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https://github.com/openSUSE/catatonit

A container init that is so simple it's effectively brain-dead.
https://github.com/openSUSE/catatonit

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A container init that is so simple it's effectively brain-dead.

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## catatonit ##



[![Build Status](https://github.com/openSUSE/catatonit/actions/workflows/ci.yml/badge.svg)](https://github.com/openSUSE/catatonit/actions/workflows/ci.yml)



A container init that is so simple it's effectively brain-dead. This is a

rewrite of [initrs][initrs] in C, because we found that it is not possible to

statically compile Rust binaries without using musl. That was, in turn, a

reimplementation of other container inits like `tini` and `dumb-init`.



The reason for re-implementing `docker-init` is because it appears as though

all of the other implementations do not handle signals as correctly as they

should. In particular, they all appear to make use of `sigwait(2)` (`tini` does

a `sigtimedwait(2)` for an interval and then will do a `waitpid(2)` even if it

didn't detect a `SIGCHLD`). `catatonit` uses `signalfd(2)`, which [has its own

warts][signalfd-broken], but the improvements over `sigwait(2)` are significant

in terms of stability. Ideally we would just write a patch for the other

projects to use `signalfd(2)` rather than creating a new project, but after

some time spent looking at `tini` and `dumb-init` we felt that such patches

would be closer to full rewrites.



In addition, the purpose of `catatonit` is to only support the key usage by

`docker-init` which is `/dev/init -- `. With few exceptions, no

other features will be added.



[initrs]: https://github.com/cyphar/initrs

[signalfd-broken]: https://ldpreload.com/blog/signalfd-is-useless



### Usage ###



catatonit has identical usage to other basic `docker-init`'s -- you give it the

command and list of arguments to that command.



If you install `catatonit` to `/usr/bin/docker-init`, `docker run --init` will

use `catatonit` as its container pid1. Alternatively, you can configure the

Docker daemon to use `catatonit` without deleting any previously installed

`/usr/bin/docker-init` by using `--init-path` (or adding an `init-path` setting

in `/etc/docker/daemon.json`). Podman has similar options.



Catatonit supports a very limit subset of features, in order to keep the code

as simple as possible:



* If catatonit is not pid1 (in other words, you are not in a PID namespace), it

  will try to use the sub-reaper support in the kernel to act as a

  "pseudo-init" for the process you requested.



* You can pass `-g` if you want signals to be forwarded to the entire process

  group of your spawned process (otherwise it's just forwarded to the process

  spawned).



* If you wish to use catatonit as a convenient pause container (do not spawn a

  child process nor do any signal handling), you can pass `-P`.



If you want to include `catatonit` in your images, you can conveniently add it

to your Dockerfile as an entrypoint:



```dockerfile

# Runs "catatonit -- /my/amazing/script --with --args"

ENTRYPOINT ["catatonit", "--"]



# or if you use --rewrite or other cli flags

# ENTRYPOINT ["catatonit", "--rewrite", "2:3", "--"]



CMD ["/my/amazing/script", "--with", "--args"]

```



### Installation ###



catatonit uses autotools for building, so building is a fairly standard:



```

% ./autogen.sh

% ./configure

% make

% sudo make install

```



Note that this install the `catatonit` binary to `/usr/bin/catatonit`. If you

want to use `docker run --init` you may need to symlink `/usr/bin/docker-init`

to `catatonit` or configure Docker to use `catatonit`.



### License ###



catatonit is licensed under the GNU General Public License version 2 or later.



```

catatonit: a container init so simple it's effectively brain-dead

Copyright (C) 2018-2023 SUSE LLC



This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation, either version 2 of the License, or

(at your option) any later version.



This program is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU General Public License for more details.



You should have received a copy of the GNU General Public License

along with this program.  If not, see .

```