Table of contents
Hooks are an extension feature provided by all package managers that are used in
larger Linux distributions. For example, Debian uses apt, which has various
uses rpm, which has
package managers use different names for the concept, but all of them offer
package maintainers the ability to run arbitrary code during package
installation and upgrades. Example hook use cases include adding daemon user
accounts to your system (e.g.
postgres), or generating/updating cache files.
Triggers are a kind of hook which run when other packages are installed. For
example, on Debian, the
comes with a trigger which regenerates the search database index whenever any
package installs a manpage. When, for example, the
nginx(8) package is installed, a
trigger provided by the
Over the past few decades, Open Source software has become more and more uniform: instead of each piece of software defining its own rules, a small number of build systems are now widely adopted.
Hence, I think it makes sense to revisit whether offering extension via hooks and triggers is a net win or net loss.
Hooks preclude concurrent package installation
Package managers commonly can make very little assumptions about what hooks do, what preconditions they require, and which conflicts might be caused by running multiple package’s hooks concurrently.
Hence, package managers cannot concurrently install packages. At least the hook/trigger part of the installation needs to happen in sequence.
While it seems technically feasible to retrofit package manager hooks with concurrency primitives such as locks for mutual exclusion between different hook processes, the required overhaul of all hooks¹ seems like such a daunting task that it might be better to just get rid of the hooks instead. Only deleting code frees you from the burden of maintenance, automated testing and debugging.
① In Debian, there are 8620 non-generated maintainer scripts, as reported by
find shard*/src/*/debian -regex ".*\(pre\|post\)\(inst\|rm\)$" on a Debian Code Search instance.
Triggers slow down installing/updating other packages
Personally, I never use the
apropos(1) command, so I don’t
man(1) package’s trigger
which updates the database used by
apropos(1). The process takes a long
time and, because hooks and triggers must be executed serially (see previous
section), blocks my installation or update.
When I tell people this, they are often surprised to learn about the existance
apropos(1) command. I suggest
adopting an opt-in model.
Hooks run when packages are installed. If a package’s contents are not used between two updates, running the hook in the first update could have been skipped. Running the hook lazily when the package contents are used reduces unnecessary work.
As a welcome side-effect, lazy hook evaluation automatically makes the hook work in operating system images, such as live USB thumb drives or SD card images for the Raspberry Pi. Such images must not ship the same crypto keys (e.g. OpenSSH host keys) to all machines, but instead generate a different key on each machine.
Why do users keep packages installed they don’t use? It’s extra work to remember and clean up those packages after use. Plus, users might not realize or value that having fewer packages installed has benefits such as faster updates.
I can also imagine that there are people for whom the cost of re-installing packages incentivizes them to just keep packages installed—you never know when you might need the program again…
Implemented in an interpreted language
While working on hermetic packages (more on that in another blog post), where
the contained programs are started with modified environment variables
PATH) via a wrapper bash script, I noticed that the overhead of those
wrapper bash scripts quickly becomes significant. For example, when using the
excellent magit interface for Git in Emacs, I encountered
second-long delays² when using hermetic packages compared to standard
packages. Re-implementing wrappers in a compiled language provided a significant
Similarly, getting rid of an extension point which mandates using shell scripts allows us to build an efficient and fast implementation of a predefined set of primitives, where you can reason about their effects and interactions.
② magit needs to run git a few times for displaying the full status, so small overhead quickly adds up.
Incentivizing more upstream standardization
Hooks are an escape hatch for distribution maintainers to express anything which their packaging system cannot express.
Distributions should only rely on well-established interfaces such as autoconf’s
./configure && make && make install (including commonly used flags) to
build a distribution package. Integrating upstream software into a distribution
should not require custom hooks. For example, instead of requiring a hook which
updates a cache of schema files, the library used to interact with those files
should transparently (re-)generate the cache or fall back to a slower code path.
Distribution maintainers are hard to come by, so we should value their time. In particular, there is a 1:n relationship of packages to distribution package maintainers (software is typically available in multiple Linux distributions), so it makes sense to spend the work in the 1 and have the n benefit.
Can we do without them?
If we want to get rid of hooks, we need another mechanism to achieve what we currently achieve with hooks.
If the hook is not specific to the package, it can be moved to the package
manager. The desired system state should either be derived from the package
contents (e.g. required system users can be discovered from systemd service
files) or declaratively specified in the package build instructions—more on that
in another blog post. This turns hooks (arbitrary code) into configuration,
which allows the package manager to collapse and sequence the required state
changes. E.g., when 5 packages are installed which each need a new system user,
the package manager could update
/etc/passwd just once.
If the hook is specific to the package, it should be moved into the package
contents. This typically means moving the functionality into the program start
(or the systemd service file if we are talking about a daemon). If (while?)
upstream is not convinced, you can either wrap the program or patch it. Note
that this case is relatively rare: I have worked with hundreds of packages and
the only package-specific functionality I came across was automatically
generating host keys before starting OpenSSH’s
There is one exception where moving the hook doesn’t work: packages which modify state outside of the system, such as bootloaders or kernel images.
③ Even that can be moved out of a package-specific hook, as Fedora demonstrates.
Global state modifications performed as part of package installation today use hooks, an overly expressive extension mechanism.
Instead, all modifications should be driven by configuration. This is feasible because there are only a few different kinds of desired state modifications. This makes it possible for package managers to optimize package installation.