Remove yum chroot caching

Every run we are doing a full tar.gz of the chroot environment that
never gets used.

It's not suitable for CI since we use fresh images each time there.

The cache in general isn't really isn't a very safe thing to have
around, because there's no invalidation procedure and no real way to
make one -- we've no guarantee that a new chroot build even moments
after a previous one wouldn't bring in or different packages, etc (of
course this is *unlikely*, but the longer you go between builds the
worse the problem becomes.  Also, tons of packages get installed after
this not from any cache, so potential speed-up is rather marginal.

Debian turned this off with I58fc485aacacaa17243bf9ce760ed91256d1f182.
However, given the reasons above and it's complete lack of testing, I
don't see this as useful.

If we really want this type of thing, I think we should come up with a
way to use a persistent external yum/dnf cache that yum/dnf keeps in
sync with it's usual invalidation rules.

Change-Id: I66789c35db75c41bc45ea1ad2e26f87456de4e4d

elements/centos-minimal/README.rst

@@ -6,13 +6,9 @@ Create a minimal image based on CentOS 7.
 Use of this element will require 'yum' and 'yum-utils' to be installed on
 Ubuntu and Debian. Nothing additional is needed on Fedora or CentOS.
 
-The `DIB_OFFLINE` or more specific `DIB_YUMCHROOT_USE_CACHE`
+By default, ``DIB_YUM_MINIMAL_CREATE_INTERFACES`` is set to enable the
-variables can be set to prefer the use of a pre-cached root filesystem
+creation of ``/etc/sysconfig/network-scripts/ifcfg-eth[0|1]`` scripts to
-tarball.
+enable DHCP on the ``eth0`` & ``eth1`` interfaces.  If you do not have
-
-By default, `DIB_YUM_MINIMAL_CREATE_INTERFACES` is set to enable the
-creation of `/etc/sysconfig/network-scripts/ifcfg-eth[0|1]` scripts to
-enable DHCP on the `eth0` & `eth1` interfaces.  If you do not have
 these interfaces, or if you are using something else to setup the
 network such as cloud-init, glean or network-manager, you would want
-to set this to `0`.
+to set this to ``0``.

elements/fedora-minimal/README.rst

@@ -11,10 +11,6 @@ Due to a bug in the released version of urlgrabber, on many systems an
 installation of urlgrabber from git is required. The git repository
 can be found here: http://yum.baseurl.org/gitweb?p=urlgrabber.git;a=summary
 
-The `DIB_OFFLINE` or more specific `DIB_YUMCHROOT_USE_CACHE`
+This element sets the ``DIB_RELEASE`` var to 'fedora'. The release of
-variables can be set to prefer the use of a pre-cached root filesystem
+fedora to be installed can be controlled through the ``DIB_RELEASE``
-tarball.
+variable, which defaults the latest supported release.
-
-This element sets the `DIB_RELEASE` var to 'fedora'. The release of fedora
-to be installed can be controlled through the `DIB_RELEASE` variable, which
-defaults to '21'.

elements/yum-minimal/README.rst

@@ -9,10 +9,6 @@ or fedora-minimal elements to get an actual base image.
 Use of this element will require 'yum' and 'yum-utils' to be installed on
 Ubuntu and Debian. Nothing additional is needed on Fedora or CentOS.
 
-The `DIB_OFFLINE` or more specific `DIB_YUMCHROOT_USE_CACHE`
-variables can be set to prefer the use of a pre-cached root filesystem
-tarball.
-
 If you wish to have DHCP networking setup for eth0 & eth1 via
 /etc/sysconfig/network-config scripts/ifcfg-eth[0|1], set the
 environment variable `DIB_YUM_MINIMAL_CREATE_INTERFACES` to `1`.

elements/yum-minimal/root.d/08-yum-chroot

@@ -31,8 +31,6 @@ if [ $ARCH = amd64 ]; then
     ARCH=x86_64
 fi
 # Calling elements will need to set DISTRO_NAME and DIB_RELEASE
-DIB_YUMCHROOT_EXTRA_ARGS=${DIB_YUMCHROOT_EXTRA_ARGS:-}
-YUMCHROOT_TARBALL=$DIB_IMAGE_CACHE/yumchroot-${DISTRO_NAME}-${DIB_RELEASE}-${ARCH}.tar.gz
 # TODO Maybe deal with DIB_DISTRIBUTION_MIRROR
 http_proxy=${http_proxy:-}
 YUM=${YUM:-yum}
@@ -189,10 +187,6 @@ function _install_pkg_manager {
         $TARGET_ROOT/etc/yum.repos.d/*repo
 }
 
-if [ -n "$DIB_OFFLINE" -o -n "${DIB_YUMCHROOT_USE_CACHE:-}" ] && [ -f $YUMCHROOT_TARBALL ] ; then
-    echo $YUMCHROOT_TARBALL found in cache. Using.
-    sudo tar -C $TARGET_ROOT --numeric-owner -xzf $YUMCHROOT_TARBALL
-else
 # Note this is not usually done for root.d elements (see
 # lib/common-functions:mount_proc_dev_sys) but it's important that
 # we have things like /dev/urandom around inside the chroot for
@@ -286,10 +280,4 @@ else
     sudo mv $newfile $(echo $newfile | sed 's/.rpmnew$//')
 done
 
-    echo Caching result in $YUMCHROOT_TARBALL
-    sudo tar --numeric-owner \
-        -C $TARGET_ROOT \
-        -zcf $YUMCHROOT_TARBALL --exclude='./tmp/*' .
-fi
-
 sudo rm -f ${TARGET_ROOT}/.extra_settings

releasenotes/notes/yum-cache-removal-148c33012515e56e.yaml

@@ -0,0 +1,9 @@
+---
+deprecations:
+  - The ``DIB_YUMCHROOT_USE_CACHE`` variable has been removed and the
+    Fedora and CentOS ``-minimal`` initial chroot will always be
+    created by the package manager.  The default creation of a chroot
+    tarball is stopped for these elements.  This unused option was
+    unsafe; there is no guarantee that the base system will not change
+    even between runs.  Getting the package manager to reuse the cache
+    for the initial chroot install is future work.