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Forking and shelling out to curl

by Argel (Prior)
on Mar 26, 2019 at 01:50 UTC ( #1231678=perlquestion: print w/replies, xml ) Need Help??

Argel has asked for the wisdom of the Perl Monks concerning the following question:

Hello fellow Monks! I'm trying to write a daemon that runs a curl command.
use IO::Socket; use POSIX qw(WNOHANG setsid); sub daemonize { $SIG{CHLD} = 'IGNORE'; # Configure to autoreap zombies die "Can't fork" unless defined ( my $child = fork ); # FORK +<<<<<<<<<<<< CORE::exit(0) if $child; # Parent exits setsid(); # Become session leader open( STDIN, "</dev/null" ); # Detach STDIN from shell open( STDOUT, ">/dev/null" ); # Detach STDOUT from shell open( STDERR, ">&STDOUT" ); # Detach STDERR from shell chdir '/tmp'; # Change working directory umask(0); # Reset umask $ENV{PATH} = '/bin:/sbin:/usr/sbin'; # Reset PATH }

Looks like my script is hanging when I daemonize it. I'm logging to a CSV file, and I finally added some code in tracing this back to where it is hanging, and it looks like it's when I shell out.

sub cmd_wrapper { my $cmd = shift; my @out; eval { local $SIG{ALRM} = sub { die "alarm\n" }; alarm 15; ## (CSV output logged) ## @out = qx[$cmd]; #<<<<< Shell out ## (CSV output *NOT* logged) ## alarm 0; }; return $@ if $@ ne "" return wantarray ? @out : join '', @out; }
What's even stranger is if I uncomment my $? == -1 check in the subroutine that calls cmd_wrapper I see log entries indicating it failed (though I appear to be getting valid results back; guessing $? doesn't work well when forking). But I'm at loss to explain how code appearing after I shell out affects the shelling out behavior.
## Run the Curl command ## my $restURL = $opt_ref->{restURL} . $opt_ref->{pagingURL}; say "DEBUG: \$restURL: \]$restURL\[" if $cfg{debug} or $cfg{test}; my $cmd = "$curlCommand -X $opt_ref->{method} $restURL"; my $results = cmd_wrapper( $cmd, $opt_ref, $info_ref, $cfh{csv_exp +ort}, \@rpt_hdr, 1 ); say 'DEBUG: $results: ', Dumper $results if $cfg{debug}; #or $cfg{ +test} > 1; $info_ref->{name} = $?; $info_ref->{primary} = $cmd; $info_ref->{status_msg} = $results; #=pod # Uncommented I see results, Commented out cmd_wrapper seems to hang. ## CASE: Connection issue if( $? == -1 ) { # WARN: Will this still work now that I'm call +ing cmd_wrapper()??? $info_ref->{entry_level} = 'DEBUG'; $info_ref->{status} = 'runCurl connection failed'; csv_log( $opt_ref, $info_ref, $cfh{csv_export}, \@rpt_hdr, 1 ) +; return; #<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< +RETURN } #=cut

Am I missing something simple? Something with buffering? Or hitting an optimization problem? Something to do with that return? Or....?

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-li +nux Red Hat Enterprise Linux Server release 5.11 (Tikanga)

Elda Taluta; Sarks Sark; Ark Arks
My deviantART gallery

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Re: Forking and shelling out to curl
by ikegami (Pope) on Mar 26, 2019 at 05:07 UTC

    Why not access libcurl via Net::Curl instead of via curl?

      Thanks! We have a bug that we're trying to monitor for, and I'm trying to mash a modified perl+curl script I inherited with an monitoring script I wrote over a decade ago. So ugly-but-working is the bar I'm trying to hit. I'll take a look at Net::Curl -- my call to curl is in one sub, so I may be able to drop this in without too much pain. Ill update you on how it goes!

      Elda Taluta; Sarks Sark; Ark Arks
      My deviantART gallery

        Looks like our libcurl is too old (3.x). I started looking into LWP::UserAgent, but the SSL headers are not on the server, which I assume rules out just about every other https module. :( :/

        So, I returned back to figuring out why shelling out is causing problems, and it looks like a large part of my problem was a sleep 300 that was supposed to be a sleep 3 for testing and the rest was my $? check, which doesn't work as expected when forking (or maybe it's tied to signal handling). The downsides of not having a second pair of eyes to spotcheck the code.

        Thanks for the help though! Trying to get Net::Curl and then LWP::USerAgent working let me return back to the code with a fresh set of eyes.

        EDIT: For some reason, nothing gets logged the first run through the main while(1), so the sleep makes it look like the script is hanging, so I ended up chasing the wrong issue.

        Elda Taluta; Sarks Sark; Ark Arks
        My deviantART gallery

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