travis: cache the chroot we use for package builds (#429)

Prior to this change, we would debootstrap for every single integration
test run in Travis.  This changes that, so we will store the chroot
created by debootstrap in Travis' cache, and use it if available.  This
saves 2-3 minutes, or ~1/3rd of the integration test run time (which is
our longest run).

1 files changed, 59 insertions, 11 deletions

diff --git a/.travis.yml b/.travis.yml
index 84f38533..b600eccd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,18 +3,25 @@ dist: bionic
 
 # We use two different caching strategies.  The default is to cache pip
 # packages (as most of our jobs use pip packages), which is configured here.
-# For the integration tests, we instead want to cache the lxd images.  The
-# directory in which the images are stored (/var/snap/lxd/common/lxd/images/)
-# is not readable/writeable by the default user (which is a requirement for
-# caching), so we instead cache the `lxd_images/` directory.  We move lxd
-# images out of there before we run tests and back in once tests are complete.
-# We _move_ the images out and only copy the most recent lxd image back into
-# the cache, to avoid our cache growing without bound.  (We only need the most
-# recent lxd image because the integration tests only use a single image.)
+# For the integration tests, we instead want to cache the lxd images and
+# package build schroot.
 #
 # We cache the lxd images because this saves a few seconds in the general
 # case, but provides substantial speed-ups when cloud-images.ubuntu.com, the
-# source of the images, is under heavy load.
+# source of the images, is under heavy load.  The directory in which the lxd
+# images are stored (/var/snap/lxd/common/lxd/images/) is not
+# readable/writeable by the default user (which is a requirement for caching),
+# so we instead cache the `lxd_images/` directory.  We move lxd images out of
+# there before we run tests and back in once tests are complete.  We _move_ the
+# images out and only copy the most recent lxd image back into the cache, to
+# avoid our cache growing without bound.  (We only need the most recent lxd
+# image because the integration tests only use a single image.)
+#
+# We cache the package build schroot because it saves 2-3 minutes per build.
+# Without caching, we have to perform a debootstrap for every build.  We update
+# the schroot before storing it back in the cache, to ensure that we aren't
+# just using an increasingly-old schroot as time passes.  The cached schroot is
+# stored as a tarball, to preserve permissions/ownership.
 cache: pip
 
 install:
@@ -37,6 +44,7 @@ matrix:
           cache:
               - directories:
                   - lxd_images
+                  - chroots
           before_cache:
               - |
                   # Find the most recent image file
@@ -45,6 +53,14 @@ matrix:
                   latest_file="$(basename $latest_file .rootfs)"
                   # Find all files with that prefix and copy them to our cache dir
                   sudo find /var/snap/lxd/common/lxd/images/ -name $latest_file* -print -exec cp {} lxd_images/ \;
+              - |
+                  # If a schroot exists (i.e. we didn't fail before its
+                  # creation), tar it up (to preserve ownership/permissions)
+                  # and move it into the cached dir; no need to compress it
+                  # because Travis will do that anyway
+                  if [ -e /var/lib/schroot/chroots/xenial-amd64 ]; then
+                    sudo tar --sparse --xattrs --xattrs-include=* -cf "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64 .
+                  fi
           install:
             - git fetch --unshallow
             - sudo apt-get build-dep -y cloud-init
@@ -68,8 +84,40 @@ matrix:
           script:
             # Ubuntu LTS: Build
             - ./packages/bddeb -S
-            # Use this to get a new shell where we're in the sbuild group
-            - sudo -E su $USER -c 'mk-sbuild xenial'
+            - |
+                if [ -e "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" ]; then
+                    # If we have a cached chroot, move it into place
+                    sudo mkdir -p /var/lib/schroot/chroots/xenial-amd64
+                    sudo tar --sparse --xattrs --preserve-permissions --numeric-owner -xf "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64
+                    # Write its configuration
+                    cat > sbuild-xenial-amd64 << EOM
+                [xenial-amd64]
+                description=xenial-amd64
+                groups=sbuild,root,admin
+                root-groups=sbuild,root,admin
+                # Uncomment these lines to allow members of these groups to access
+                # the -source chroots directly (useful for automated updates, etc).
+                #source-root-users=sbuild,root,admin
+                #source-root-groups=sbuild,root,admin
+                type=directory
+                profile=sbuild
+                union-type=overlay
+                directory=/var/lib/schroot/chroots/xenial-amd64
+                EOM
+                    sudo mv sbuild-xenial-amd64 /etc/schroot/chroot.d/
+                    sudo chown root /etc/schroot/chroot.d/sbuild-xenial-amd64
+                    # And ensure it's up-to-date.  (TODO: At the moment, even
+                    # if there were no upgrades applied, because we tar up the
+                    # schroot, Travis always detects changes and so
+                    # recompresses/stores the cache every build.  If we only
+                    # replaced the cached tarball if upgrades occurred, we
+                    # could save an extra ~30s on most builds.)
+                    sudo schroot -c source:xenial-amd64 -d / -- sh -c "apt-get update && apt-get -qqy upgrade"
+                else
+                    # Otherwise, create the chroot
+                    sudo -E su $USER -c 'mk-sbuild xenial'
+                fi
+            # Use sudo to get a new shell where we're in the sbuild group
             - sudo -E su $USER -c 'sbuild --nolog --no-run-lintian --verbose --dist=xenial cloud-init_*.dsc'
             # Ubuntu LTS: Integration
             - sg lxd -c 'tox -e citest -- run --verbose --preserve-data --data-dir results --os-name xenial --test modules/apt_configure_sources_list.yaml --test modules/ntp_servers --test modules/set_password_list --test modules/user_groups --deb cloud-init_*_all.deb'