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Re: A Critique of Shepherd Design

From: Mark H Weaver
Subject: Re: A Critique of Shepherd Design
Date: Sat, 20 Mar 2021 01:07:45 -0400


raid5atemyhomework <> writes:

> GNU Shepherd is the `init` system used by GNU Guix.  It features:
> * A rich full Scheme language to describe actions.
> * A simple core that is easy to maintain.
> However, in this critique, I contend that these features are bugs.
> The Shepherd language for describing actions on Shepherd daemons is a
> Turing-complete Guile language.  Turing completeness runs afoul of the
> Principle of Least Power.  In principle, all that actions have to do
> is invoke `exec`, `fork`, `kill`, and `waitpid` syscalls.

These 4 calls are already enough to run "sleep 100000000000" and wait
for it to finish, or to rebuild your Guix system with an extra patch
added to glibc.

> Yet the language is a full Turing-complete language, including the
> major weakness of Turing-completeness: the inability to solve the
> halting problem.
> The fact that the halting problem is unsolved in the language means it
> is possible to trivially write an infinite loop in the language.  In
> the context of an `init` system, the possibility of an infinite loop
> is dangerous, as it means the system may never complete bootup.

Limiting ourselves to strictly total functions wouldn't help much here,
because for all practical purposes, computing 10^100 digits of Pi is
just as bad as an infinite loop.

That said, I certainly agree that Shepherd could use improvement, and
I'm glad that you've started this discussion.

At a glance, your idea of having Shepherd do more within subprocesses
looks promising to me, although this is not my area of expertise.


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