I am trying to write a simple script that spawns a thread that performs a task that may timeout. (For the sake of writing a simple example for StackOverflow I replaced the actual process with a sleep command).
This program spawns a thread and then uses a cond_timedwait to monitor the thread and check if it has timed out. If a timeout occurs it calls the kill method on the thread with a "STOP" signal to notify the thread that it should exit.
use strict;
use threads;
use threads::shared;
use warnings;
my $var :shared;
my $thread = threads->create(sub {
# Tell the thread how to handle the STOP signal
local $SIG{'STOP'} = sub {
print "Stop signal received\n";
threads->exit();
};
# Perform a process that takes some time
sleep 10;
# Signal that the thread is complete
lock($var); cond_signal($var);
});
# Current time + 1 second
my $wait_time = time() + 1;
my $timeout;
{
# Wait for the thread to complete or until a timeout has occurred
lock($var); $timeout = !cond_timedwait($var, $wait_time);
}
# Check if a timeout occurred
if ($timeout) {
print "A timeout has occurred\n";
# Signal the thread to stop
$thread->kill('STOP')->join();
}
else {
$thread->join();
}
This code runs successfully and prints the following output:
1 second passes...
A timeout has occurred
9 seconds pass...
Stop signal received
The problem is, even though a timeout is detected and the "STOP" signal is sent to the thread, the program still seems to be waiting the full 10 seconds before printing "Stop signal received" and exiting.
I tried changing it so it calls detach instead of join after killing the thread, but then the "Stop signal received" message is never printed which means the program is exiting before the thread cleanly exits. I want to make sure the thread is actually interrupted and exits, because in the real program I will want to kill and retry the process after the timeout has occurred and the process won't work if there is another instance already running on a detached thread.
How can I make it so the thread instantly prints the message and exits when it receives the "STOP" signal?
Signals can only be sent to processes. As such, $thread->kill('STOP') can't possibly be sending an actual signal. As such, nothing interrupts sleep.
Between each statement, Perl checks if a "signal" came in. If it has, it handles it. So the "signal" is only handled once sleep completes.
If you had ten one second sleeps instead of one ten second sleep, the wait would be at most one second.