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When building Ubuntu-based images, apt-get update fails with "Could not get lock"

Problem:

If an Ubuntu image is used as a baseline for a new EC2 instance, and the EC2 build scripts attempt to update the packages during the build process, apt-get will sometimes fail updating, causing the build script to crash and fail.


Symptoms:

During the build process, you may see the following messages:

E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?


Cause:

There are maintenance scripts within the default Ubuntu Server installation that occasionally update the apt package database on their own, to inform system administrators of recommended and necessary updates.  When a new instance is launched, this process sometimes starts before you expect it to, and only one copy of apt can modify the package database at a time.  This causes your apt-get update process to exit, since there is no retry option and apt does not know how to handle this error beyond exiting.


Solution:

Use the flock command to wait for the background update job to finish before your own operation is executed.

# flock "$(apt-config shell StateDir Dir::State/d | perl -ne "print /'(.*)\/'/")"/daily_lock apt-get update
Hit:1 http://us-east-1.ec2.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://us-east-1.ec2.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:3 http://us-east-1.ec2.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
Reading package lists...
# flock "$(apt-config shell StateDir Dir::State/d | perl -ne "print /'(.*)\/'/")"/daily_lock apt-get upgrade -y
Reading package lists...
Building dependency tree...
Reading state information...
Calculating upgrade...
The following packages have been kept back:
  base-files ubuntu-server
The following packages will be upgraded:
  apport bcache-tools cloud-init cryptsetup cryptsetup-bin grub-common grub-pc
  grub-pc-bin grub2-common initramfs-tools initramfs-tools-bin
...

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