* Single-node remote execution service: ~just execute~

~just execute~ starts a single-node remote build execution service in
the environment in which the command has been issued. Having the
possibility to easily create a remote build execution service can
improve the developing experience where the build environment (and the
cache) can/should be shared among the developers. For example (and
certainly not limited to)

- when developers build on the same machine. It will allow multiple
  users to build in the same environment and share the cache, thus
  avoiding duplicated work.

- quickly set up a testing environment that can be used by other
  developers.

For the sake of completeness, these are the files used to compile the
examples
#+BEGIN_SRC
latex-hello-world/
+--hello.tex
+--repos.json
+--TARGETS
#+END_SRC

They read as follows

File ~repos.json~:
#+SRCNAME: repos.json
#+BEGIN_SRC js
{ "main": "tutorial"
, "repositories":
  { "latex-rules":
    { "repository":
      { "type": "git"
      , "branch": "master"
      , "commit": "ffa07d6f3b536f1a4b111c3bf5850484bb9bf3dc"
      , "repository": "https://github.com/just-buildsystem/rules-typesetting"
      }
    }
  , "tutorial":
    { "repository": {"type": "file", "path": "."}
    , "bindings": {"latex-rules": "latex-rules"}
    }
  }
}
#+END_SRC

File ~TARGETS~:
#+SRCNAME: TARGETS
#+BEGIN_SRC js
{ "tutorial":
  { "type": ["@", "latex-rules", "latex", "latexmk"]
  , "main": ["hello"]
  , "srcs": ["hello.tex"]
  }
}
#+END_SRC

File ~hello.tex~:
#+SRCNAME: hello.tex
#+BEGIN_SRC tex
\documentclass[a4paper]{article}

\author{JustBuild developers}
\date{}
\title {just execute}

\begin{document}
\maketitle

Hello from \LaTeX!
\end{document}
#+END_SRC

** Simple usage of ~just execute~ in the same environment

In this first example, we simply call ~just execute~ and the
environment of the caller is made available. We therefore recommend to
have a dedicated non-privileged ~build~ user to run the execution
service. In the following, we will use ~%~ to indicate the prompt of
the ~build~ user, ~$~ for a /normal/ user.

To enable such a single-node execution service, it is sufficient to
type on one shell (as ~build~ user)
#+BEGIN_SRC bash
% just execute -p <N>
#+END_SRC
Where ~<N>~ is a port number which is supposed to be available.
By default, the native ~git~-based protocol will be used, but it
is also possible to use the original protocol with ~sha256~ hashes
by providing the ~--compatible~ option.
#+BEGIN_SRC bash
% just execute --compatible -p <N>
#+END_SRC
This is particularly useful when providing the remote-execution service
to a different build tool.

To use it, as a /normal/ user, on a different shell type
#+BEGIN_SRC bash
$ just [...] -r localhost:<N>
#+END_SRC
Let's run these commands to understand the output.

#+BEGIN_SRC bash
% just execute -p 8080
INFO: execution service started: {"interface":"127.0.0.1","pid":4911,"port":8080}
#+END_SRC

Once the execution service is started, it logs out three essential
data:
- which interface is used (in this case, the default one, which is the
  loopback device)
- the pid number (number will always change)
- the used port

To exploit the execution service, run from a different shell
#+BEGIN_SRC bash
$ just [...] -r localhost:8080
#+END_SRC

*** Use a random port

If we don't need (or know) a fixed port number, we can simply omit the
~-p~ option. In this case, ~just execute~ will listen to a random free
port.

#+BEGIN_SRC bash
% just execute
INFO: execution service started: {"interface":"127.0.0.1","pid":7217,"port":33841}
#+END_SRC

The port number can be different each time we invoke the
above command.

Finally, to connect to the remote endpoint, type
#+BEGIN_SRC bash
$ just [...] -r localhost:33841
#+END_SRC

*** Info file

Copying and pasting port numbers and pids can be
error-prone/unfeasible if we manage several/many execution service
instances. Therefore, the invocation of ~just execute~ can be
decorated with the option ~--info-file <PATH>~, which will store, in
JSON format, in ~<PATH>~ the interface, pid, and port bound to the
running instance. The user can then easily parse this file to extract
the required information.

For example
#+BEGIN_SRC bash
% just execute --info-file /tmp/foo.json
INFO: execution service started: {"interface":"127.0.0.1","pid":7680,"port":44115}
#+END_SRC

#+BEGIN_SRC bash
$ cat /tmp/foo.json
{"interface":"127.0.0.1","pid":7680,"port":44115}
#+END_SRC

Please note that the info file will /not be automatically deleted/
when the user terminates the service. The user is responsible for
eventually removing it from the file system.


