user: do not allow refollowing somebody who has blocked a user

lib/pleroma/user.ex

@@ -170,9 +170,14 @@ def register_changeset(struct, params \\ %{}) do
   def follow(%User{} = follower, %User{info: info} = followed) do
     ap_followers = followed.follower_address
 
-    if following?(follower, followed) or info["deactivated"] do
+    cond do
+      following?(follower, followed) or info["deactivated"] ->
         {:error, "Could not follow user: #{followed.nickname} is already on your list."}
-    else
+
+      blocks?(followed, follower) ->
+        {:error, "Could not follow user: #{followed.nickname} blocked you."}
+
+      true ->
         if !followed.local && follower.local && !ap_enabled?(followed) do
           Websub.subscribe(follower, followed)
         end

test/user_test.exs

@@ -46,6 +46,15 @@ test "can't follow a deactivated users" do
     {:error, _} = User.follow(user, followed)
   end
 
+  test "can't follow a user who blocked us" do
+    blocker = insert(:user)
+    blockee = insert(:user)
+
+    {:ok, blocker} = User.block(blocker, blockee)
+
+    {:error, _} = User.follow(blockee, blocker)
+  end
+
   # This is a somewhat useless test.
   # test "following a remote user will ensure a websub subscription is present" do
   #   user = insert(:user)