external command, progress bar and exit value

[svenXY]

Hi,

I know that questions about progress bars have been answered here before, but they did not seem to address my special case here (forgive me if I'm wrong). Anyway, I tried to roll my own solution that works partly, but I ran into some trouble.

Here's what I try to achieve:

run an external command (e.c.) and dispaly some kind of "notification" to the user (that - yes - has nothing to do with how "far" the e.c. actually is)

capture output AND exit value from the e.c.

the current design is as follows

the sub forks.

the child runs the command via open-pipe

the parent outputs some kind of progress bar

child communicates output and exit value via pipes to the parent

parent outputs the retrieved stuff after the child has exited

My code works fine for some commands, but - for whatever reason - not for all of them.

($result, $exitcode) = run_w_progress(1, 'for i in `seq -w 4`; do slee
+p 3; /bin/echo $i; done');

returns

Result:
1
2
3
4

Exitcode:0
finishing ...

whereas

($result, $exitcode) = run_w_progress(1, '/bin/echo -e "foo\nbar"');

only returns (note the missing 'bar')

Result:
foo

Exitcode:0
finishing ...

Here's my code (fully working example):

#!/usr/bin/perl                                  
 
use strict;
use warnings;
use POSIX ":sys_wait_h";
use IO::Select;         
use IO::Handle;         

my ($result, $exitcode) = run_w_progress(1, 'for i in `seq -w 4`; do s
+leep 3; /bin/echo $i; done');
print "Result:\n",$result,"\nExitcode:",$exitcode,"\n";               
+                             

($result, $exitcode) = run_w_progress(1, '/bin/echo -e "foo\nbar"');
print "Result:\n",$result,"\nExitcode:",$exitcode,"\n";             

sub run_w_progress {
        my ($sleep, $cmd) = @_;
        my($reader, $writer);  
        pipe $reader, $writer;
        my($reader2, $writer2);
        pipe $reader2, $writer2;
        $writer->autoflush(1);
        $writer2->autoflush(1);
        my @watch = qw(| / - + \ | / - \ |);
        my $pos = 0;
        $|++;
        if ( fork()) {
                my $kid;
                my $output = '';
                my $exitval;
                my $c=0;
                my $handles = IO::Select->new();
                $handles->add($reader, $reader2);
                do {
                        sleep $sleep;
                        my @fhs;
                        while (@fhs = $handles->can_read(0)) {
                                for my $fh (@fhs){
                                        if (fileno($fh) == fileno($rea
+der)) {
                                                my $line = scalar <$re
+ader>;
                                                        $output .= $li
+ne;
                                        }
                                        else {
                                                        $exitval = sca
+lar <$reader2>;
                                                        chomp $exitval
+;
                                        }

                                }
                        }
                        print "\r",'|'x$pos,$watch[$c++];
                        if ($c == 10) {
                                $c=0;
                                $pos++;
                        }
                        $kid = waitpid(-1, WNOHANG);
                } until $kid > 0;
                print "|\n";
                return ($output, $exitval);
        }
        else {
                open(my $pipe, '-|', $cmd);
                while (<$pipe>) {
                        # pass output line to parent
                        print $writer $_;
                }
                close $pipe;
                close $writer;
                # pass exit value to parent 
                print $writer2 $?>>8, "\n";
                close $writer2;
                exit 0;
        }
}

print "finishing ...\n";

Any hints greatly appreciated!
svenXY

Todo: Make this also work with STDERR and such...

[almut]

The problem is that whenever can_read() says there's something to read, perl will (under the hood) read everything into an internal buffer, from which you then get one line at a time via scalar <$reader>. This only matters if there's more than one line being written faster than you're reading on the other end — i.e. in your last case, where you're writing "foo\nbar\n" without a delay in between.

You could have discovered this using strace:

$ strace -etrace=select,read ./754685.pl >/dev/null
...
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "1\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "2\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "3\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
--- SIGCHLD (Child exited) @ 0 (0) ---
select(16, [6 9], NULL, NULL, {0, 0})   = 2 (in [6 9], left {0, 0})
read(6, "4\n", 4096)                    = 2
read(9, "0\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
--- SIGCHLD (Child exited) @ 0 (0) ---
select(16, [6 9], NULL, NULL, {0, 0})   = 2 (in [6 9], left {0, 0})
read(6, "foo\nbar\n", 4096)             = 8                         <-
+-- !!
read(9, "0\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)

This works fine as long as there's more to read (in which case your loop would take care of reading everything from the internal buffer line by line), but if there's nothing more to read (after the last block (e.g. "foo\nbar\n") has been read under the hood, further select()s will just time out) you'll only read the first line of the block...

One workaround would be to use non-blocking read()s:

        pipe $reader, $writer;
        $reader->blocking(0);
        ...

            if (fileno($fh) == fileno($reader)) {
                read $reader, my $line, 1e5;
                $output .= $line;
            }
            ...

[svenXY]

many thanks, almut and a warm ++! That completely solved my problem!

I had realized that scalar <$fh> was the problem and also had tried to slurp it instead (to get "all" lines at once), but then it blocked. That's when I started composing this node.

Just out of curiosity - how would you have implemented this? Similarly? Or is what I did very unusual?


Regards,
svenXY