// Package zfsadm performs the few mutating ZFS operations the UI offers
// (create / destroy datasets). Commands go through a Runner so they can be
// executed on the host via systemd-run: nasmon runs in a sandboxed mount
// namespace (ProtectSystem=strict), where a plain "zfs create" would mount the
// new dataset only inside the service and leave the host without it.
package zfsadm

import (
	"bufio"
	"context"
	"errors"
	"fmt"
	"regexp"
	"strconv"
	"strings"
	"time"

	"nasmon/internal/hostrun"
)

// Runner executes a command and returns its combined output; err is non-nil on
// a non-zero exit or when the command could not be started.
type Runner func(ctx context.Context, name string, args ...string) (out string, err error)

// NoInput adapts a hostrun.Runner (which takes stdin) to the Runner used here.
func NoInput(r hostrun.Runner) Runner {
	return func(ctx context.Context, name string, args ...string) (string, error) {
		return r(ctx, "", name, args...)
	}
}

// DirectRunner runs the command in-process (development, tests, non-systemd hosts).
var DirectRunner = NoInput(hostrun.Direct)

// HostRunner runs the command as a transient systemd unit, i.e. in the host's
// mount namespace and outside this service's sandbox.
var HostRunner = NoInput(hostrun.Host)

// AutoRunner picks HostRunner when systemd is managing the system and
// systemd-run is available, DirectRunner otherwise.
func AutoRunner() Runner { return NoInput(hostrun.Auto()) }

// Admin runs dataset operations through Run.
type Admin struct {
	Run     Runner
	Timeout time.Duration
}

func New(run Runner) *Admin { return &Admin{Run: run, Timeout: 60 * time.Second} }

func (a *Admin) ctx() (context.Context, context.CancelFunc) {
	t := a.Timeout
	if t <= 0 {
		t = 60 * time.Second
	}
	return context.WithTimeout(context.Background(), t)
}

// CreateOpts are the user-visible knobs of "zfs create".
type CreateOpts struct {
	Compression string // "", lz4, zstd, off ("" = inherit)
	Quota       string // "", or a size like 100G / 1.5T
	Owner       string // uid:gid applied to the mountpoint; "" = leave root
}

// Create creates dataset full (pool/name[/sub]) and chowns its mountpoint.
func (a *Admin) Create(full string, o CreateOpts) error {
	if err := ValidateFullName(full); err != nil {
		return err
	}
	if err := ValidateCompression(o.Compression); err != nil {
		return err
	}
	if err := ValidateSize(o.Quota); err != nil {
		return err
	}
	if err := ValidateOwner(o.Owner); err != nil {
		return err
	}
	args := []string{"create", "-p"}
	if o.Compression != "" {
		args = append(args, "-o", "compression="+o.Compression)
	}
	if o.Quota != "" {
		args = append(args, "-o", "quota="+o.Quota)
	}
	args = append(args, full)
	ctx, cancel := a.ctx()
	defer cancel()
	if _, err := a.Run(ctx, "zfs", args...); err != nil {
		return err
	}
	if o.Owner == "" {
		return nil
	}
	mp, err := a.Run(ctx, "zfs", "get", "-H", "-o", "value", "mountpoint", full)
	if err != nil {
		return fmt.Errorf("created, but mountpoint lookup failed: %w", err)
	}
	if !strings.HasPrefix(mp, "/") {
		return fmt.Errorf("created, but dataset is not mounted (mountpoint=%s); chown skipped", mp)
	}
	if _, err := a.Run(ctx, "chown", o.Owner, mp); err != nil {
		return fmt.Errorf("created, but chown failed: %w", err)
	}
	return nil
}

// EmptyRefer is the upper bound of "referenced" bytes for a dataset we still
// consider empty (a fresh dataset refers ~96-128 KiB of metadata).
const EmptyRefer = 1 << 20

// DestroyEmpty destroys full only if, on the live system, it has no children,
// no snapshots and refers less than EmptyRefer bytes.
func (a *Admin) DestroyEmpty(full string) error {
	if err := ValidateFullName(full); err != nil {
		return err
	}
	if !strings.Contains(full, "/") {
		return errors.New("refusing to destroy a pool root dataset")
	}
	ctx, cancel := a.ctx()
	defer cancel()
	out, err := a.Run(ctx, "zfs", "list", "-Hp", "-r", "-t", "filesystem", "-o", "name,refer", full)
	if err != nil {
		return err
	}
	sc := bufio.NewScanner(strings.NewReader(out))
	rows := 0
	var refer uint64
	for sc.Scan() {
		f := strings.Split(sc.Text(), "\t")
		if len(f) < 2 {
			continue
		}
		rows++
		if f[0] == full {
			refer, _ = strconv.ParseUint(f[1], 10, 64)
		}
	}
	if rows != 1 {
		return fmt.Errorf("%s has child datasets", full)
	}
	if refer >= EmptyRefer {
		return fmt.Errorf("%s is not empty (%d bytes referenced)", full, refer)
	}
	snaps, err := a.Run(ctx, "zfs", "list", "-H", "-t", "snapshot", "-o", "name", "-r", full)
	if err != nil {
		return err
	}
	if strings.TrimSpace(snaps) != "" {
		return fmt.Errorf("%s has snapshots", full)
	}
	_, err = a.Run(ctx, "zfs", "destroy", full)
	return err
}

// ---- validation (pure)

var (
	reComponent = regexp.MustCompile(`^[A-Za-z0-9_.:-]+$`)
	reSize      = regexp.MustCompile(`^[0-9]+(\.[0-9]+)?[KMGTPkmgtp]?$`)
	reOwner     = regexp.MustCompile(`^[0-9]+:[0-9]+$`)
)

// ValidateRelName checks a dataset name relative to its pool, e.g. "media/video".
func ValidateRelName(s string) error {
	if s == "" {
		return errors.New("name is required")
	}
	if len(s) > 200 {
		return errors.New("name is too long")
	}
	for _, c := range strings.Split(s, "/") {
		switch {
		case c == "":
			return errors.New("name must not start or end with / or contain //")
		case c == "." || c == "..":
			return errors.New("name must not contain . or .. components")
		case strings.HasPrefix(c, "-"):
			return errors.New("name components must not start with -")
		case !reComponent.MatchString(c):
			return errors.New("name may only contain letters, digits, _ . : - and /")
		}
	}
	return nil
}

// ValidateFullName checks "pool/name..." (or a bare pool).
func ValidateFullName(s string) error {
	return ValidateRelName(s)
}

func ValidateSize(s string) error {
	if s == "" || reSize.MatchString(s) {
		return nil
	}
	return errors.New("size must look like 100G, 1.5T or 512M")
}

func ValidateCompression(s string) error {
	switch s {
	case "", "lz4", "zstd", "off":
		return nil
	}
	return errors.New("compression must be inherit, lz4, zstd or off")
}

func ValidateOwner(s string) error {
	if s == "" || reOwner.MatchString(s) {
		return nil
	}
	return errors.New("owner must be numeric uid:gid, e.g. 1000:1000")
}