# open_ex: ['/usr/bin/ssh','-S','/var/lib/wwwrun/.libnet-openssh-perl/
+root-192.168.100.8-20273-555951','-l','root','192.168.100.8','--']
Pseudo-terminal will not be allocated because stdin is not a terminal.
# _waitpid(20296) => pid: 20296, rc:
# open_ex: ['/usr/bin/ssh','-O','check','-T','-S','/var/lib/wwwrun/.li
+bnet-openssh-perl/root-192.168.100.8-20273-373748','-l','root','192.1
+68.100.8','--']
# waiting for slave, timeout: 5, remaining: 5, sleep: 1
# _waitpid(20299) => pid: 20299, rc: Interrupted system call
# open_ex: ['/usr/bin/ssh','-S','/var/lib/wwwrun/.libnet-openssh-perl/
+root-192.168.100.8-20273-373748','-l','root','192.168.100.8','--']
Pseudo-terminal will not be allocated because stdin is not a terminal.
# _waitpid(20300) => pid: 20300, rc:
# open_ex: ['/usr/bin/ssh','-O','exit','-T','-S','/var/lib/wwwrun/.lib
+net-openssh-perl/root-192.168.100.8-20273-373748','-l','root','192.16
+8.100.8','--']
# waiting for slave, timeout: 5, remaining: 5, sleep: 1
# _waitpid(20301) => pid: 20301, rc: Interrupted system call
# open_ex: ['/usr/bin/ssh','-O','exit','-T','-S','/var/lib/wwwrun/.lib
+net-openssh-perl/root-192.168.100.8-20273-555951','-l','root','192.16
+8.100.8','--']
# waiting for slave, timeout: 5, remaining: 5, sleep: 1
# _waitpid(20302) => pid: 20302, rc: Interrupted system call
[download]

Looks like your system isn't automatically resuming interrupted system calls.¹

You could try modifying the module's code in order to do it manually.  Resuming a system call manually means to simply call it again if $! is EINTR or ERESTART.  In this particular case, look for line 755 in OpenSSH.pm

    $r = waitpid($pid, WNOHANG) and last;
[download]

and modify it to read

    do {
        $r = waitpid($pid, WNOHANG);
    } while $r<0 and ($! == Errno::EINTR or $! == Errno::ERESTART);
    last if $r;
[download]

This should take care of this particular waitpid call which produced the message in the debug log.  There are more, though, which would likely need a similar treatment, so it's probably best to get in touch with the module's author... (he frequents this place regularly, so he'll probably see the thread sooner or later, anyway).

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¹ System calls may be interrupted for a variety of reasons. The more loaded a system, the more often it happens.  Most systems these days automatically resume them, so code isn't always written to handle the rare case where you have to do it manually...

The return value of waitpid is positive so the value of $! is useless. The EINTR probably cames from the previous select call on the loop that is interrupted by the SIGCHLD arriving.

IMO, the way to solve this problem involves strace or some similar utility.