Fix io:columns/0 timeout when invoked via user

This patch fixes an issue where io:columns/0 times out when invoked from
any application callback (or any supervisor/supervised module since the
group leader is inherited).

To reproduce the issue, one must simply call io:columns() from any
application callback. You will notice the process will block for 2 seconds
which then times out and returns {:error, :enotsup}.

Note this bug only happens inside the erlang shell (using -noshell or
escripts do not trigger the bug).

To fix the bug, it is important to understand how io requests flow from
application callback processes. Here are the steps followed:

1. Since io:columns/1 is timing out, the first step is to find out who is the
group leader for the application callback process. Using process_info/1, we
can see the parent process is the application_master and handles
io_requests by delegating them to the group_leader.

2. By inspecting the application_master process, we can find the group_leader
the message is sent is the registered process named user. The process is
running the group module which does handle io:columns/1 requests delegating
them to user_drv process.

3. The user_drv process does handle tty_geometry requests, except that a
clause above ends up short-circuiting all tty_geometry requests from the
user process.

This patch moves the user clause below the specific driver messages.

OTP-12241

1 files changed, 0 insertions, 5 deletions

diff --git a/lib/kernel/src/user_drv.erl b/lib/kernel/src/user_drv.erl
index 79f80f6604..e6ce85c379 100644
--- a/lib/kernel/src/user_drv.erl
+++ b/lib/kernel/src/user_drv.erl
@@ -173,11 +173,6 @@ server_loop(Iport, Oport, Curr, User, Gr, IOQueue) ->
 	{Iport,eof} ->
 	    Curr ! {self(),eof},
 	    server_loop(Iport, Oport, Curr, User, Gr, IOQueue);
-        {User,Req} when element(1,Req) == tty_geometry;
-                        element(1,Req) == get_unicode_state;
-                        element(1,Req) == set_unicode_state ->
-            %% We ignore these requests from User, could be a bug?
-            server_loop(Iport, Oport, Curr, User, Gr, IOQueue);
         Req when element(1,Req) =:= User orelse element(1,Req) =:= Curr,
                  tuple_size(Req) =:= 2 orelse tuple_size(Req) =:= 3 ->
             %% We match {User|Curr,_}|{User|Curr,_,_}