echo expect script is non-deterministic - expect

Question

The following expect / code pair only works sometimes:
Simple cpp echo program:
#include <iostream>
#include <string>
using namespace std;
int main()
{
string buffer;
while (1)
{
getline(cin, buffer, '\n');
if (cin.eof())
break;
cout<<buffer<<endl;
}
return 0;
}
Expect script
#!/usr/bin/expect
spawn ./echo
exp_internal 1
set timeout 1
send "a\n"
expect {
-re {^a\r\n$}
}
Success match:
spawn ./echo
send: sending "a\n" to { exp6 }
Gate keeper glob pattern for '^a\r\n$' is 'a
'. Activating booster.
expect: does "" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n"? gate=no
a
expect: does "a\r\n" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n"? gate=yes re=yes
expect: set expect_out(0,string) "a\r\n"
expect: set expect_out(spawn_id) "exp6"
expect: set expect_out(buffer) "a\r\n"
Failure match:
spawn ./echo
send: sending "a\n" to { exp6 }
Gate keeper glob pattern for '^a\r\n$' is 'a
'. Activating booster.
expect: does "" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n" gate=no
a
a
expect: does "a\r\na\r\n" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n"? gate=yes re=no
expect: timed out
I don't understand why on some runs I get the double line "a", and some runs I get the single one? I would have assumed the expect hooks up spawned process such that send will pipe everything to stdin and expect gets everything from stdout.
Any clarifications would be appreciated.

Answer 1

It is because your tty have echo enabled. In your first case you send a\n and your program read it and return a\n but expect did read before std::cout is flushed so the buffer contain only a\n. In the other case it read and send didn't finished so it read nothing and the second run it does read and your std::cout was flushed before. So you read two times. You could modify your program to output something else (like with a prefix) or add a dummy expect "<non existing string>" as last instruction so you force to drain stdout.
$ ./expect.sh
spawn ./echo
send: sending "a\n" to { exp6 }
Gate keeper glob pattern for '^a\r\n$' is 'a
'. Activating booster.
expect: does "" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n"? gate=no
a
expect: does "a\r\n" (spawn_id exp6) match regular expression "^a\r\n$"? Gate "a\r\n"? gate=yes re=yes
expect: set expect_out(0,string) "a\r\n"
expect: set expect_out(spawn_id) "exp6"
expect: set expect_out(buffer) "a\r\n"
expect: does "" (spawn_id exp6) match glob pattern "<non existing string>"? no
a
expect: does "a\r\n" (spawn_id exp6) match glob pattern "<non existing string>"? no
expect: timed out
To fix the root cause you can first disabling echo with stty -echo and then spawn with option -nottyinit used like this spawn -nottyinit ./echo because
-nottyinit skips the "sane" initialization
An alternative way is to set set stty_init -echo then spawn
https://github.com/aeruder/expect/blob/ce11b8121e6900888d137fec0b990bd1484cbb7c/pty_unicos.c#L147-L155 here is where tty is setup. stty_init is applied after tty sane. So it might be a preferred method.