// Package resetpw implements the `webhooker resetpw` subcommand,
// which sets an existing account's password from the command line.
//
// It exists because the bootstrap password is shown exactly once. If it
// is lost — the boot's output rotated away, the terminal closed — the
// deployment has no other way in: there is no second account, no
// forgot-password flow, and no environment override. The only recovery
// before this command was deleting the row from webhooker.db with a
// SQLite client so the next start would re-seed.
//
// It operates on a stopped deployment only. The password is read from
// standard input or generated, never taken from argv, and it reuses the
// service's own Argon2id hashing rather than reimplementing it.
package resetpw

import (
	"bufio"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/fs"
	"log/slog"
	"os"
	"path/filepath"
	"strings"

	"gorm.io/gorm"
	"sneak.berlin/go/webhooker/internal/banner"
	"sneak.berlin/go/webhooker/internal/config"
	"sneak.berlin/go/webhooker/internal/database"
	"sneak.berlin/go/webhooker/internal/datadir"
)

// Name is the subcommand's name on the command line.
const Name = "resetpw"

const (
	// generatedPasswordLen is the length of a -generate password. It
	// is longer than the 16 characters the first boot generates: this
	// one is typed or pasted once by an operator recovering a
	// deployment, not carried around.
	generatedPasswordLen = 24

	// minPasswordLen is the shortest password accepted on standard
	// input. It is a floor against a stray keystroke or a truncated
	// pipe silently becoming the account's credential, not a password
	// policy.
	minPasswordLen = 8
)

// Exit statuses. Usage errors are distinguished from operational ones
// so that a script can tell "you called it wrong" from "it refused".
const (
	exitOK      = 0
	exitFailure = 1
	exitUsage   = 2
)

// Sentinel errors, exported so a test can assert on the refusal rather
// than on the wording of a message.
var (
	// ErrNoDataDir reports that DATA_DIR names nothing, or names
	// something that is not a directory.
	ErrNoDataDir = errors.New("no data directory")

	// ErrNoDatabase reports that the data directory holds no main
	// database, so there is no deployment to reset a password in.
	ErrNoDatabase = errors.New("no webhooker database")

	// ErrLiveInstance reports that a running webhooker holds the data
	// directory.
	ErrLiveInstance = errors.New(
		"data directory is held by a running webhooker",
	)

	// ErrNoSuchUser reports that the named account does not exist.
	ErrNoSuchUser = errors.New("no such user")

	// ErrEmptyPassword reports that standard input carried nothing.
	ErrEmptyPassword = errors.New("empty password")

	// ErrPasswordTooShort reports a password below minPasswordLen.
	ErrPasswordTooShort = errors.New("password too short")

	// ErrNotUpdated reports that the update matched no row, which
	// means the account disappeared between the lookup and the write.
	ErrNotUpdated = errors.New("password was not updated")
)

// usage describes the subcommand. It is written to the same stream as
// the error that provoked it.
func usage(w io.Writer, flags *flag.FlagSet) {
	_, _ = fmt.Fprintf(w, `usage: webhooker %s [-generate] <username>

Set an existing account's password. The deployment must be stopped:
webhooker %s takes the same exclusive DATA_DIR lock the server does
and refuses to run while a live instance holds it.

The password is read as one line from standard input, or generated
with -generate. It is never taken as a command-line argument, which
on Linux would publish it in /proc to every account on the host.

DATA_DIR selects the deployment exactly as it does for the server
(default %s). The directory and its database must already exist;
nothing is created.

Flags:
`, Name, Name, config.DefaultDataDir)

	flags.PrintDefaults()
}

// Run executes the subcommand and returns the process exit status.
func Run(
	args []string,
	stdin io.Reader,
	stdout, stderr io.Writer,
) int {
	flags := flag.NewFlagSet("webhooker "+Name, flag.ContinueOnError)
	flags.SetOutput(stderr)

	generate := flags.Bool(
		"generate", false,
		"generate a random password instead of reading one from "+
			"standard input, and print it",
	)

	flags.Usage = func() { usage(stderr, flags) }

	err := flags.Parse(args)
	if err != nil {
		// flag has already reported the error and printed the usage.
		return exitUsage
	}

	if flags.NArg() != 1 {
		_, _ = fmt.Fprintf(
			stderr,
			"webhooker %s: exactly one username is required\n",
			Name,
		)
		usage(stderr, flags)