*** Enable mTLS

It is worth mentioning that mTLS must be enabled when the execution
service starts, and it cannot be activated (or deactivated) while the
instance runs.
#+BEGIN_SRC bash
% just execute [...] --tls-ca-cert <path_to_CA_cert> --tls-server-cert <path_to_server_cert> --tls-server-key <path_to_server_key>
#+END_SRC

When a client connects, it must pass the same ~CA certificate~ and
its pair of certificate and private key, which the used certified
authority has signed.
#+BEGIN_SRC bash
$ just [...] --tls-ca-cert <path_to_CA_cert> --tls-client-cert <path_to_client_cert> --tls-client-key <path_to_client_key>
#+END_SRC

**** How to generate self-signed certificates

This section does not pretend to be an exhaustive guide to the
generation and management of certificates, which is well beyond the
aim of this tutorial. We just want to provide a minimal reference for
let users start using mTLS and having the benefits of mutual
authentication.

***** Certification Authority certificate

As a first step, we need a Certification Authority certificate (~ca.crt~)
#+BEGIN_SRC bash
% openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:4096 -keyout ca.key -out ca.crt
#+END_SRC

***** Server certificate and key

If the clients will connect using the loopback device, i.e., the users
are logged in the same machine where ~just execute~ will run, the
/server certificates/ can be generate with the following instructions
#+BEGIN_SRC bash
% openssl req -new -nodes -newkey rsa:4096 -keyout server.key -out server.csr -subj "/CN=localhost"
% openssl x509 -req -days 365 -in server.csr -CA ca.crt -CAkey ca.key -set_serial 0 -out server.crt
% rm server.csr
#+END_SRC bash

On the other hand, if the clients will connect from a different
machine, and ~just execute~ will use a different interface (see [[Expose
a particular interface]] below), the steps are a bit more involved. We
need an additional configuration file where we state the ip address of
the used interface. For example, if the interface ip address is
~192.168.1.14~, we will write
#+BEGIN_SRC bash
% cat << EOF > ssl-ext-x509.cnf
[v3_ca]
subjectAltName = IP.1:192.168.1.14
EOF
#+END_SRC

Then, the pair of certificate and pair can be obtained with
#+BEGIN_SRC bash
% openssl req -new -nodes -newkey rsa:4096 -keyout server.key -out server.csr -subj "/CN=localhost"
% openssl x509 -req -days 365 -in server.csr -CA ca.crt -CAkey ca.key -set_serial 0 -out server.crt -extensions v3_ca -extfile ssl-ext-x509.cnf
% rm server.csr
#+END_SRC bash

***** Client certificate and key

The client, which needs the ~ca.crt~ and ~ca.key~ files, can run the
following

#+BEGIN_SRC bash
$ openssl req -new -nodes -newkey rsa:4096 -keyout client.key  -out client.csr
$ openssl x509 -req -days 365 -signkey client.key -in client.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out client.crt
$ rm client.csr
#+END_SRC


*** Expose a particular interface

To use an interface different from the loopback one, we have to list
it with the ~-i~ option
#+BEGIN_SRC bash
$ just execute -i 192.168.1.14 -p 8080 --tls-ca-cert <path_to_CA_cert> --tls-server-cert <path_to_server_cert> --tls-server-key <path_to_server_key>
INFO: execution service started: {"interface":"192.168.1.14","pid":7917,"port":8080}
#+END_SRC

If the interface is accessible from another machine, it is also
recommended to enable mutual TLS (mTLS) authentication.

** Managing multiple build environments

Since multiple instances of ~just execute~ can run in parallel
(listening at different ports), the same machine can be the worker for
various projects. However, to avoid conflicts between the dependencies
and to guarantee a clean environment for each project, it is
recommended that ~just execute~ is invoked from within a container or
a chroot environment.

In the following sections, we will set up, step by step, a dedicated
execution service for compiling latex documents in these two
scenarios.

*** How to run ~just execute~ inside a chroot environment

**** TL;DR
- create a suitable chroot environment
- chroot into it
- run ~just execute~ from there
- in a different shell, ~just build -r <interface>:<port>~

**** Full latex chroot: walkthrough

This short tutorial will use ~debootstrap~ and ~schroot~ to create and
enter the chroot environment. Of course, different strategies/programs
can be used.

