Test that previously installed APT packages are restored

.travis.yml

@@ -62,6 +62,10 @@ jobs:
       name: Test extensions on Debian 10 (buster)
       env: DOCKER_DISTRO=buster
 
+    - <<: *test-extensions-job-template
+      name: Test restoring previous packages
+      script: docker run --rm --volume "$TRAVIS_BUILD_DIR:/app" --workdir /app php:7.3-stretch ./scripts/test-restore-apt
+
     - stage: Update docs
       name: Update README.md
       if: branch = master AND type = push

scripts/invoke-shfmt

@@ -23,6 +23,7 @@ shfmt $PARAMS \
 	scripts/common \
 	scripts/invoke-shfmt \
 	scripts/lint \
+	scripts/test-restore-apt \
 	scripts/travisci-test-extensions \
 	scripts/travisci-update-readme \
 	scripts/update-readme

scripts/test-restore-apt

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+# Let's set a sane environment
+set -o errexit
+set -o nounset
+
+apt-get update -qq
+apt-get install -qqy libssl-dev
+
+./install-php-extensions zip
+
+if test -z "$(dpkg --get-selections | grep -E '^libssl-dev[:\s].*install$')"; then
+	echo 'The libssl-dev APT package has not been restored.' >&2
+	exit 1
+fi
+
+echo 'The libssl-dev APT package has been correctly restored.'