tests: cache glibc version check

131b94a ("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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phillipwood

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"