Fix RPM/DEB clean install (#816) (#839)

* Fix RPM/DEB clean install

* Improve the post install script

* Do not try to copy the state files if the agent directory is the same,
  this causes the error.
* Check the existance of symlink instead of the file it is pointing to
  for the state file migration.

* Update check for symlink existance for the cases where the symlink points to non-existent file

(cherry picked from commit 12c7d15)

Co-authored-by: Aleksandr Maus <[email protected]>

dev-tools/packaging/templates/linux/postinstall.sh.tmpl

@@ -3,16 +3,26 @@
 set -e
 
 symlink="/usr/share/elastic-agent/bin/elastic-agent"
-old_agent_dir="$( dirname "$(readlink -f -- "$symlink")" )"
+old_agent_dir=""
+
+# check if $symlink exists for the previous install
+# and derive the old agent directory
+if test -L "$symlink"; then
+    resolved_symlink="$(readlink -f -- "$symlink")"
+    # check if it is resolved to non empty string
+    if ! [ -z "$resolved_symlink" ]; then
+        old_agent_dir="$( dirname "$resolved_symlink" )"
+    fi
+fi
 
 commit_hash="{{ commit_short }}"
 
-yml_path="$old_agent_dir/state.yml"
-enc_path="$old_agent_dir/state.enc"
+new_agent_dir="/var/lib/elastic-agent/data/elastic-agent-$commit_hash"
 
-new_agent_dir="$( dirname "$old_agent_dir")/elastic-agent-$commit_hash"
-
-if ! [[ "$old_agent_dir" -ef "$new_agent_dir" ]]; then
+# copy the state files if there was a previous agent install
+if ! [ -z "$old_agent_dir" ] && ! [ "$old_agent_dir" -ef "$new_agent_dir" ]; then
+    yml_path="$old_agent_dir/state.yml"
+    enc_path="$old_agent_dir/state.enc"
     echo "migrate state from $old_agent_dir to $new_agent_dir"
 
     if test -f "$yml_path"; then
@@ -24,15 +34,17 @@ if ! [[ "$old_agent_dir" -ef "$new_agent_dir" ]]; then
         echo "found "$enc_path", copy to "$new_agent_dir"."
         cp "$enc_path" "$new_agent_dir"
     fi
+fi
 
-    if test -f "$symlink"; then
-        echo "found symlink $symlink, unlink"
-        unlink "$symlink"
-    fi
-
-    echo "create symlink "$symlink" to "$new_agent_dir/elastic-agent""
-    ln -s "$new_agent_dir/elastic-agent" "$symlink"
+# delete symlink if exists
+if test -L "$symlink"; then
+    echo "found symlink $symlink, unlink"
+    unlink "$symlink"
 fi
 
+# create symlink to the new agent
+echo "create symlink "$symlink" to "$new_agent_dir/elastic-agent""
+ln -s "$new_agent_dir/elastic-agent" "$symlink"
+
 systemctl daemon-reload 2> /dev/null
 exit 0