From 15ce4cff1c83b852963a9f2782e56a5c62b3647f Mon Sep 17 00:00:00 2001
From: Daniel Watkins <oddbloke@ubuntu.com>
Date: Mon, 15 Jun 2020 16:42:09 -0400
Subject: [PATCH] .travis.yml: only store new schroot if something has changed

Prior to this change, the process of tarring up would mean that Travis
would always detect that the cache had changed, and we would incur ~30s
of packing/transferring at the end of every build.

Instead, we now check if there was any change to the installed contents
of the schroot, and only generate a new tarball if there were changes.
---
 .travis.yml | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 56b4b113a44..9ee23f8ebba 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -53,14 +53,6 @@ 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 $latest_file* -print -exec cp {} "$TRAVIS_BUILD_DIR/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 "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64 .
-                  fi
           install:
             - git fetch --unshallow
             - sudo apt-get build-dep -y cloud-init
@@ -85,6 +77,7 @@ matrix:
             # Ubuntu LTS: Build
             - ./packages/bddeb -S
             - |
+                needs_caching=false
                 if [ -e "$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
@@ -106,16 +99,24 @@ matrix:
                 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.)
+                    # And ensure it's up-to-date.
+                    before_pkgs="$(sudo schroot -c source:xenial-amd64 -d / dpkg -l | sha256sum)"
                     sudo schroot -c source:xenial-amd64 -d / -- sh -c "apt-get update && apt-get -qqy upgrade"
+                    after_pkgs=$(sudo schroot -c source:xenial-amd64 -d / dpkg -l | sha256sum)
+                    if [ "$before_pkgs" != "$after_pkgs" ]; then
+                        needs_caching=true
+                    fi
                 else
                     # Otherwise, create the chroot
                     sudo -E su $USER -c 'mk-sbuild xenial'
+                    needs_caching=true
+                fi
+                # If there are changes to the schroot (or it's entirely new),
+                # tar up the schroot (to preserve ownership/permissions) and
+                # move it into the cached dir; no need to compress it because
+                # Travis will do that anyway
+                if [ "$needs_caching" = "true" ]; then
+                    sudo tar --sparse --xattrs --xattrs-include=* -cf "$TRAVIS_BUILD_DIR/chroots/xenial-amd64.tar" -C /var/lib/schroot/chroots/xenial-amd64 .
                 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'