About posttrans-triggers

RPM package maintainers are familiar with the various scriptlets to control an RPM transaction, such as %post, %postun, %posttrans, etc...

One thing missing though, is the possibility to run some commands at the end of a transaction, and only once.

Consider the following example:

• the package httpd provides the service httpd.service
• the packages mod_perl and mod_ssl provide resources for this service

When the mod_perl or mod_ssl modules are installed, updated or removed, the service httpd.service should be reloaded. This could be effectively done in the %postun or %posttrans scriplet of each package.

However, if both mod_perl and mod_ssl are present in the same transaction, then httpd.service will be reloaded twice in a very short time, which might cause some trouble. Even if it doesn't, it is inefficient.

This plugin allows packages to specify paths to be watched, and actions to take after a transaction during which files matching those paths were changed.

Of course, each action will be executed only once, which means that in the above example, httpd.service will only be reloaded once.

In essence, this plugin implements what %posttrans should really be.

Install

To use this YUM plugin, you need:

• a RPM-managed operating system
• Yum == 3.2.29 This is the version on EL 6, but it will probably work with other versions too, e.g on Fedora. I've simply never tested those, feedback is warmly welcome.
• Python == 2.6 Again, this is the version on EL 6, but it would probably work with other versions. Let me know if you try it.

Installing this plugin from the sources should be as simple as running one command, as root:

# python setup.py install

Hopefully, this plugin is already packaged in your favorite distribution, so you can just run:

# yum install yum-plugin-posttrans-triggers

Usage

The plugin is enabled by default, but won't do anything unless it is told to.

There are two ways to let the plugin know you want it to run the configured triggers:

• only for one transaction, passing the --posttrans-triggers option to Yum
• permanently, by setting the always_run_triggers option to 1 in the /etc/yum/pluginconf.d/posttrans-triggers.conf file.

Configuration syntax

Each watcher file must be installed in the /etc/yum/pluginconf.d/posttrans-triggers.conf.d/ folder and be named with the .conf extension.

The syntax is very similar to the INI one:

[/path/to/watch]
exec=/path/to/command option1 option2

Each file can have as many sections as paths to watch.

Paths are watched recursively, and the section is in fact the beginning of the path to watch. That means that the following paths will be matched by the above example section:

• /path/to/watch
• /path/to/watch/subfolder/file
• /path/to/watched/file
• /path/to/watch-anything/here/really

One can specify as many commands to execute as desired, simply by specifying more than one exec= line:

[/path/to/watch]
exec=/path/to/command option1 option2
exec=/path/to/anothercommand option3 option4

Of course, several files can watch the same path, and specify different commands to be run.

If several triggers are configured with identical exec= commands, it will be executed only once. As an example, the following commands are all considered identical, since a shell would parse them all the same way:

exec= /bin/systemctl reload httpd.service
exec=/bin/systemctl  reload httpd.service
exec=/bin/systemctl reload  httpd.service
exec=/bin/systemctl reload 'httpd.service'
exec=/bin/systemctl reload "httpd.service"

The commands are executed in a minimal environment, not the full environment of the parent process (yum). At the moment, the environment passed to each command contains only the LC_* and LANG variables.

As a consequence, the exec= command must be specified with its full path, since the PATH variable is removed from its environment before execution.

To implement the example cited in the preamble, the httpd package would provide the /etc/yum/pluginconf.d/posttrans-triggers.conf.d/httpd.conf file with the following content:

[/usr/%(libarch)/httpd/modules/]
exec=/bin/systemctl reload httpd.service

Then every time a package like mod_perl, containing an Apache module, gets installed, updated or removed, the Apache server would reload all its modules to take the change into account automatically.

Legal

This project is distributed under the terms of the GNU General Public License version 3 or later.

It was written as part of my work at Network Box as we needed it for our own products.

We do not require you to assign your copyright or sign a legal document of any kind before accepting your contributions to this project, so send us patches or pull requests!