cowboy_rest: Allow last_modified to return undefined

1 files changed, 6 insertions, 1 deletions

diff --git a/src/cowboy_rest.erl b/src/cowboy_rest.erl
index 9f30fcf..7629b3e 100644
--- a/src/cowboy_rest.erl
+++ b/src/cowboy_rest.erl
@@ -130,7 +130,7 @@
 -optional_callbacks([languages_provided/2]).
 
 -callback last_modified(Req, State)
-	-> {calendar:datetime(), Req, State}
+	-> {calendar:datetime() | undefined, Req, State}
 	when Req::cowboy_req:req(), State::any().
 -optional_callbacks([last_modified/2]).
 
@@ -1531,6 +1531,11 @@ last_modified(Req, State=#state{last_modified=undefined}) ->
 	case unsafe_call(Req, State, last_modified) of
 		no_call ->
 			{undefined, Req, State#state{last_modified=no_call}};
+		%% We allow the callback to return 'undefined',
+		%% in which case the generated header would be missing
+		%% as if the callback was not called.
+		{undefined, Req2, State2} ->
+			{undefined, Req2, State2#state{last_modified=no_call}};
 		{LastModified, Req2, State2} ->
 			{LastModified, Req2, State2#state{last_modified=LastModified}}
 	end;