		return exitUsage
	}

	err = reset(flags.Arg(0), *generate, stdin, stdout, stderr)
	if err != nil {
		_, _ = fmt.Fprintf(stderr, "webhooker %s: %v\n", Name, err)

		return exitFailure
	}

	return exitOK
}

// reset performs the whole operation against the configured data
// directory.
func reset(
	username string,
	generate bool,
	stdin io.Reader,
	stdout, stderr io.Writer,
) error {
	dir := config.DataDir()

	err := checkDataDir(dir)
	if err != nil {
		return err
	}

	lock, err := acquire(dir)
	if err != nil {
		return err
	}

	// The kernel drops the lock when this process exits, whatever
	// happens below; releasing explicitly is what makes a long-running
	// caller — a test — see it freed. A release error tells the
	// operator nothing they can act on.
	defer func() { _ = lock.Release() }()

	db, err := database.Open(dir, cliLogger(stderr))
	if err != nil {
		return err
	}

	password, err := setPassword(db, username, generate, stdin, stderr)

	closeErr := db.Close()

	if err != nil {
		return err
	}

	if closeErr != nil {
		return fmt.Errorf("closing the database: %w", closeErr)
	}

	return report(stdout, stderr, dir, username, password, generate)
}

// report tells the operator what happened.
//
// A generated password is printed in the same banner the first boot
// uses, because this is the only time it is ever shown. A password the
// operator supplied is not echoed back: they already have it, and
// writing it to standard output a second time would put it in another
// log for no gain.
func report(
	stdout, stderr io.Writer,
	dir, username, password string,
	generate bool,
) error {
	if generate {
		write := func(w io.Writer) error {
			return banner.Credentials(
				w,
				"WEBHOOKER PASSWORD RESET: this account's new "+
					"password is",
				username,
				password,
				"Save this password now: it is shown only here.\n"+
					"The database stores only its Argon2id hash.",
			)
		}

		err := write(stdout)
		if err == nil {
			return nil
		}

		// The password is already stored. Standard output failing
		// here is the difference between a recovered deployment and
		// one locked out behind a password nobody has ever seen, so
		// try the other stream before giving up.
		if write(stderr) == nil {
			return nil
		}

		return err
	}

	_, err := fmt.Fprintf(
		stdout,
		"password updated for user %q in %s\n", username, dir,
	)
	if err != nil {
		return fmt.Errorf("writing the result: %w", err)
	}

	return nil
}

// acquire takes the DATA_DIR lock, translating the contended case into
// the refusal this command owes the operator.
//
// Resetting a password underneath a live instance would not corrupt
// anything, but the running process keeps serving every session that
// authenticated with the old one, so the operator would be told the
// password changed while the deployment still behaved as though it had
// not. Refusing is also what the lock is for.
func acquire(dir string) (*datadir.Lock, error) {
	lock, err := datadir.Acquire(dir)
	if err == nil {
		return lock, nil
	}

	if errors.Is(err, datadir.ErrLocked) {
		return nil, fmt.Errorf(
			"%w: %s. Stop it and run this again: a password reset "+
				"does not reach a running process, whose existing "+
				"sessions stay authenticated",
			ErrLiveInstance, dir,
		)
	}

	return nil, err
}

// checkDataDir refuses to act on a path that does not already hold a
// deployment.
//
// This runs before datadir.Acquire on purpose. Acquire calls
// os.MkdirAll, so a mistyped DATA_DIR would otherwise be built out —
// the directory tree, the lock file, and then an empty migrated
// database — and the command would report success against a deployment
// that does not exist while the real one stayed locked out. Nothing
// here creates anything.
func checkDataDir(dir string) error {
	info, err := os.Stat(dir)

	switch {
	case errors.Is(err, fs.ErrNotExist):
		return fmt.Errorf(
			"%w: %s (DATA_DIR). This acts on an existing "+
				"deployment and creates nothing",
			ErrNoDataDir, dir,
		)
	case err != nil:
		return fmt.Errorf("checking data directory %s: %w", dir, err)
	case !info.IsDir():
		return fmt.Errorf(
			"%w: %s (DATA_DIR) is not a directory", ErrNoDataDir, dir,
		)
	}

	dbPath := filepath.Join(dir, database.MainDBFileName)

