Use XZ compression for DEB packages

We have been using the default Gzip compression method for DEB and RPM packages
up to this point. However, XZ compression is available in every supported Linux
distribution, which reduces the final package size by nearly 30%. We'll use
XZ compression for DEB packages by default, with a documented variable
to revert to the prior behavior in cases where XZ compression is not supported.

We are not using XZ compression for RPM packages right now due to an unknown
issue causing the packaging times to increase more that we can afford.
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8222 is open to
investigate more.

Changelog: changed
MR: https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/7128

Author: Ryan Egesdahl

config/projects/gitlab.rb

@@ -273,11 +273,25 @@
 package :rpm do
   vendor 'GitLab, Inc. <[email protected]>'
   signing_passphrase Gitlab::Util.get_env('GPG_PASSPHRASE')
+
+  # Enable XZ compression if selected
+  compress_xz = Gitlab::Util.get_env('COMPRESS_XZ') || 'false'
+  if compress_xz == 'true'
+    compression_type :xz
+    compression_level 6
+  end
 end
 
 package :deb do
   vendor 'GitLab, Inc. <[email protected]>'
   signing_passphrase Gitlab::Util.get_env('GPG_PASSPHRASE')
+
+  # Enable XZ compression if selected
+  compress_xz = Gitlab::Util.get_env('COMPRESS_XZ') || 'true'
+  if compress_xz == 'true'
+    compression_type :xz
+    compression_level 6
+  end
 end
 
 resources_path "#{Omnibus::Config.project_root}/resources"

doc/build/build_package.md

@@ -64,6 +64,20 @@ project's [Container Registry](https://gitlab.com/gitlab-org/gitlab-omnibus-buil
    export COMPILE_ASSETS=true
    ```
 
+1. By default, XZ compression is used to produce the final DEB package,
+   which reduces the package size by nearly 30% in comparison to Gzip, with
+   little to no increase in build time. However, the system's package
+   manager must also support the format. If your system's package manager does
+   not support XZ packages, set the `COMPRESS_XZ` environment variable to `false`:
+
+   ```shell
+   export COMPRESS_XZ=false
+   ```
+   
+   NOTE: This is feature is disabled by default on RPM platforms due to an
+   as-yet-unexplained increase in packaging time. It can be enabled by setting
+   the `COMPRESS_XZ` environment variable to `true`.
+
 1. Install the libraries and other dependencies:
 
    ```shell