[ruby-core:98159] [Ruby master Bug#16835] SIGCHLD + system new behavior on 2.6

Issue #16835 has been updated by maths22 (Jacob Burroughs).


Also I have tested and the old behavior (not calling the CLD signal handler) does appear to match the glibc behavior

``` c
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

void proc_exit()
{
  printf ("Handler\n");
}
int main ()
{
  signal(SIGCHLD, proc_exit);
  system("true");
  sleep(1);
}
```


----------------------------------------
Bug #16835: SIGCHLD + system new behavior on 2.6
https://bugs.ruby-lang.org/issues/16835#change-85398

* Author: maths22 (Jacob Burroughs)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin19]`
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
In rubies < 2.5, the `system` command did not trigger sigchld signal traps.  On ruby >= 2.6 it does.  This appears to have been introduced by https://github.com/ruby/ruby/commit/054a412d540e7ed2de63d68da753f585ea6616c3 .  To observe the change in behavior, run the following code on ruby 2.5 and 2.6:
``` ruby
Signal.trap("CLD")  { puts "Child died" }; system("true")
```
On ruby 2.5 it won't print anything.  On ruby 2.6 it will print "Child died".  I believe this is an unintended, undocumented change in behavior, but I may be wrong about that.





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