Where is official documentation about locking mechanisms for etckeeper, apt, and/or dpkg?

**Context:** Want to put a lock on *etckeeper/apt* hook activity during special backup. Objective is to preserve whole package integrity, e.g., wait until any package installation is complete, and then prevent new installation from starting until special backup is complete. Found shell script under *cron* which appears to be attempting to a lock on /var/cache/etckeeper/packagelist.pre-install but actually it is not performed atomically and so it is flawed. I presume the *cron* shell script is part of *Ubuntu 16.04* installation, not a part of *etckeeper* release. **Flawed lock code shown below.** Searched for *etckeeper* documentation about use of /var/cache/etckeeper/packagelist.pre-install as lock file. Found no documentation. But did find a piece of script file which writes to /var/cache/etckeeper/packagelist.pre-install without treating it as a lock file. At this time I am presuming that /var/cache/etckeeper/packagelist.pre-install is not intended to serve as a lock file interface for *etckeeper*. **Etckeeper internal script not treating /var/cache/etckeeper/packagelist.pre-install as a lock file shown below.** **Question 1: Is there (and if so where is) documentation on etckeeper locking mechanism, or a developers portal to issue a requestion for clarification?** There are plenty of questions and much discussion on *stackexchange* sites about the use of /var/lib/apt/lists/lock (we call it apt lock below) and /var/lib/dpkg/lock (we call it dpkg lock below) as locks for *apt* and *dpkg* respectively. All of the communications are concerned with stuck locks, how to diagnose them, and how to unstick them. However, I kind find no references to official *apt* and *dpkg* documentation specifying those lock files use as a formal interface. **Question 2: Is there (and if so where is) documentation on *apt* lock mechanism and/or *dpkg* lock mechanism as public interfaces?** Flawed lock attempt shell script, probably provided by *Ubuntu 16.04*: $ sudo cat /etc/cron.daily/etckeeper #!/bin/sh set -e if [ -x /usr/bin/etckeeper ] && [ -e /etc/etckeeper/etckeeper.conf ]; then . /etc/etckeeper/etckeeper.conf if [ "$AVOID_DAILY_AUTOCOMMITS" != "1" ]; then # avoid autocommit if an install run is in progress lockfile=/var/cache/etckeeper/packagelist.pre-install if [ -e "$lockfile" ] && [ -n "$(find "$lockfile" -mtime +1)" ]; then rm -f "$lockfile" # stale fi if [ ! -e "$lockfile" ]; then AVOID_SPECIAL_FILE_WARNING=1 export AVOID_SPECIAL_FILE_WARNING if etckeeper unclean; then etckeeper commit "daily autocommit" >/dev/null fi fi fi fi *Etckeeper* internal shell scipt writing to packagelist.pre-install and not treating it as a lock - hence I don't think it was intended as a lock interface. $ sudo cat /etc/etckeeper/pre-install.d/10packagelist #!/bin/sh # This list will be later used when committing. mkdir -p /var/cache/etckeeper/ etckeeper list-installed > /var/cache/etckeeper/packagelist.pre-install etckeeper list-installed fmt > /var/cache/etckeeper/packagelist.fmt