Why doesn't die in alarm signal handler kill the process? - perl

From How can I specify timeout limit for Perl system call?
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($#) {
die unless $# eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
If a timeout happens, should sub { die "alarm\n" }; cause the end of a process. I guess I am not able to understand die. This http://www.cs.cf.ac.uk/Dave/PERL/node111.html says that "The die() function is used to quit your script and display a message for the user to read". However, in the case of the script above, the script will process the code in #timed out. Also sysread continues to work. Instead of sysread, I had a perl script that slept for 30 seconds. My timeout was set to 10 seconds. As expected, the code in #timed out is executed but the script continued to sleep.Any inputs appreciated

die doesn't cause the end of a process, it throws an exception.
Now, if nothing catches an exception, that ends a process, but you have code in place to catch this very exception.
The process doesn't end because you explicitly prevent it from ending.
Since you're not very clear on what behaviour you are getting, there could be another possibility: That you are using a Windows build of Perl.
The alarm is a Unix system call. It's very purpose (sending a signal after a certain amount of time has passed) makes no sense on Windows since Windows doesn't have signals.
Perl emulates alarm to some extent, but only in a very limited manner. sleep could very well be the only operation that's interruptable by alarm. Otherwise, the timeout is only checked between statements.
So it won't interrupt sysread, but once sysread returns, Perl notices the timeout expired and emulate a signal then.

From man alarm
alarm() arranges for a SIGALRM signal to be delivered to the calling process in seconds seconds.
Before sigalarm is delivered execution reaches else block. Insert a STDIN before sysread so that sigalarm triggers resulting expected results.

"Instead of sysread, I had a perl script that slept for 30 seconds. My
timeout was set to 10 seconds. As expected, the code in #timed out is
executed but the script continued to sleep."
Really?
#!/usr/bin/perl
use strict;
use warnings FATAL => qw(all);
eval {
open my $fh, '<', $0 || die;
local $SIG{ALRM} = sub {
print STDERR "hello!\n";
die "bye!";
};
alarm 3;
while (<$fh>) {
print $_;
sleep 1;
}
close $fh;
};
if ($#) {
print "HERE: $#\n";
}
The output:
#!/usr/bin/perl
use strict;
use warnings FATAL => qw(all);
hello!
HERE: bye! at ./test.pl line 9, <$fh> line 3.
Over in the expected 3 seconds; this is still the case if I just use "sleep 100" instead of the file read. Note that if you spawn a subprocess, alarm will not kill that and the parent process must wait. In that case, the "hello!" in the signal handler will appear when alarm fires, but the eval which catches the die will not complete until the subprocess does.

I had the same issue when porting a Linux Perl script to Windows.
I solved it by ...
Creating a non-blocking socket
$recsock = IO::Socket::INET->new(
LocalPort => 68,
Proto => "udp",
Broadcast => 1,
Blocking => 0,
) or die "socket: $#";
Adding $continue variable to the timeout handle
# Timeout handle
$SIG{ALRM} = sub {
print "timeout\n";
$continue = 1;
};
and checking for the $continue to become true when the timeout occurs:
alarm($timeout);
while(1){
$recsock->recv($newmsg, 1024);
eval {
$packet = Net::Package->new($newmsg);
...
};
sleep 0.1;
last if ($continue);
}
alarm(0);

Related

Alarm is not triggering at time - perl

I have used an Alarm function in my script which is not triggering at the time which it should be :
Here is my code :
$SIG{ALRM} = sub{
print"*****Test Fail*****";
};
eval{
alarm(10);
getTheBootTime();
alarm(0);
};
die $# if $#;
getTheBootTime(); is taking 5 mints to gets execute. Am i doing anything wrong here?
Assuming that getTheBootTime() is a computation and does NOT tackle with alarm and/or sleep itself, the answer is like follows.
print "something"; w/o trailing \n may not output anything, as printed string gets stuck in the buffer until a newline is printed. This is the default behavior (unless $| is set to true or specific file descriptor has autoflush turned on).
Also the specified $SIG{ALRM} does NOT interrupt execution (no die there) which is what eval/alarm combination expects.
So the following may be in $SIG{ALRM}:
$SIG{ALRM} = sub {
print "alarm, no interruption, newline\n";
};
or
$SIG{ALRM} = sub {
die "alarm, interruption";
};