***** Prepare the root file system
Install debian bullseye in directory ~/chroot/bullseye-latex~
#+BEGIN_SRC bash
sudo debootstrap bullseye /chroot/bullseye-latex
#+END_SRC

***** Create a configuration file

~schroot~ needs a proper configuration file, which can be generated as
follows
#+BEGIN_SRC bash
$ echo "[bullseye-latex]
description=bullseye latex env
directory=/chroot/bullseye-latex
root-users=$(whoami)
users=$(whoami)
type=directory" | sudo tee /etc/schroot/chroot.d/bullseye-latex
#+END_SRC

Note that ~type=directory~, apart from performing the necessary
bindings, will make ~$HOME~ shared between the host and chroot
environment. While this can be useful for sharing artifacts, the user
should specify a ~--local-build-root~ (aka, the cache root) different
from the default one to avoid conflicts between the host and the
chroot environment.

***** Install required packages in the chroot environment

~schroot~ also allows running commands inside the environment by
stating it after the ~--~
#+BEGIN_SRC bash
$ schroot -c bullseye-latex -u root -- sh -c 'apt update && apt install -y texlive-full'
#+END_SRC

***** Start the execution service

To start the execution service inside the chroot environment run
#+BEGIN_SRC bash
$ schroot -c bullseye-latex -- /bin/just execute --local-build-root ~/.cache/chroot/bullseye-latex -p 8080
#+END_SRC

We assumed that the binary ~just~ is available in the chroot
environment at the path ~/bin/just~. If you don't know how to make
~just~ available in the chroot environment, read the section [[How to
have the binary just inside the chroot environment]] below.

Since the ~$HOME~ is shared, specifying a local build root (aka, cache
root) different from the default is highly recommended. For
convenience, we also set a port (using the flag ~-p~) that the
execution service will listen to.

If the chosen port is available, the following output should be
produced (note that the pid number might be different).
#+BEGIN_SRC bash
INFO: execution service started: {"interface":"127.0.0.1","pid":48880,"port":8080}
#+END_SRC

For example, let's compile the example listed in the introduction
#+BEGIN_SRC bash
$ just-mr -C repos.json install -o . -r localhost:8080
#+END_SRC

which should report
#+BEGIN_SRC bash
INFO: Performing repositories setup
INFO: Found 2 repositories to set up
INFO: Setup finished, exec ["just","install","-C","...","-o",".","-r","localhost:8080"]
INFO: Requested target is [["@","tutorial","doc/just-execute/latex-hello-world","tutorial"],{}]
INFO: Analysed target [["@","tutorial","doc/just-execute/latex-hello-world","tutorial"],{}]
INFO: Discovered 1 actions, 0 trees, 1 blobs
INFO: Building [["@","tutorial","doc/just-execute/latex-hello-world","tutorial"],{}].
INFO: Processed 1 actions, 0 cache hits.
INFO: Artifacts can be found in:
        /tmp/work/doc/just-execute/latex-hello-world/hello.pdf [25e05d3560e344b0180097f21a8074ecb0d9f343:37614:f]
#+END_SRC

In the shell where ~just execute~ is running, this line should have
appeared, witnessing that the compilation happened on the remote side
#+BEGIN_SRC bash
INFO (execution-service): Execute 6237d87faed1ec239512ad952eeb412cdfab372562
#+END_SRC

*** How to start ~just execute~ inside a docker container

Building inside a container is another strategy to ensure no
undeclared dependencies are pulled and to build in a fixed
environment.

We will replicate what we did for the chroot environment and create a
suitable docker image.

**** Build a suitable docker image

Let's write a ~Dockerfile~ that has ~just execute~ as ~ENTRYPOINT~. We
assume the binary ~just~ is available inside the container at path
~/bin/just~. The easiest way is to use a
[[https://github.com/just-buildsystem/justbuild-static-binaries][static just binary]]
and copy it into the container.

#+SRCNAME: Dockerfile
#+BEGIN_SRC docker
FROM debian:bullseye-slim

COPY ./just /bin/just

RUN apt update
RUN apt install -y --no-install-recommends texlive-full

ENTRYPOINT ["/bin/just", "execute"]
#+END_SRC

We build the image with
#+BEGIN_SRC bash
$ sudo docker image build -t bullseye-latex .
#+END_SRC

Finally, we can start the execution service
#+BEGIN_SRC bash
$ docker run --network host --name execute-latex -p 8080
#+END_SRC

From a different shell, we can build the latex hello world example
listed in the introduction running
#+BEGIN_SRC bash
$ just-mr -C repos.json install -o . -r localhost:8080
#+END_SRC

Note that the cache that ~just execute~ populates is confined within
the container. The cache is gone if the container is restarted (or the
pc rebooted). If you want the cache to survive the container life
cycle, you can bind a "host directory" within the container as
follows
#+BEGIN_SRC bash
$ docker run --network host --name execute-latex --mount type=bind,source="${HOME}/.cache",target=/cache bullseye-latex -p 8080 --local-build-root /cache/docker/latex
#+END_SRC
