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A tiny init for docker containers written in C

If your Dockerfile contains something like this:

CMD ["/bin/node", "app.js"]

This can lead to some undesirable consequences - in short: your process can get shutdown in a bad way and zombie processes will pile up.

To solve this you can use smell-baron and change your Dockerfile to:

ADD smell-baron /bin/smell-baron
ENTRYPOINT ["/bin/smell-baron"]
CMD ["/bin/node", "app.js" ]

smell-baron is written in c, it needs no dependencies on the host machine and consumes almost no resources.

smell-baron supplies some extra features:

  • Multiple commands can be run, smell-baron will exit when all the watched processes have exited.
  • Whether a spawned process is watched can be configured.
  • smell-baron can be told to signal all child processes on termination, this allows it to cleanly deal with processes that spawn a subprocess in a different process group then fail to clean it up on exit.

Running more than one process

To run more than one command separate each one with the argument ---:

ADD smell-baron /bin/smell-baron
ENTRYPOINT ["/bin/smell-baron"]
CMD ["/bin/runit", "---", "/bin/node", "app.js" ]

smell-baron will wait for every process to exit before sending a SIGTERM to the remaining (reaped) processes. It will exit when all child processes exit.

Waiting for processes to exit

This behaviour can be altered with the -f command-line argument:

smell-baron -f sleep 1 --- sleep 2

When -f is used then only the processes marked with -f will be watched, any other processes will be killed with SIGTERM after the watched process(es) have exited. In the above example sleep 2 would be killed after one second. -f can be used many times and not specifying it is the same as specifying it before every command.

Configuration processes

-c can be used to mark a command as a configuring command. smell-baron will delay running all commands not marked with -c until all commands marked with -c have exited.

smell-baron -c sleep 1 --- echo hello

In the above example hello will be echoed after one second.

Cleaning up

After smell-baron's supervised processes have exited it uses kill(0, SIGTERM) to kill remaining processes. This sends a kill signal to every process in the same process group. Some processes create processes in new process groups and then fail to terminate them when they are killed. The -a flag can be used to kill all reachable processes (by using kill(-1, SIGTERM)). This argument can only be used from the init process (a process with pid 1).

Building

On a machine with gcc installed a release version can be built with:

make release

To build a debug version:

make

smell-baron can be built on a machine without gcc installed but with docker in a Centos 5 container:

./build-release.sh

To to link against musl rather than glibc an Alpine Linux container can be used:

./build-release.sh alpine

A prebuilt binary can be obtained from github:

wget https://github.com/ohjames/smell-baron/releases/download/v0.4.2/smell-baron
chmod a+x smell-baron