	_, err = os.Stat(dbPath)

	switch {
	case errors.Is(err, fs.ErrNotExist):
		return fmt.Errorf(
			"%w: %s does not exist. The admin account is created by "+
				"the first server start",
			ErrNoDatabase, dbPath,
		)
	case err != nil:
		return fmt.Errorf("checking %s: %w", dbPath, err)
	}

	return nil
}

// setPassword looks the account up, obtains the new password, and
// writes its hash.
//
// The order matters: the account is resolved before an operator is
// asked to type anything, and the hash is computed in full before the
// single UPDATE that stores it. A failure at any step therefore leaves
// the stored credential exactly as it was — there is no half-written
// state to recover from.
func setPassword(
	db *database.Database,
	username string,
	generate bool,
	stdin io.Reader,
	stderr io.Writer,
) (string, error) {
	var user database.User

	err := db.DB().Where("username = ?", username).First(&user).Error
	if errors.Is(err, gorm.ErrRecordNotFound) {
		return "", fmt.Errorf(
			"%w: %q. This changes an existing account's password "+
				"and never creates an account",
			ErrNoSuchUser, username,
		)
	}

	if err != nil {
		return "", fmt.Errorf("looking up user %q: %w", username, err)
	}

	password, err := newPassword(generate, stdin, stderr)
	if err != nil {
		return "", err
	}

	hash, err := database.HashPassword(password)
	if err != nil {
		return "", fmt.Errorf("hashing the new password: %w", err)
	}

	result := db.DB().Model(&database.User{}).
		Where("id = ?", user.ID).
		Update("password", hash)
	if result.Error != nil {
		return "", fmt.Errorf(
			"updating user %q: %w", username, result.Error,
		)
	}

	if result.RowsAffected != 1 {
		return "", fmt.Errorf(
			"%w: %q matched %d rows",
			ErrNotUpdated, username, result.RowsAffected,
		)
	}

	return password, nil
}

// newPassword returns the password to store: generated, or read from
// standard input.
func newPassword(
	generate bool,
	stdin io.Reader,
	stderr io.Writer,
) (string, error) {
	if generate {
		password, err := database.GenerateRandomPassword(
			generatedPasswordLen,
		)
		if err != nil {
			return "", fmt.Errorf("generating a password: %w", err)
		}

		return password, nil
	}

	return readPassword(stdin, stderr)
}

// readPassword reads the new password as one line from standard input,
// minus its line ending.
//
// Standard input rather than an argument: on Linux argv is readable
// through /proc by every account on the host for as long as the process
// lives, and a password typed as an argument lands in shell history
// besides.
//
// When standard input is a terminal the input is echoed — no attempt is
// made to put the terminal into no-echo mode — so the prompt says so
// rather than letting an operator assume otherwise.
func readPassword(stdin io.Reader, stderr io.Writer) (string, error) {
	if f, ok := stdin.(*os.File); ok && isTerminal(f) {
		_, _ = fmt.Fprintf(
			stderr,
			"New password (echoed as you type), then Enter: ",
		)
	}

	line, err := bufio.NewReader(stdin).ReadString('\n')
	if err != nil && !errors.Is(err, io.EOF) {
		return "", fmt.Errorf(
			"reading the password from standard input: %w", err,
		)
	}

	password := strings.TrimRight(line, "\r\n")

	if password == "" {
		return "", fmt.Errorf(
			"%w: standard input carried no password. Pipe one in, "+
				"or pass -generate",
			ErrEmptyPassword,
		)
	}

	if len(password) < minPasswordLen {
		return "", fmt.Errorf(
			"%w: %d bytes, minimum %d",
			ErrPasswordTooShort, len(password), minPasswordLen,
		)
	}

	return password, nil
}

// isTerminal reports whether f is a character device, which is as much
// as this needs to know to decide whether to prompt.
func isTerminal(f *os.File) bool {
	info, err := f.Stat()
	if err != nil {
		return false
	}

	return info.Mode()&os.ModeCharDevice != 0
}

// cliLogger builds the logger the database layer writes through while
// this command runs. It is deliberately quiet: connecting and migrating
// are steps the operator did not ask about, and the one thing they need
// to see is the outcome on standard output.
func cliLogger(stderr io.Writer) *slog.Logger {
	return slog.New(slog.NewTextHandler(stderr, &slog.HandlerOptions{
		Level: slog.LevelWarn,
	}))
}