// Command mesh-bootstrap brings a mesh into existence on a bare machine.
//
// Tier 0, beside `mesh-host` and not inside it. Bootstrapping is done by hand and it changes a
// machine, which is what tier 0 is (novox/hq 03-DESIGN/01-to-be/05-the-node-host.md) — but
// `mesh-host` states of itself that it connects to nothing and listens on nothing and that what it
// applies comes from a file, and that is the whole reason an always-running root daemon can be
// audited by reading one page. An installer that loads images and interrogates a control plane
// cannot be folded into it without making that sentence false. Same tier, same repository,
// different program.
//
// Genesis is a pivot (novox/hq ADR 0067). It raises a substrate whose control plane is named by the
// digest of its own configuration — legal exactly where nothing could have served an image — then
// enrols this machine, installs the registry module, pushes that image into it to get the manifest
// digest it has never had, reinstalls the control plane as an ordinary module pinned to it, and
// drops the temporary one. Without --catalog it stops after the substrate and says why.
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"net"
	"net/http"
	"os"
	"os/signal"
	"syscall"
	"time"

	"github.com/novox/mesh-host/internal/apply"
	"github.com/novox/mesh-host/internal/bootstrap"
	"github.com/novox/mesh-host/internal/store"
)

// version is stamped at build time. Unset in a development build, and said so rather than
// defaulted to something that looks like a release.
var version = "development build"

const (
	defaultTemplate = "substrate.lock"
	defaultOut      = "/var/lib/mesh-host/substrate.lock"
	defaultRegistry = "127.0.0.1:5000"
	defaultHost     = "/usr/local/bin/mesh-host"
	defaultService  = "mesh-host.service"
)

const usage = `mesh-bootstrap — make a bare machine into a mesh

  bootstrap   the ten steps below   (the default)
  version

     1 preflight  what has to be true before anything is changed
     2 load       the control plane's image, carried in this installer
     3 bundle     the substrate, named for this machine
     4 apply      raise it
     5 verify     it is up, and the control plane replies
     6 enrol      this machine becomes the mesh's first node
     7 registry   install the module that gives this mesh an image store
     8 publish    push the control plane's image into it, for its first digest
     9 control    reinstall the control plane as an ordinary module, pinned to that digest
    10 retire     drop the temporary control plane; the host removes it

  --bundle    the substrate template to build this machine's bundle from
              (default ` + defaultTemplate + `)
  --out       where the produced bundle is written, for a person to read
              (default ` + defaultOut + `)
  --state     where this node records what it has applied
              (default ` + store.DefaultPath + `)
  --catalog   a checkout of the mesh's catalogue, holding the registry's and the
              control plane's manifests. Without it this stops after step 5
  --node      the name this machine is known by (default: its hostname)
  --registry  where this mesh keeps its own images (default ` + defaultRegistry + `)
              every node pulls the control plane from this, so on a mesh of more
              than one machine it must be an address the others can reach
  --host      the mesh-host binary on this machine (default ` + defaultHost + `)
  --host-service      the unit that supervises it (default ` + defaultService + `)
  --host-in-background  start the host unsupervised instead. It does not survive
              a reboot. This is what a lab does and what no real machine should
  --system    which operating system this is; by default it is asked
  --timeout   how long any single probe may take (default 30s)
  --wait      how long a thing that is merely starting is given (default 3m)
  --dry-run   everything that does not change the machine
  --json      machine-readable output

Genesis is a pivot: a temporary control plane installs the registry that makes it
permanent. The temporary one is called temp-mesh-control and the permanent one is
called mesh-control, so they are two containers with two owners and there is nothing
to hand over.

Every step is idempotent: run it again after fixing whatever it named, and the steps
that already succeeded say so.
`

func main() {
	// Ctrl-C must stop the installer, not be swallowed by whatever it is waiting for — and it
	// waits on pulls, on a runtime starting, and on a control plane opening its stores.
	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
	defer stop()

	command, opts, jsonOut, err := parseArgs(os.Args[1:])
	if err == nil {
		err = run(ctx, command, opts, jsonOut)
	}
	if err != nil {
		fmt.Fprintf(os.Stderr, "mesh-bootstrap: %v\n", err)
		os.Exit(1)
	}
}

// parseArgs takes an optional subcommand first, then its flags.
//
// Parsed in a loop for the reason `mesh-host` records: the standard library stops at the FIRST
// non-flag argument, so a flag sitting after one is silently dropped and the command exits zero
// having ignored what it was asked. That fault has been paid for twice in this repository and is
// not being paid for a third time.
func parseArgs(args []string) (string, bootstrap.Options, bool, error) {
	opts := bootstrap.Options{
		Template: defaultTemplate,
		Out:      defaultOut,
		State:    store.DefaultPath,
		Registry: defaultRegistry,
		Host:     defaultHost,
		// The machine's own name, because that is what a person already calls it and an installer
		// inventing a different one would leave the mesh naming a machine nobody recognises. It is
		// read here rather than inside the bootstrap so that --node overrides a fact rather than a
		// default computed halfway through.
		Node:        hostname(),
		HostService: defaultService,
		// Longer than the host's 10s: these probes reach a container runtime that may be busy
		// pulling, and a probe that times out on a working machine is a false refusal.
		Timeout: 30 * time.Second,
		// A socket-activated runtime queued behind the network, and a control plane running its
		// first `initdb`-shaped wait, are both minutes rather than seconds.
		Wait: 3 * time.Minute,
	}
	var jsonOut bool

	command := "bootstrap"
	if len(args) > 0 && len(args[0]) > 0 && args[0][0] != '-' {
		command = args[0]
		args = args[1:]
	}

	set := newFlagSet(&opts, &jsonOut)
	var positionals []string
	rest := args
	for {
		if err := set.Parse(rest); err != nil {
			return "", opts, false, err
		}
		rest = set.Args()
		if len(rest) == 0 {
			break
		}
		positionals = append(positionals, rest[0])
		rest = rest[1:]
	}

