This test will assume a Unix/Linux system, but later we are going to change it to work on Windows as well.

The test script

The test script saved in the t/01-simple-echo-server.t file looks like this:

use strict;
use warnings;

my $pid = fork();
die 'Could not fork' if not defined $pid;

if (not $pid) {
    close STDERR;
    exec("$^X bin/simple_echo_server.pl");
}

END {
  if ($pid) {
    kill 2, $pid;
    wait;
  }
}

sleep 1;
require Test::More;
import Test::More;
require Net::Telnet;
plan(tests => 7);
my $t = Net::Telnet->new();
ok($t->open(Host => 'localhost', Port => 8000, Timeout => 10), 'connected');

{
    my ($pre, $match) = $t->waitfor('/(?<=if you want to leave)/');
    is($pre, "Welcome to the Echo server, please type in some text and press enter. Say 'bye' if you want to leave", 'banner');
    is($match, '', 'empty');
}

{
    $t->print('hello');
    my ($pre, $match) = $t->waitfor('/You said "hello"/');
    like($pre, qr/^\s*$/, 'just newline');
    is($match, 'You said "hello"', 'reply by the server');
}
{
    $t->print('bye');
    my ($pre, $match) = $t->waitfor('/You said "bye"/');
    like($pre, qr/^\s*$/, 'just newline');
    is($match, 'You said "bye"', 'reply by the server');
}

It is placed next to the lib and bin directories we created for the Simple echo server. We can run the test using prove -l t/01-simple-echo-server.t and it will print out the following:

t/01-simple-echo-server.t .. ok
All tests successful.
Files=1, Tests=7,  1 wallclock secs ( 0.03 usr  0.01 sys +  0.12 cusr  0.01 csys =  0.17 CPU)
Result: PASS

(Actually in this case we could leave the -l flag out, but I think it is better to have it in our muscle memory.)

If we want a more verbose output we can add a -v flag and run: prove -lv t/01-simple-echo-server.t getting the following output:

t/01-simple-echo-server.t ..
1..7
ok 1 - connected
ok 2 - banner
ok 3 - empty
ok 4 - just newline
ok 5 - reply by the server
ok 6 - just newline
ok 7 - reply by the server
ok
All tests successful.
Files=1, Tests=7,  1 wallclock secs ( 0.03 usr  0.01 sys +  0.12 cusr  0.01 csys =  0.17 CPU)
Result: PASS

The first showed us a summary of the test cases collected by the Test::Harness, the second one was the raw TAP output.

Explaining the test script

We need to test a client-server application. For this we are going to launch our server from the test script in a forked process. This is useful as we then know the process id of the server and we can shut it down at the end of our test.

We also have to deal with a couple of other issues. If we have Test::More loaded in a script it will expect to have a "plan" and to have test assertions in it. In our case only the client is a real test script, the server is not. So we have to make sure we load Test::More only in the parent process, which is going to be the client. We also have to make sure that none of the output from the server will be mixed in with the output of our test script or it can confuse the Test::Harness.

Let's go over the test script.

Forking to launch the server

This code forks, and the child process runs the server. Because the server will print various things on the Standard Error channel of its console, first close STDERR in the child process. Then we call exec to run the external script. This will make sure the external script has the same process id as the child process and when the external script exits, the child process itself exits. For that reason there is no need to call exit in the child process.

my $pid = fork();
die 'Could not fork' if not defined $pid;

if (not $pid) {
    close STDERR;
    exec("$^X bin/simple_echo_server.pl");
}

Closing the server when the test ends

We want to make sure the server stops working after the test script finishes, even if the test script died an unnatural death. (e.g. by throwing an exception.) So we send kill 2 to the process id of the child process. We do it only if $pid had a value so this block will only run in the parent process. We wrap our code in and END block. This will ensure the code is executed even if something throws an exception in the test script:

END {
  if ($pid) {
    kill 2, $pid;
    wait;
  }
}

Loading Test::More late and planning late

Then we sleep a bit, just to let the server launch - this is a bit problematic. On very slow systems 1 second might not be enough for the server to launch, on fast systems this is a waste of time. Even if it is only one second. The good thing is that there is a simple solution.

After sleeping we load Test::More, using the require function. We don't call use as that would load Test::More at compile-time and thus it would be loaded even before the forking. Regardless of the physical location of the use-statement. require on the other hand will load the module during run-time. import will, well, import the various testing functions such as ok() and is() and plan().

We also load Net::Telnet in the client. We don't need to import anything from there as it is an object oriented module. Then we declare the expected number of test cases using plan(tests => 7);. In this case we need to put parentheses after the parameters of plan() just as after ok() and all the other testing functions, because they are not declared yet when perl compiles this part of the code. We would not need the parentheses if we loaded Test::More with use during compile time, but we do need the parentheses now.

sleep 1;
require Test::More;
import Test::More;
require Net::Telnet;
plan(tests => 7);

Connect to the server

This is just creating a Net::Telnet object and then opening a connection to the server. The result is going to be true if the connection succeeded.

Nothing fancy here:

my $t = Net::Telnet->new();
ok($t->open(Host => 'localhost', Port => 8000, Timeout => 10), 'connected');

Waiting for the banner

Once our connection is established, the server is supposed to send a welcome message to the client. It is sometimes called a banner. So we tell the telnet client to wait for a sign that should be the end of the banner. As our application does not have a prompt we have to rely on the expected content. So we call the waitfor method of the Net::Telnet object with a regular expression. For historical reasons this needs to be a regex inside a string. That's why we have both single-quote ' and a slash /.

The telnet client collects everything the server sends to the client in a buffer. The waitfor method watches that buffer and tries to match the regex. Once the regex is matched the waitfor method returns two values. (At least when it is called in list context as we called it.) The second value is the actual match. (What would be in $& normally. The first value is the content of the buffer up to the beginning of the match. In this case we used (?<= ... ) around the regex. This is a fixed-width look-behind. It means we are looking for something that fits the regex between the parentheses, but when it matches it won't be part of $&. This means we are expecting $& and thus the $match variable to be empty and the $pre variable to contain the whole banner. Even the part that was in our regex.

Once we have $pre and $match we can use the regular testing functions is() to compare them to the expected values.

{
    my ($pre, $match) = $t->waitfor('/(?<=if you want to leave)/');
    is($pre, "Welcome to the Echo server, please type in some text and press enter. Say 'bye' if you want to leave", 'banner');
    is($match, '', 'empty');
}

One more thing about this code. We put the 3 lines in a pair of curly braces because in the next few tests we will want to use the same variables ($pre and $match) and we want to avoid accidentally checking an older version of these values in a later test case. Putting them in a block restricts their scope.

Testing the echo

The rest of the test script is the main part of the testing, but we have covered almost all the aspects of the test script already. One thing remains. In both of the cases below, first we call the print method of Net::Telnet. This simply sends its parameter to the server.

Then we go back our previous ritual, waiting for specific responses from the server.

{
    $t->print('hello');
    my ($pre, $match) = $t->waitfor('/You said "hello"/');
    like($pre, qr/^\s*$/, 'just newline');
    is($match, 'You said "hello"', 'reply by the server');
}
{
    $t->print('bye');
    my ($pre, $match) = $t->waitfor('/You said "bye"/');
    like($pre, qr/^\s*$/, 'just newline');
    is($match, 'You said "bye"', 'reply by the server');
}