msa-agent/internal/install/service.go

// Package install puts the agent on a node and takes it off again, as the two halves of one sequence.
//
// It is one package because the two have to agree about every path. An install that creates something
// the uninstall does not remove leaves a machine with a stray credential on it, and an uninstall that
// removes something the install did not create fails on a fresh node. Keeping the path list in one
// place is what makes that agreement checkable rather than remembered.
//
// The systemd unit is embedded rather than fetched. Two reasons, and the second is the one that
// decided it: a node installing the agent may have no route to anything except the relay, and a unit
// file downloaded from a URL is a second thing that can 404 at the exact moment the operator has the
// least context to debug it. Embedding also means the unit an operator runs is the one the binary was
// built with, so a node cannot end up with a unit from a different release than its agent.
package install

import (
	_ "embed"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

// The vendored unit. It is copied from agent/systemd/ by the Makefile, and
// TestVendoredUnitMatchesSystemd fails when the copy drifts — the same arrangement the agent's policy
// uses, and for the same reason: an embedded file that is edited in place is a file the build
// overwrites, and the mistake should be loud rather than silent.
//
//go:embed msa-agent.service
var embeddedUnit []byte

// UnitFile returns the unit this binary installs, for tests and for `install --print-unit`.
func UnitFile() []byte { return embeddedUnit }

// InstallOptions configures an install.
type InstallOptions struct {
	Paths Paths
	// DryRun prints the plan and changes nothing.
	DryRun bool
	Runner Runner
	Log    func(format string, args ...any)
}

// Install makes a node ready to run the agent as a service, assuming the binary is already in place
// and the config has already been written by `msa-agent enroll`.
//
// It is idempotent: every step checks what is already true, so re-running it after an upgrade or a
// failed attempt is safe. That matters because the alternative — a command that only works on a clean
// node — makes every partial failure a manual cleanup.
//
// The order is the reverse of the uninstall's, and for the same reason: the user has to exist before
// anything can be owned by it, and the config has to be readable by that user before the service can
// start.
func Install(opts InstallOptions) error {
	if opts.Runner == nil {
		opts.Runner = ExecRunner{}
	}
	if opts.Log == nil {
		opts.Log = func(string, ...any) {}
	}
	paths := opts.Paths

	// The two preconditions, reported rather than enforced during a dry run.
	//
	// A dry run is asked before the work is done — that is the whole point of it — so refusing it for
	// the very conditions it exists to reveal would make it useless on exactly the machine an operator
	// runs it on. The real run refuses, because a service whose ExecStart points at nothing fails with
	// "no such file or directory" in the journal, which reads like a broken build.
	binaryMissing := !exists(paths.BinaryPath)
	configMissing := !exists(paths.ConfigPath)
	if !opts.DryRun {
		if binaryMissing {
			return fmt.Errorf("install: %s is not there; run this from the installed binary, not a copy in /tmp", paths.BinaryPath)
		}
		if configMissing {
			return fmt.Errorf("install: %s is not there; run `msa-agent enroll` first", paths.ConfigPath)
		}
	} else {
		if binaryMissing {
			opts.Log("note: %s is not there yet; the real run would refuse", paths.BinaryPath)
		}
		if configMissing {
			opts.Log("note: %s is not there yet; run `msa-agent enroll` first", paths.ConfigPath)
		}
	}

	// 1. The system user. `--system` gives it no login shell and no home, which is what a process that
	//    only dials out needs; `--no-create-home` keeps the filesystem as it was.
	if stepWith(opts.DryRun, opts.Log, "creating the %s system user", paths.User) {
		if err := opts.Runner.Run("getent", "passwd", paths.User); err == nil {
			opts.Log("  %s already exists", paths.User)
		} else if err := opts.Runner.Run("useradd", "--system", "--no-create-home",
			"--shell", "/usr/sbin/nologin", paths.User); err != nil {
			return fmt.Errorf("install: creating the %s user: %w", paths.User, err)
		}
	}

	// 2. The log directory, owned by that user. The unit's ProtectSystem=strict makes everything else
	//    read-only, so this is one of the two paths the service can write.
	if stepWith(opts.DryRun, opts.Log, "creating %s", paths.LogDir) {
		if err := os.MkdirAll(paths.LogDir, 0o750); err != nil {
			return fmt.Errorf("install: creating the log directory: %w", err)
		}
	}

	// 3. Ownership of the config, and of the log directory, and that step is load-bearing rather than
	//    tidy: `enroll` ran under sudo, so the config is root-owned 0600 and the service — which runs as
	//    the user above — cannot read its own private keys. The failure is a service that starts and
	//    immediately exits with a permission error.
	if stepWith(opts.DryRun, opts.Log, "giving %s ownership of the config and logs", paths.User) {
		for _, path := range []string{paths.ConfigPath, filepath.Dir(paths.ConfigPath), paths.LogDir} {
			if err := chownTo(path, paths.User, opts); err != nil {
				return err
			}
		}
	}

	// 4. The unit, from this binary.
	if stepWith(opts.DryRun, opts.Log, "writing %s", paths.UnitPath) {
		if err := os.MkdirAll(filepath.Dir(paths.UnitPath), 0o755); err != nil {
			return fmt.Errorf("install: creating the unit directory: %w", err)
		}
		if err := os.WriteFile(paths.UnitPath, embeddedUnit, 0o644); err != nil {
			return fmt.Errorf("install: writing the unit: %w", err)
		}
	}

	// 5. Enable and start. `daemon-reload` first, because systemd caches unit files and would otherwise
	//    start whatever it read last.
	if stepWith(opts.DryRun, opts.Log, "reloading systemd") {
		if err := opts.Runner.Run("systemctl", "daemon-reload"); err != nil {
			return fmt.Errorf("install: systemctl daemon-reload: %w", err)
		}
	}
	if stepWith(opts.DryRun, opts.Log, "enabling and starting %s", paths.ServiceName) {
		if err := opts.Runner.Run("systemctl", "enable", "--now", paths.ServiceName); err != nil {
			return fmt.Errorf("install: systemctl enable --now: %w", err)
		}
	}

	if opts.DryRun {
		opts.Log("dry run: nothing was changed")
		return nil
	}
	opts.Log("msa-agent installed and running")
	opts.Log("check it with: journalctl -u %s -n 20", paths.ServiceName)
	opts.Log("next: run `msa-agent --pair` once, then pair the phone from the app")
	return nil
}

// exists reports whether a path is present, without caring why it might not be.
func exists(path string) bool {
	_, err := os.Stat(path)
	return err == nil
}

// chownTo gives a path to the service user.
//
// It shells out to chown rather than calling os.Chown, and that is a deliberate consistency rather than
// a preference: the whole install is a sequence of system commands, so routing all of them through the
// Runner is what makes the sequence testable without root. os.Chown would work and would also be the one
// step a test could not intercept.
func chownTo(path, user string, opts InstallOptions) error {
	if err := opts.Runner.Run("chown", user+":"+user, path); err != nil {
		return fmt.Errorf("install: giving %s to %s: %w", path, user, err)
	}
	opts.Log("  %s -> %s", path, user)
	return nil
}

// PrintUnit writes the embedded unit to w, so an operator can read what will be installed before it is.
func PrintUnit(w io.Writer) error {
	_, err := w.Write(embeddedUnit)
	return err
}