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List:       busybox
Subject:    Re: ash buildin "read" interrupted on SIGCHLD
From:       Kang-Che Sung <explorer09 () gmail ! com>
Date:       2017-06-01 15:35:30
Message-ID: CADDzAfP+JFzRXtjvj2sjoQ6Z3m4Pp-jnvE7prc_Bp2rnnF3_rA () mail ! gmail ! com
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On Thu, Jun 1, 2017 at 5:36 PM, Guido Classen <clagi.x+busybox@gmail.com> wrote:
> I was curious how to catch SIGCHLD using trap and what behavior we should
> expect. Could you also provide a test for trap in conjunction with SIGCHLD.

I was about to ask the same question, until I realize that the SIGCHLD
handling behavior with respect to `trap` command is never standardized
and varies among shells.

> Here my tries with results from busybox-git with your fix:
>
> # sleep 1& read x
> => no trap signal handler installed, read waits now forever for input

This is expected behavior in all shells and is included in BusyBox testsuite.

> # trap "echo --TRAP---" CHLD; sleep 1& read x
> --TRAP---
> [1]+  Done                       sleep 1
>
> => Okay, read will be interrupted after one second
>    (this works now thanks your fixes in signal handling coder.
>     It would not work correctly in busybox 1.24 versions)

For `ash` (and `dash` also), this would interrupt `read`.
But for `hush`, (and `ksh93` according to my testing), this will run the
SIGCHLD handler, but after that the `read` command is executed again and it
waits for user input.
(The least desirable behavior I've ever seen is in `bash`, where it _blocks_
SIGCHLD altogether during the `read` command, and executes the handler
only after the `read` is finished.)

> # sleep 1& read x
> --TRAP---
> [1]+  Done                       sleep 1
>
> => Okay, trap handler still installed, same in subsequent commands

A shell's trap handler is not reset after executing the handler, unless you tell
to reset it within the handler. Required by POSIX, actually.

(http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#trap
"Traps shall remain in place for a given shell until explicitly
changed with another trap command.")

> # trap '' CHLD
> # sleep 1& read x
> [1]+  Done                       sleep 1
>
> => Is this correct? Set SIGCHLD to ignore, read still will be
>    interrupted after one second.

I'm not sure about this. Sorry.
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