Re: [PATCH 2/4] tests: replace chainlint subshell with a function

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> To test that we don't break the &&-chain, test-lib.sh does something
> like:
>
>    (exit 117) && $test_commands
>
> and checks that the result is exit code 117. We don't care what that
> initial command is, as long as it exits with a unique code. Using "exit"
> works and is simple, but is a bit expensive since it requires a subshell
> (to avoid exiting the whole script!). This isn't usually very
> noticeable, but it can add up for scripts which have a large number of
> tests.
>
> Using "return" naively won't work here, because we'd return from the
> function eval-ing the snippet (and it wouldn't find &&-chain breakages).
> But if we further push that into its own function, it does exactly what
> we want, without extra subshell overhead.

Cute.

> According to hyperfine, this produces a measurable improvement when
> running t3070 (which has 1800 tests, all of them quite short):
>
>   'HEAD' ran
>     1.09 ± 0.01 times faster than 'HEAD~1'

That is certainly a respectable improvement.

> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  t/test-lib.sh | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/t/test-lib.sh b/t/test-lib.sh
> index 09789566374..cfcbd899c5a 100644
> --- a/t/test-lib.sh
> +++ b/t/test-lib.sh
> @@ -1086,6 +1086,10 @@ test_eval_ () {
>  	return $test_eval_ret_
>  }
>  
> +fail_117 () {
> +	return 117
> +}
> +
>  test_run_ () {
>  	test_cleanup=:
>  	expecting_failure=$2
> @@ -1097,7 +1101,7 @@ test_run_ () {
>  		trace=
>  		# 117 is magic because it is unlikely to match the exit
>  		# code of other programs
> -		if test "OK-117" != "$(test_eval_ "(exit 117) && $1${LF}${LF}echo OK-\$?" 3>&1)"
> +		if test "OK-117" != "$(test_eval_ "fail_117 && $1${LF}${LF}echo OK-\$?" 3>&1)"
>  		then
>  			BUG "broken &&-chain or run-away HERE-DOC: $1"
>  		fi