package vaultik

import (
	"errors"
	"fmt"
	"strings"

	"sneak.berlin/go/vaultik/internal/storage"
)

// destinationMarkerKey is the object every backup writes at the root of
// the backup destination store. While the local index holds a snapshot,
// a destination without it is treated as not present: an unplugged
// volume leaves an empty directory at its mount point, which would
// otherwise read as a store whose snapshots are all gone.
const destinationMarkerKey = "vaultik-destination.txt"

// destinationMarkerText is the whole content of the marker. It names no
// host, path or time.
const destinationMarkerText = "This is a vaultik backup destination store.\n"

// errDestinationNotPresent is returned when the local index holds a
// snapshot and the backup destination store has no marker.
var errDestinationNotPresent = errors.New("backup destination store is not present")

// checkDestinationMarker returns errDestinationNotPresent when the local
// index holds a snapshot and the backup destination store has no marker.
// With no snapshot in the local index, as on a first backup or on
// another machine, there is nothing to check.
func (v *Vaultik) checkDestinationMarker() error {
	ids, err := v.localSnapshotIDs()
	if err != nil {
		return fmt.Errorf("listing local snapshots: %w", err)
	}

	if len(ids) == 0 {
		return nil
	}

	_, err = v.Storage.Stat(v.ctx, destinationMarkerKey)
	if errors.Is(err, storage.ErrNotFound) {
		return fmt.Errorf("%w: %s has no %s, but the local index holds "+
			"snapshots stored there; mount the volume if it is removable, "+
			"or if the destination was emptied on purpose, run "+
			"'vaultik database delete' and back up again",
			errDestinationNotPresent, v.Storage.Info().Location,
			destinationMarkerKey)
	}

	if err != nil {
		return fmt.Errorf("checking for %s: %w", destinationMarkerKey, err)
	}

	return nil
}

// markDestination checks the backup destination store with
// checkDestinationMarker and then writes the marker, so a first backup
// marks its destination and a later one writes the same text again.
func (v *Vaultik) markDestination() error {
	err := v.checkDestinationMarker()
	if err != nil {
		return err
	}

	err = v.Storage.Put(v.ctx, destinationMarkerKey,
		strings.NewReader(destinationMarkerText))
	if err != nil {
		return fmt.Errorf("writing %s to the backup destination store: %w",
			destinationMarkerKey, err)
	}

	return nil
}