Fix Websocket compression for OTP 20.1.3+

This issue only concerns zlib contexts that are created in
a different process from the one they are used in.

Unfortunately compression will have to remain disabled if you
are using OTP 20.1, 20.1.1 or 20.1.2 due to missing functionality.
If you need compression you will need to use another version.

src/cow_ws.erl

@@ -151,12 +151,27 @@ init_permessage_deflate(InflateWindowBits, DeflateWindowBits, Opts) ->
 		maps:get(mem_level, Opts, 8),
 		maps:get(strategy, Opts, default)),
 	%% Set the owner pid of the zlib contexts if requested.
+	%%
+	%% The zlib port became a reference in OTP 20.1+. There
+	%% was however no way to change the controlling process
+	%% until the OTP 20.1.3 patch version. Since we can't
+	%% enable compression for 20.1, 20.1.1 and 20.1.2 we
+	%% explicitly crash. The caller should ignore this extension.
 	_ = case Opts of
-		#{owner := Pid} ->
+		#{owner := Pid} when is_port(Inflate) ->
 			true = erlang:port_connect(Inflate, Pid),
 			true = unlink(Inflate),
 			true = erlang:port_connect(Deflate, Pid),
 			unlink(Deflate);
+		#{owner := Pid} ->
+			case erlang:function_exported(zlib, set_controlling_process, 2) of
+				true ->
+					zlib:set_controlling_process(Inflate, Pid),
+					zlib:set_controlling_process(Deflate, Pid);
+				false ->
+					exit({error, incompatible_zlib_version,
+						'OTP 20.1, 20.1.1 and 20.1.2 are missing required functionality.'})
+			end;
 		_ ->
 			true
 	end,