// mesh-nats-snapshot: a consistent copy of the mesh bus's streams, and the way back from one
// (novox/hq ADR 0235, to-be 43).
//
// The bus keeps everything it holds in JetStream: every stream, and every key-value bucket, which is
// a stream too (conditions and their history, calls, hand-acts, the controller's lease, assignments,
// events, every module's state). Copying the store's files while the server writes them is not a
// copy that can be trusted to restore — a block half-written, an index from before the block it
// indexes. The server has its own way to hand a stream out whole: the snapshot API, a chunked
// transfer with flow control that the server answers from a consistent view of the stream while
// it goes on taking messages. This program asks for it, one stream at a time, and nothing else.
//
// It lives in the bus's image because that is where it runs: the machine's backup holder runs the
// nats module's declared dump — `docker exec` into the server's container — and the archive comes
// out on stdout, so nothing is mounted into the server for it and the credential arrives on stdin.
//
//	mesh-nats-snapshot snapshot  [--server URL] [--credential FILE|-]   > bus.tar
//	mesh-nats-snapshot verify    [--from FILE]
//	mesh-nats-snapshot restore   --into DIR [--account MESH] [--from FILE]
//	mesh-nats-snapshot restore   --server URL --credential FILE [--from FILE]
//
// **Read-only on the live bus.** A snapshot neither pauses nor reconfigures a stream; the server
// keeps accepting writes while it streams the copy out. The credential it uses is the bus's own
// module's, granted the snapshot API and nothing that writes (mesh-controller internal/broker).
package main

import (
	"context"
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"time"
)

func main() {
	if len(os.Args) < 2 {
		usage()
		os.Exit(2)
	}
	var err error
	switch os.Args[1] {
	case "snapshot":
		err = runSnapshot(os.Args[2:])
	case "verify":
		err = runVerify(os.Args[2:])
	case "restore":
		err = runRestore(os.Args[2:])
	case "-h", "--help", "help":
		usage()
		return
	default:
		usage()
		os.Exit(2)
	}
	if err != nil {
		fmt.Fprintln(os.Stderr, "mesh-nats-snapshot:", err)
		os.Exit(1)
	}
}

func usage() {
	fmt.Fprint(os.Stderr, `mesh-nats-snapshot — a consistent copy of the bus's streams, and the way back

  snapshot  [--server tls://127.0.0.1:4222] [--credential -]       every stream, one at a time, as a tar on stdout
  verify    [--from bus.tar]                                       every archive against the manifest
  restore   --into DIR [--account MESH] [--from bus.tar]           into a new store directory, by a server of its own
  restore   --server URL --credential FILE [--from bus.tar]        into a running server that holds none of them

The credential is the JSON the mesh seals to the bus's own module: url, fingerprint, user, password.
`)
}

func runSnapshot(args []string) error {
	fs := flag.NewFlagSet("snapshot", flag.ContinueOnError)
	server := fs.String("server", "tls://127.0.0.1:4222", "the bus, as seen from where this runs")
	credential := fs.String("credential", "-", "the bus's own module's credential, a file or - for stdin")
	streamTimeout := fs.Duration("stream-timeout", 10*time.Minute, "the longest one stream may take")
	timeout := fs.Duration("timeout", 30*time.Minute, "the longest the whole snapshot may take")
	pause := fs.Duration("pause", 250*time.Millisecond, "a rest between two streams")
	tmp := fs.String("tmp", "", "where each stream's archive waits before it is written out (default: the system's)")
	if err := fs.Parse(args); err != nil {
		return err
	}
	cred, err := readCredential(*credential)
	if err != nil {
		return err
	}
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	nc, err := connect(*server, cred)
	if err != nil {
		return err
	}
	defer nc.Close()
	opts := SnapshotOptions{StreamTimeout: *streamTimeout, Pause: *pause, TempDir: *tmp, Log: logf}
	m, err := Snapshot(ctx, nc, os.Stdout, opts)
	if err != nil {
		return err
	}
	logf("snapshot: %d stream(s), %d message(s), %s of streams in %s of archives, %.1fs",
		len(m.Streams), m.Messages, human(m.StreamBytes), human(m.ArchiveBytes), m.Seconds)
	return nil
}

func runVerify(args []string) error {
	fs := flag.NewFlagSet("verify", flag.ContinueOnError)
	from := fs.String("from", "-", "the archive, a file or - for stdin")
	if err := fs.Parse(args); err != nil {
		return err
	}
	in, closer, err := openFrom(*from)
	if err != nil {
		return err
	}
	defer closer()
	a, err := ReadArchive(in, "")
	if err != nil {
		return err
	}
	defer a.Close()
	if err := a.Verify(); err != nil {
		return err
	}
	logf("verified: %d stream(s), taken %s, every archive matches the manifest and reads to its end",
		len(a.Manifest.Streams), a.Manifest.Finished.Format(time.RFC3339))
	return nil
}

func runRestore(args []string) error {
	fs := flag.NewFlagSet("restore", flag.ContinueOnError)
	from := fs.String("from", "-", "the archive, a file or - for stdin")
	into := fs.String("into", "", "a new store directory, filled by a server of this program's own on loopback")
	account := fs.String("account", "MESH", "the account the streams belong to (the bus's one account)")
	server := fs.String("server", "", "a running server to restore into instead; it must hold none of the streams")
	credential := fs.String("credential", "", "the credential for --server, a file")
	timeout := fs.Duration("timeout", time.Hour, "the longest the whole restore may take")
	if err := fs.Parse(args); err != nil {
		return err
	}
	if (*into == "") == (*server == "") {
		return errors.New("restore needs exactly one of --into (a new store directory) or --server (a running server)")
	}
	in, closer, err := openFrom(*from)
	if err != nil {
		return err
	}
	defer closer()
	a, err := ReadArchive(in, "")
	if err != nil {
		return err
	}
	defer a.Close()
	if err := a.Verify(); err != nil {
		return fmt.Errorf("the archive does not match its own manifest, so nothing is restored: %w", err)
	}
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	if *into != "" {
		where, err := RestoreOffline(ctx, a, *into, *account, logf)
		if err != nil {
			return err
		}
		logf("restored %d stream(s) into %s. With the bus stopped, put this directory in place of the "+
			"jetstream directory in the bus's store, then start it (novox/hq to-be 43).", len(a.Manifest.Streams), where)
		return nil
	}
	if *credential == "" {
		return errors.New("--server needs --credential")
	}
	cred, err := readCredential(*credential)
	if err != nil {
		return err
	}
	nc, err := connect(*server, cred)
	if err != nil {
		return err
	}
	defer nc.Close()
	return RestoreAll(ctx, nc, a, logf)
}

func openFrom(path string) (io.Reader, func(), error) {
	if path == "-" || path == "" {
		return os.Stdin, func() {}, nil
	}
	f, err := os.Open(path)
	if err != nil {
		return nil, nil, err
	}
	return f, func() { f.Close() }, nil
}

func logf(format string, args ...any) {
	fmt.Fprintf(os.Stderr, format+"\n", args...)
}

func human(n int64) string {
	const unit = 1024
	if n < unit {
		return fmt.Sprintf("%d B", n)
	}
	div, exp := int64(unit), 0
	for v := n / unit; v >= unit; v /= unit {
		div *= unit
		exp++
	}
	return fmt.Sprintf("%.1f %ciB", float64(n)/float64(div), "KMGTPE"[exp])
}