debian/debhelper.in/libc.preinst: skip kernel check in chrootless mode

If glibc is installed in chrootless mode (i.e. $DPKG_ROOT is not empty), then checking the currently running kernel is not useful. At the point where the chroot is created on system X, the kernel running on X can be wildly different from the system Y that the chroot is supposed to run on.

This was discovered when creating hurd chroots on linux. The tools creating the chroot have no information about the kernel which might run on the system that the chroot is built for.

This commits adds an additional condition to the preinst block doing the kernel checks. It is only executed if $DPKG_ROOT is empty, in other words, only if this is a normal installation.

Closes: #1063624

debian/debhelper.in/libc.preinst

@@ -11,7 +11,7 @@ kernel_compare_versions () {
     test $verA -$2 $verB
 }
 
-if [ "$type" != abort-upgrade ] && [ -z "$DPKG_ROOT" ]
+if [ "$type" != abort-upgrade ]
 then
     # Load debconf module if available and usable
     if [ -f /usr/share/debconf/confmodule ] && \
@@ -25,7 +25,10 @@ then
     # Only change LC_ALL after loading debconf to ensure the debconf templates
     # are properly localized.
     export LC_ALL=C
+fi
 
+if [ "$type" != abort-upgrade ] && [ -z "$DPKG_ROOT" ]
+then
     # See if LD_LIBRARY_PATH contains the traditional /lib, but not the
     # multiarch path
     dirs=$(echo $LD_LIBRARY_PATH | sed 's/:/ /g')