patchtests: cache glibc version check

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
> 
> 131b94a10a ("test-lib.sh: Use GLIBC_TUNABLES instead of MALLOC_CHECK_
> on glibc >= 2.34", 2022-03-04) introduced a check for the version of
> glibc that is in use. This check is performed as part of
> setup_malloc_check() which is called at least once for each test. As
> the test involves forking `getconf` and `expr` cache the result and
> use that within setup_malloc_check() to avoid forking these extra
> processes for each test.
> 
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
>      tests: cache glibc version check
>      
>      A recent discussion on the list[1] reminded me that this patch was
>      waiting to be sent

> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1311%2Fphillipwood%2Fwip%2Ftest-cache-glibc-tunables-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1311/phillipwood/wip/test-cache-glibc-tunables-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/1311
> 
>   t/test-lib.sh | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/t/test-lib.sh b/t/test-lib.sh
> index 7726d1da88a..ad81c78fce7 100644
> --- a/t/test-lib.sh
> +++ b/t/test-lib.sh
> @@ -557,14 +557,19 @@ then
>   		: nothing
>   	}
>   else
> +	_USE_GLIBC_TUNABLES=
> +	if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
> +	   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
> +	   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
> +	then
> +		_USE_GLIBC_TUNABLES=YesPlease
> +	fi
>   	setup_malloc_check () {
>   		local g
>   		local t
>   		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
>   		export MALLOC_CHECK_ MALLOC_PERTURB_
> -		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
> -		   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
> -		   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
> +		if test -n "$_USE_GLIBC_TUNABLES"
>   		then
>   			g=
>   			LD_PRELOAD="libc_malloc_debug.so.0"
> 
> base-commit: 23b219f8e3f2adfb0441e135f0a880e6124f766c

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>
> 131b94a10a ("test-lib.sh: Use GLIBC_TUNABLES instead of MALLOC_CHECK_
> on glibc >= 2.34", 2022-03-04) introduced a check for the version of
> glibc that is in use. This check is performed as part of
> setup_malloc_check() which is called at least once for each test. As
> the test involves forking `getconf` and `expr` cache the result and
> use that within setup_malloc_check() to avoid forking these extra
> processes for each test.
>
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
>     tests: cache glibc version check
>     
>     A recent discussion on the list[1] reminded me that this patch was
>     waiting to be sent.
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1311%2Fphillipwood%2Fwip%2Ftest-cache-glibc-tunables-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1311/phillipwood/wip/test-cache-glibc-tunables-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/1311
>
>  t/test-lib.sh | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/t/test-lib.sh b/t/test-lib.sh
> index 7726d1da88a..ad81c78fce7 100644
> --- a/t/test-lib.sh
> +++ b/t/test-lib.sh
> @@ -557,14 +557,19 @@ then
>  		: nothing
>  	}
>  else
> +	_USE_GLIBC_TUNABLES=
> +	if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
> +	   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
> +	   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
> +	then
> +		_USE_GLIBC_TUNABLES=YesPlease
> +	fi
>  	setup_malloc_check () {
>  		local g
>  		local t
>  		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
>  		export MALLOC_CHECK_ MALLOC_PERTURB_
> -		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
> -		   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
> -		   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
> +		if test -n "$_USE_GLIBC_TUNABLES"
>  		then
>  			g=
>  			LD_PRELOAD="libc_malloc_debug.so.0"

> Between USE_LIBC_MALLOC_DEBUG, which is the name Peff originally
> gave this intermediate variable, and the one you use here, I am
> undecided.  If the only thing the GLIBC_TUNABLES mechanism can do
> were to tweak the malloc checking, then both names are good, but
> that is not the case.  We are only seeing if we are going to use the
> malloc check feature given by glibc here, so the original name feels
> more to the point, and use of GLIBC_TUNABLE mechanism to trigger
> that malloc check feature is a mere implementation detail.
> 
> But that is minor.  Let's queue the patch to help me not to forget
> about it, and we'll amend it if necessary, as we'd probably need a
> helped-by or signed-off-by from Peff anyway before this hits 'next'.