[collectd] collectd leaks SIGCHLD == SIG_IGN into plugins

[Richard W.M. Jones]

Peter Dimitrov and myself were debugging a very peculiar bug when
libguestfs is run as a plugin from collectd:

  https://www.redhat.com/archives/libguestfs/2018-November/thread.html#00023

The long story short is that collectd leaks SIGCHLD == SIG_IGN setting
into plugins:

  https://www.redhat.com/archives/libguestfs/2018-November/msg00095.html

This means that any plugin that does the usual pattern of:

  pid = fork ();
  ...
  if (waitpid (pid, NULL, 0) == -1) {
    perror ("waitpid");
    exit (EXIT_FAILURE);
  }

will fail, because the forked subprocess is automatically reaped
before waitpid is called, resulting in the wait failing with errno ==
ECHILD.

It is possible to work around this by adding:

  signal (SIGCHLD, SIG_DFL);

to the plugin.  However I believe this is a bug in collectd, and it
should sanitize signals (and maybe other things) before running
plugins.

Rich.

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