	// Refused rather than ignored: a mistyped argument that changes nothing and reports success is
	// worse than an error, and this program's whole job is to change a machine.
	if len(positionals) > 0 {
		return "", opts, false, fmt.Errorf(
			"unexpected argument %q — try `mesh-bootstrap help`", positionals[0])
	}
	return command, opts, jsonOut, nil
}

func newFlagSet(opts *bootstrap.Options, jsonOut *bool) *flag.FlagSet {
	set := flag.NewFlagSet("mesh-bootstrap", flag.ContinueOnError)
	set.SetOutput(os.Stderr)
	set.Usage = func() { fmt.Fprint(os.Stderr, usage) }
	set.StringVar(&opts.Template, "bundle", opts.Template, "the substrate template to build from")
	set.StringVar(&opts.Out, "out", opts.Out, "where the produced bundle is written")
	set.StringVar(&opts.State, "state", opts.State, "where this node records what it has applied")
	set.StringVar(&opts.Catalogue, "catalog", opts.Catalogue,
		"a checkout of the mesh's catalogue; without it this stops after the substrate")
	set.StringVar(&opts.Node, "node", opts.Node, "the name this machine is known by")
	set.StringVar(&opts.Registry, "registry", opts.Registry, "where this mesh keeps its own images")
	set.StringVar(&opts.Host, "host", opts.Host, "the mesh-host binary on this machine")
	set.StringVar(&opts.HostService, "host-service", opts.HostService, "the unit that supervises it")
	set.BoolVar(&opts.HostInBackground, "host-in-background", false,
		"start the host unsupervised; it does not survive a reboot")
	set.StringVar(&opts.System, "system", opts.System, "which operating system this is")
	set.DurationVar(&opts.Timeout, "timeout", opts.Timeout, "how long any single probe may take")
	set.DurationVar(&opts.Wait, "wait", opts.Wait, "how long something merely starting is given")
	set.BoolVar(&opts.DryRun, "dry-run", false, "everything that does not change the machine")
	set.BoolVar(jsonOut, "json", false, "machine-readable output")
	return set
}

func run(ctx context.Context, command string, opts bootstrap.Options, jsonOut bool) error {
	switch command {
	case "bootstrap":
		say := func(line string) {
			if !jsonOut {
				fmt.Println(line)
			}
		}
		result, err := bootstrap.Run(ctx, opts, bootstrap.Deps{
			Run:   apply.ExecRunner,
			Dial:  dial,
			Fetch: fetch,
		}, say)

		// Printed whichever way it went. What the installer got through before it stopped is on
		// the machine either way, and a report that only exists on success describes a machine
		// nobody has (novox/hq ADR 0018).
		if jsonOut {
			encoder := json.NewEncoder(os.Stdout)
			encoder.SetIndent("", "  ")
			if encodeErr := encoder.Encode(result); encodeErr != nil && err == nil {
				return encodeErr
			}
		}
		return err

	case "version":
		fmt.Println(version)
		return nil

	case "help", "-h", "--help":
		fmt.Fprint(os.Stderr, usage)
		return nil

	default:
		return fmt.Errorf("unknown command %q — try `mesh-bootstrap help`", command)
	}
}

// hostname is what this machine calls itself, or empty.
//
// Empty rather than a guess: a machine that cannot say its own name is one the installer must be
// told about, and `mesh-bootstrap-0` would be a name in the mesh's records that matches nothing
// anybody types anywhere else. The refusal happens at step 6, where the name is first needed.
func hostname() string {
	name, err := os.Hostname()
	if err != nil {
		return ""
	}
	return name
}

// fetch asks an HTTP endpoint and reports what it said.
//
// Plain HTTP, and only at the mesh's own registry: it is reached over the mesh's private network,
// which is already the encrypted and authenticated thing, and a second layer inside it would be
// certificates to issue and rotate for no property the first does not have (mesh-control's
// `internal/builder` pushes to it on the same reasoning).
//
// The body is read with a limit. What is asked for is a status and a short JSON answer, and a
// registry that answered with a gigabyte would otherwise be an installer that never returns.
func fetch(ctx context.Context, url string) (int, string, error) {
	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return 0, "", err
	}
	response, err := http.DefaultClient.Do(request)
	if err != nil {
		return 0, "", err
	}
	defer response.Body.Close()

	said, err := io.ReadAll(io.LimitReader(response.Body, 1<<20))
	if err != nil {
		return response.StatusCode, "", err
	}
	return response.StatusCode, string(said), nil
}

// dial answers whether a TCP address responds.
//
// A connection rather than a ping or a name lookup: what has to work is a pull, and a pull opens a
// connection to exactly this address. A machine whose DNS resolves and whose route is missing
// passes a lookup and fails the thing that matters.
func dial(ctx context.Context, address string) error {
	var dialer net.Dialer
	conn, err := dialer.DialContext(ctx, "tcp", address)
	if err != nil {
		return err
	}
	return conn.Close()
}