Signal handler not working with double eval

I have this code to timeout a long-running process (sleep in this case):
#!/usr/bin/env perl
use strict;
use warnings;
die "Usage: $0 SLEEP TIMEOUT\n" unless #ARGV == 2;
my ( $sleep, $timeout ) = #ARGV;
$|++;
eval {
local $SIG{ALRM} = sub { die "TIMEOUT\n" };
alarm $timeout;
eval {
# long-running process
print "Going to sleep ... ";
sleep $sleep;
print "DONE\n";
};
alarm 0; # cancel timeout
};
die $# if $#;
When I run it as ./alarm 5 2, I expect it to die saying "TIMEOUT". However it exits with 0 and says nothing. It works as expected when I remove the inner eval block (not the block's content, just the eval) though. Can someone explain why is that? Thanks.
Because you trap the error in the first eval block and the second eval block does not have an exception and clears $#.
eval {
local $SIG{ALRM} = sub { die "TIMEOUT\n" };
alarm $timeout;
eval {
# long-running process
print "Going to sleep ... ";
A: sleep $sleep;
print "DONE\n";
};
B:
alarm 0; # cancel timeout
C:};
die $# if $#;
$sleep > $timeout, so at A: your program throws a SIGALRM. The signal is caught by your local signal handler and calls die "TIMEOUT\n". So Perl sets $# to "TIMEOUT\n" and resumes execution at B:. Your program then makes it to C: without any additional errors. Since your outer eval block completed normally, Perl clears $#, and your final die statement does not execute.
To do what it seems like you want to do, you could either
don't use eval on the outer block
put another die $# if $# call at the end of the outer block

Use alarm to set a timeout for reading stdin

I have this code:
#!/usr/bin/perl
use strict;
use warnings;
my ($timeout, $size, $buffer) = (10, 10, undef);
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
my $nread = sysread STDIN, $buffer, $size;
# !!!! race condition !!!!!
alarm 0;
print "$nread: $buffer";
};
if ($#) {
warn $#;
}
Is it correct?
May be there is a race condition between 8 and 9 line?
Let's look, what's going on:
my ($timeout, $size, $buffer) = (10, 10, undef);
eval {
#establish ALRM signal handler
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
#send alarm signal to program in 10 second
alarm $timeout;
#try yo read 10 bytes of data into $buffer
my $nread = sysread STDIN, $buffer, $size;
#cancel the previous timer without starting a new one
#if we returned from sysread. Yes, if 10 seconds pass
#before the next function is executed, the script will
#die even though the data was read
alarm 0;
#print number of bytes read (will be 10) and the string,
#read from input
print "$nread: $buffer";
};
$# is set if the string to be eval-ed did not compile, or if Perl code executed during evaluation die()d. In these cases the value of $# is the compile error, or the argument to die:
if ($#) {
warn $#;
}
So, this will print die message "alarm\n" if we didn't return from sysread in 10 second.
In the very unlikely case, when the input will be received just before 10 seconds elapse and we won't be able to run alarm 0;, I suggest to use the following code:
my ($timeout, $size, $buffer) = (10, 10, undef);
#I define $nread before the signal handler as undef, so if it's defined
#it means, that sysread was executed and the value was assigned
my $nread = undef;
eval {
local $SIG{ALRM} = sub {
#if it's not defined - it means, that sysread wasn't executed
unless(defined($nread))
{
die "alarm\n";
}
};
alarm $timeout;
$nread = sysread STDIN, $buffer, $size;
alarm 0;
print "$nread: $buffer";
};
Unfortunately, it doesn't save us from the case, when assignment operator wasn't executed.
Links:
http://perldoc.perl.org/functions/alarm.html
http://perldoc.perl.org/perlvar.html
http://perldoc.perl.org/functions/sysread.html
Your usage of alarm introduces a potential race condition.
The normal solution is to add alarm 0; after your eval block, so if the first alarm 0 isn't executed, you could still close the alarm.
Or you can use Time::Out package on CPAN to wrap up your code, it's better and safer.
What OS are you running this on? What version of perl?
Works fine for me on Mac OS X 10.8.3 with perl 5.12.4.
If you're using perl on Windows, you'll find that signals don't work the same as on POSIX and POSIX-like operating systems, and you might need to use the 4-argument version of select() instead.

Time out when using system cmd in perl

I'm using PSEXEC to run remote process with system($CMD) in perl.
I have a computer (lets call it-#1) that runs system cmd, and another computer (lets call it-#2), which "receives" commands from computer #1.
Sometimes the process in the second computer (#2) gets stuck.
How can I set a timeout to the system cmd in computer #1 that will force-terminate the cmd after several minutes?
thanks for the answers, but:
i'm tring to do somthing very simple, I have 2 perl files.
file#1 that counting seconds 1 to 10. (+print to the screen)
file#2-Timeout file that cal file#1 (system command) that should terminate file #1 after 5 sec.
the results...:
timeout occurred, but process #1 still running...
file#2
$timeout=5;
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = system('F:\perl1.pl');
alarm 0;
};
if ($#) {
die unless $# eq "alarm\n"; # propagate unexpected errors
print "process terminated\n";}
else {
# didn't
}
file#1
$i=0;
while($
i<10){
sleep(1);
$i++;
print "$i\n";
}
CMD window results:
C:\>F:\perl2.pl
1
2
3
4
process terminated
C:\>5
6
7
8
9
10
Let IPC::Run handle that for you.
use IPC::Run qw( run timeout );
run \#cmd, timeout( 120 )
or die "cat: $?";
You could use alarm in your situation:
If you want to use alarm to time out a system call you need to use an
eval/die pair. You can't rely on the alarm causing the system call to
fail with $! set to EINTR because Perl sets up signal handlers to
restart system calls on some systems. Using eval/die always works,
modulo the caveats given in Signals in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($#) {
die unless $# eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}

