io:getline(Prompt) in spawned process: Prompt does not display - erlang

Question

Here's the code:
-module(my).
-compile(export_all).
test() ->
register(go, spawn(my, init, []) ).
init() ->
Reply = io:get_line("enter:"),
io:format("Reply= ~s~n", [Reply]).
In the shell:
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:test().
true
3>
If I add a timer:sleep() here:
test() ->
register(go, spawn(my, init, []) ).
init() ->
timer:sleep(1000), %%<<<==========HERE
Reply = io:get_line("enter:"),
io:format("Reply= ~s~n", [Reply]).
and run the new code in the shell, then after 1 second the shell prompt 3> in the first example's output suddenly disappears and is replaced by enter::
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:test().
true
enter:
And if I enter something, then as expected I see Reply= xx.
If I move the timer:sleep() to here:
test() ->
register(go, spawn(my, init, []) ),
timer:sleep(1000). %%<<======HERE
init() ->
Reply = io:get_line("enter:"),
io:format("Reply= ~s~n", [Reply]).
and I run the code in the shell, I see:
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:test().
enter:
then enter: suddently disappears and is replaced by ok:
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:test().
ok
3>
This does work as expected:
1> Reply = io:get_line("enter: ").
enter: hello
"hello\n"
2> Reply.
"hello\n"
3>
Can someone explain what race condition is occurring in the first example?

Answer 1

Actually this is not exactly race condition. The issue is raised from fact that we have only one std_in stream and only one process in given time can use it.
1) In first code snippet process 'go' takes control on std_in with get_line/1 function first. But after that test/0 function returns to Erlang shell and shell takes control under std_in (Erlang shell is waiting for user input with prompt like ">"). So process 'go' now unlinked from std_in and does not receive any chars from std_in. All input goes to Erlang shell.
2) Second snippet has sleep/1 function in init/1. So situation is opposite. We allow Erlang shell reach waiting for input status and AFTER that execute get_line/1 in 'go' process. So now 'go' process takes control on std_in and receives chars from keyboard. But after 'go' process terminates then Erlang shell restores its state.
3) Third snippet is just 1st but aggravate situation with sleep/1 function.
4) Nothing explain for forth snippet. It works as expected.
5) To illustrate this point of view you can run code below:
-module(my).
-compile(export_all).
test() ->
spawn_link(my, init1, []),
spawn_link(my, init2, []).
init1() ->
timer:sleep(100),
init(0, "0").
init2() ->
timer:sleep(100),
init(0, "1").
init(N, Title) ->
% io:format(">>init(~p)~n", [N]),
Reply = io:get_line(Title ++ "> enter:"),
io:format("[~s] (e - exit) Reply= ~s", [Title, Reply]),
case Reply of
"e\n" -> ok;
_ -> init(N+1, Title)
end.
The module starts two processes with get_line/1. But only one process can except input from std_in in recent moment:
50> my:test().
<0.192.0>
1> enter:w
[1] (e - exit) Reply= w
1> enter:d
[1] (e - exit) Reply= d
1> enter:e
[1] (e - exit) Reply= e
0> enter:dd
[0] (e - exit) Reply= dd
0> enter:e
[0] (e - exit) Reply= e
51>