Re: [PATCH] _AS_REEXEC_WITH_SHELL: don’t use AS_EXIT.

On 3/13/20 12:52 PM, Zack Weinberg wrote:
> If _AS_REEXEC_WITH_SHELL fails to exec the selected “better” shell
> interpreter, and that failure somehow doesn’t terminate the process,
> it calls AS_EXIT([255]).  This expands to an invocation of as_fn_exit.
> However, the definition of as_fn_exit goes into the M4SH-INIT-FN
> diversion, whereas _AS_REEXEC_WITH_SHELL goes into the M4SH-SANITIZE
> diversion, so as_fn_exit won’t be defined at the point of this use.
> We can’t move the definition of as_fn_exit earlier, because we don’t
> know that the shell supports shell functions until after we get to the
> end of the M4SH-SANITIZE diversion.
> 
> This is only a theoretical bug because, as the comments say, “all the
> known shells bail out after a failed exec.”  However, a shell that
> doesn’t bail out will instead give the user a flood of nonsensical
> error messages (starting with “as_fn_exit: not found” and then going
> on to choke on the rest of the script) so I think we should fix it
> anyway.  There shouldn’t be any problem with using a plain ‘exit’ at
> this point; no traps are active yet, and we are exiting with an
> explicit error code.
> 
> OK to commit?
> 
> zw
> 
> ---
>   lib/m4sugar/m4sh.m4 | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)

ACK

> 
> diff --git a/lib/m4sugar/m4sh.m4 b/lib/m4sugar/m4sh.m4
> index 92999f58..4af31517 100644
> --- a/lib/m4sugar/m4sh.m4
> +++ b/lib/m4sugar/m4sh.m4
> @@ -294,7 +294,10 @@ exec $1 $as_opts "$as_myself" ${1+"$[@]"}
>   # Admittedly, this is quite paranoid, since all the known shells bail
>   # out after a failed `exec'.
>   AS_ECHO(["$[]0: could not re-execute with $1"]) >&2
> -AS_EXIT([255])])# _AS_REEXEC_WITH_SHELL
> +dnl AS_EXIT cannot be used here because as_fn_exit is not yet defined;
> +dnl code inserted by AS_REQUIRE_SHELL_FN will appear _after_ this point.
> +dnl We shouldn't have to worry about any traps being active at this point.
> +exit 255])# _AS_REEXEC_WITH_SHELL
>   
>   
>   # _AS_PREPARE
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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