Error message "Signal SIGCHLD received, but no signal handler set."

Question

I am trying to debug a complex Perl application that terminates with the error message "Signal SIGCHLD received, but no signal handler set". I know that it comes from the Perl interpreter itself, notably from the file mg.c and it cannot be caught. But I don't understand the exact nature of it.

Is this an internal error in the interpreter of the kind "should not happen"?

Or is there a (simple) way to reproduce this error with recent Perl versions?

I have already tried to reproduce it with hints given in https://lists.gnu.org/archive/html/bug-parallel/2016-10/msg00000.html by setting and unsetting a signal handler in an endless loop and constantly firing that signal in and endless loop from another script. But I could not reproduce the behavior described there with Perl versions 5.18.4, 5.26.0, 5.26.2, 5.28.2, and 5.30.2.

I have also found Signal SIGSTOP received, but no signal handler set in perl script where somebody assigned to $SIG{SIGSTOP} instead of $SIG{STOP} but that also does not help to make the problem reproducible with a simple script.

All of the perls I have tested were built without thread support:

$ perl -Mthreads
This Perl not built to support threads
Compilation failed in require.
BEGIN failed--compilation aborted.

Answer 1

I am answering my own question here (to the best of my knowledge to date):

The error vanished by inserting these two lines:

$SIG{CHLD} ||= 'DEFAULT';
$SIG{HUP} ||= 'DEFAULT';

I would not call this a fix but rather a workaround because the value of "DEFAULT" should trigger the exact same behavior as no value resp. undef.

The error message is an internal error of Perl. The interpreter bails out here as a guard against signal handling bugs in Perl.

That being said, there is also no simple example that reproduces the error. And if there was one, it would be a bug in Perl.

A similar error was reported for GNU parallel some time ago: https://lists.gnu.org/archive/html/bug-parallel/2016-10/msg00000.html

The error reported there has a couple of things in common with the error that I encountered, notably that it occurred after fork()ing.

My application is a server based on Net::Server, and the error occurs, when a request handler is spawning a child process. Interestingly, the error message (and exit) happens before the child terminates.

The child process can potentially run for a very long time. Therefore it is made a session leader with setsid(), all open file descriptors are closed, and standard input is redirected to /dev/null, before exec() is being called. In other words, it is kind of daemonized.

It should also be noted that the error vanishes, when small modifications to the code are done, for example dumping the contents of %SIG for debugging purposes.

The error also did not occur, with Perl versions 5.8.9, 5.14.4, and 5.16.3. With 5.18.4, 5.26.2, and 5.30.2 it can always be reproduced. All of these executables had been built without interpreter thread support.

Answer 2

The "but no signal handler set" message is particular to threads, and signal handling works differently inside threads than it does in your main process.

The main process can receive a signal from any number of places -- a kill call in the program, a kill call from another Perl program, from the operating system, from a /usr/bin/kill command from the command line, etc. Perl has "default" signal handlers for all of the signals that can be sent from your system. You can trap some of them by setting up handlers in the global %SIG variable, though some signals (notable SIGKILL and SIGSTOP) cannot be trapped by your program.

Signals within threads are an emulation of signals in your parent operating system. They can only be sent by a call to threads::kill, which means they can only be signalled from within your Perl program. Perl does not setup any default signal handlers for thread signals, and warns you when an unhandled signal is given to a thread. And unlike the regular signal set, you may trip KILL and STOP signals to threads.

To be defensive, set a signal handler for all signals. Maybe something like:

use Carp;

sub thr_signal_handler {
    Carp::cluck("Received SIG$_[0] in ",threads->tid(),"\n";
}

# inside thread initialization function
$SIG{$_} = \&thr_signal_handler for keys %SIG;
...