activity: %{data: %{"object" => _object}} = activity
 
     bookmarked = Activity.get_bookmark(activity, opts[:for]) != nil
 
+    client_posted_this_activity = opts[:for] && user.id == opts[:for].id
+
+    expires_at =
+      with true <- client_posted_this_activity,
+           expiration when not is_nil(expiration) <-
+             ActivityExpiration.get_by_activity_id(activity.id) do
+        expiration.scheduled_at
+      end
+
     thread_muted? =
       case activity.thread_muted? do
         thread_muted? when is_boolean(thread_muted?) -> thread_muted?
@@ -262,7 +272,8 @@ def render("status.json", %{activity: %{data: %{"object" => _object}} = activity
         conversation_id: get_context_id(activity),
         in_reply_to_account_acct: reply_to_user && reply_to_user.nickname,
         content: %{"text/plain" => content_plaintext},
-        spoiler_text: %{"text/plain" => summary_plaintext}
+        spoiler_text: %{"text/plain" => summary_plaintext},
+        expires_at: expires_at
       }
     }
   end

test/web/mastodon_api/mastodon_api_controller_test.exs

@@ -166,10 +166,11 @@ test "posting a status", %{conn: conn} do
           "expires_at" => expires_at
         })
 
-      assert %{"id" => fourth_id} = json_response(conn_four, 200)
+      assert fourth_response = %{"id" => fourth_id} = json_response(conn_four, 200)
       assert activity = Activity.get_by_id(fourth_id)
       assert expiration = ActivityExpiration.get_by_activity_id(fourth_id)
       assert expiration.scheduled_at == expires_at
+      assert fourth_response["pleroma"]["expires_at"] == NaiveDateTime.to_iso8601(expires_at)
     end
 
     test "replying to a status", %{conn: conn} do

test/web/mastodon_api/status_view_test.exs

@@ -133,7 +133,8 @@ test "a note activity" do
         conversation_id: convo_id,
         in_reply_to_account_acct: nil,
         content: %{"text/plain" => HtmlSanitizeEx.strip_tags(object_data["content"])},
-        spoiler_text: %{"text/plain" => HtmlSanitizeEx.strip_tags(object_data["summary"])}
+        spoiler_text: %{"text/plain" => HtmlSanitizeEx.strip_tags(object_data["summary"])},
+        expires_at: nil
       }
     }