Exit install early if nothing to do

This is intended to match the behavior of puppetlabs-puppet_agent's
install script, which has similar logic.

The logic also appears necessary for OpenBolt's tests to pass.

In OpenBolt, a remote node with an agent installed on it, but which has
not had apply_prep executed will be initially identified as "agentless".
That causes this install script to be invoked via the puppet_library
hook. In at least one OpenBolt test this does not _run_as root, so it
relies on this bail-out logic to avoid failing when attempting to
install the package. After that initial apply_prep run, the puppet-agent
feature is added to the Target configuration, which bypasses invoking
this installation script in subsequent runs.

Initially I thought this might be a bug, but nothing else seems to add
the puppet-agent feature to a target configuration so my best guess is
that this is how OpenBolt is intended to "detect" the existence of an
agent on a remote node.

Author: austb

tasks/install_linux.sh

@@ -14,6 +14,32 @@ stop_service=${PT_stop_service:-'false'}
 # shellcheck source=files/common.sh
 source "${PT__installdir}/openvox_bootstrap/files/common.sh"
 
+# This function will cause the script to exit if one of the conditions is met
+#
+# If the requested $version is
+#   1. latest and there is already an agent installed
+#   2. not latest, but $version matches the agent's version number
+skip_if_installed() {
+  if [ -f /opt/puppetlabs/puppet/VERSION ]; then
+    installed_version=$(cat /opt/puppetlabs/puppet/VERSION)
+  elif command -v puppet >/dev/null 2>&1; then
+    installed_version=$(puppet --version)
+  else
+    installed_version=none
+  fi
+
+  if [ "$installed_version" != 'none' ]; then
+    if [ "$version" = 'latest' ]; then
+      info 'Specific agent version not requested and the agent was detected. Skipping install.'
+      exit 0
+    elif [ "$version" = "$installed_version" ]; then
+      # installed agent version matched specific version requested
+      info "Requested agent version $version. Found agent version $installed_version. Skipping install."
+      exit 0
+    fi
+  fi
+}
+
 # Based on platform family set:
 #   repository - the package repository to download from
 set_repository() {
@@ -65,6 +91,8 @@ install_release_package() {
   refresh_package_cache
 }
 
+# quit early if nothing to do
+skip_if_installed
 # Get platform information
 set_platform_globals
 # Set collection release package url based on platform