Kill a hung child process

My Perl script runs an external program (which takes a single command-line parameter) and processes its output. Originally, I was doing this:
my #result = `prog arg`;
However, turns out that the program is buggy and hangs unpredictably in rare cases. How can I kill the program if it hasn't exited after a certain amount of time? The script has to work both in Windows and in Linux, and it is my understanding that alarms and forks don't work well (or at all) in Windows.
I found a module called IPC::Run but I can't figure out how to use it properly from its documentation. :-( I tried this:
use strict;
use warnings;
use IPC::Run qw(run timeout);
my $in;
my $out;
my $err;
my #result;
my #cmd = qw(prog arg);
run \#cmd, \$in, \$out, \$err, timeout (10) or die "#cmd: $?";
push #result, $_ while (<$out>);
close $out;
print #result;
As a test, I created a program that just sleeps 60 seconds, prints a string to stdout and exits. When I try to run it with the above code, it hangs for 60 seconds (instead of for 10 seconds, as specified in the timeout) and aborts with a bizarre error:
IPC::Run: timeout on timer #1 at C:/Bin/Unix/Perl/site/lib/IPC/Run.pm line 2956
Then I found another module, Proc::Reliable. From the description, it seems to do precisely what I want. Except that it doesn't work! I tried this:
use strict;
use warnings;
use Proc::Reliable;
my $proc = Proc::Reliable->new ();
$proc->maxtime (10);
my $out = $proc->run ("prog arg");
print "$out\n";
It indeed aborts the child process after 10 seconds. So far, so good. But then I modified the external program and made it sleep for only 5 seconds. This means that the program should finish before the 10-second timeout specified in the above code and its stdout output should be captured into the variable $out. But it isn't! The above script doesn't output anything.
Any ideas how to do it properly? (Fixing the buggy external program is not an option.) Thanks in advance.
Try the poor man's alarm
my $pid;
if ($^O eq 'MSWin32') {
$pid = system 1, "prog arg"; # Win32 only, run proc in background
} else {
$pid = fork();
if (defined($pid) && $pid == 0) {
exec("proc arg");
}
}
my $poor_mans_alarm = "sleep 1,kill(0,$pid)||exit for 1..$TIMEOUT;kill -9,$pid";
system($^X, "-e", $poor_mans_alarm);
The poor man's alarm runs in a separate process. Every second, it checks whether the process with identifier $pid is still alive. If the process isn't alive, the alarm process exits. If the process is still alive after $time seconds, it sends a kill signal to the process (I used 9 to make it untrappable and -9 to take out the whole subprocess tree, your needs may vary. kill 9,... is also portable).
Edit: How do you capture the output of the process with the poor man's alarm?
Not with backticks -- then you can't get the process id and you may lose the intermediate output if the process times out and gets killed. The alternatives are
1) send output to a file, read the file when the process is done
$pid = system 1, "proc arg > some_file";
... start poor man's alarm, wait for program to finish ...
open my $fh, '<', 'some_file';
my #process_output = <$fh>;
...
2) use Perl's open to start the process
$pid = open my $proc, '-|', 'proc arg';
if (fork() == 0) {
# run poor man's alarm in a background process
exec($^X, '-e', "sleep 1,kill 0,$pid||exit ...");
}
my #process_output = ();
while (<$proc>) {
push #process_output, $_;
}
The while loop will end when the process ends, either naturally or unnaturally.
This is the best I could do. Any ideas on how to avoid the use of a temporary file on Windows would be appreciated.
#!/usr/bin/perl
use strict;
use warnings;
use File::Temp;
use Win32::Process qw(STILL_ACTIVE NORMAL_PRIORITY_CLASS);
my $pid;
my $timeout = 10;
my $prog = "prog arg";
my #output;
if ($^O eq "MSWin32")
{
my $exitcode;
my $fh = File::Temp->new ();
my $output_file = $fh->filename;
close ($fh);
open (OLDOUT, ">&STDOUT");
open (STDOUT, ">$output_file" ) || die ("Unable to redirect STDOUT to $output_file.\n");
Win32::Process::Create ($pid, $^X, $prog, 1, NORMAL_PRIORITY_CLASS, '.') or die Win32::FormatMessage (Win32::GetLastError ());
for (1 .. $timeout)
{
$pid->GetExitCode ($exitcode);
last if ($exitcode != STILL_ACTIVE);
sleep 1;
}
$pid->GetExitCode ($exitcode);
$pid->Kill (0) or die "Cannot kill '$pid'" if ($exitcode == STILL_ACTIVE);
close (STDOUT);
open (STDOUT, ">&OLDOUT");
close (OLDOUT);
open (FILE, "<$output_file");
push #output, $_ while (<FILE>);
close (FILE);
}
else
{
$pid = open my $proc, "-|", $prog;
exec ($^X, "-e", "sleep 1, kill (0, $pid) || exit for 1..$timeout; kill -9, $pid") unless (fork ());
push #output, $_ while (<$proc>);
close ($proc);
}
print "Output:\n";
print #output;
You may want to use alarm system call as in perldoc -f alarm.