/*
 * Copyright (c) 2015, Psiphon Inc.
 * All rights reserved.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package psiphon

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"math/rand"
	"sync"

	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/parameters"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/protocol"
)

var (
	datastoreServerEntriesBucket                = []byte("serverEntries")
	datastoreSplitTunnelRouteETagsBucket        = []byte("splitTunnelRouteETags")
	datastoreSplitTunnelRouteDataBucket         = []byte("splitTunnelRouteData")
	datastoreUrlETagsBucket                     = []byte("urlETags")
	datastoreKeyValueBucket                     = []byte("keyValues")
	datastoreRemoteServerListStatsBucket        = []byte("remoteServerListStats")
	datastoreSLOKsBucket                        = []byte("SLOKs")
	datastoreTacticsBucket                      = []byte("tactics")
	datastoreSpeedTestSamplesBucket             = []byte("speedTestSamples")
	datastoreLastConnectedKey                   = "lastConnected"
	datastoreLastServerEntryFilterKey           = []byte("lastServerEntryFilter")
	datastoreAffinityServerEntryIDKey           = []byte("affinityServerEntryID")
	datastorePersistentStatTypeRemoteServerList = string(datastoreRemoteServerListStatsBucket)
	datastoreServerEntryFetchGCThreshold        = 20

	datastoreMutex    sync.RWMutex
	activeDatastoreDB *datastoreDB
)

// OpenDataStore opens and initializes the singleton data store instance.
func OpenDataStore(config *Config) error {

	datastoreMutex.Lock()

	existingDB := activeDatastoreDB

	if existingDB != nil {
		datastoreMutex.Unlock()
		return common.ContextError(errors.New("db already open"))
	}

	newDB, err := datastoreOpenDB(config.DataStoreDirectory)
	if err != nil {
		datastoreMutex.Unlock()
		return common.ContextError(err)
	}

	activeDatastoreDB = newDB

	datastoreMutex.Unlock()

	_ = resetAllPersistentStatsToUnreported()

	return nil
}

// CloseDataStore closes the singleton data store instance, if open.
func CloseDataStore() {

	datastoreMutex.Lock()
	defer datastoreMutex.Unlock()

	if activeDatastoreDB == nil {
		return
	}

	err := activeDatastoreDB.close()
	if err != nil {
		NoticeAlert("failed to close database: %s", common.ContextError(err))
	}

	activeDatastoreDB = nil
}

func datastoreView(fn func(tx *datastoreTx) error) error {

	datastoreMutex.RLock()
	defer datastoreMutex.RUnlock()

	if activeDatastoreDB == nil {
		return common.ContextError(errors.New("database not open"))
	}

	err := activeDatastoreDB.view(fn)
	if err != nil {
		err = common.ContextError(err)
	}
	return err
}

func datastoreUpdate(fn func(tx *datastoreTx) error) error {

	datastoreMutex.RLock()
	defer datastoreMutex.RUnlock()

	if activeDatastoreDB == nil {
		return common.ContextError(errors.New("database not open"))
	}

	err := activeDatastoreDB.update(fn)
	if err != nil {
		err = common.ContextError(err)
	}
	return err
}

// StoreServerEntry adds the server entry to the data store.
//
// When a server entry already exists for a given server, it will be
// replaced only if replaceIfExists is set or if the the ConfigurationVersion
// field of the new entry is strictly higher than the existing entry.
//
// If the server entry data is malformed, an alert notice is issued and
// the entry is skipped; no error is returned.
func StoreServerEntry(serverEntryFields protocol.ServerEntryFields, replaceIfExists bool) error {

	// Server entries should already be validated before this point,
	// so instead of skipping we fail with an error.
	err := protocol.ValidateServerEntryFields(serverEntryFields)
	if err != nil {
		return common.ContextError(
			fmt.Errorf("invalid server entry: %s", err))
	}

	// BoltDB implementation note:
	// For simplicity, we don't maintain indexes on server entry
	// region or supported protocols. Instead, we perform full-bucket
	// scans with a filter. With a small enough database (thousands or
	// even tens of thousand of server entries) and common enough
	// values (e.g., many servers support all protocols), performance
	// is expected to be acceptable.

	err = datastoreUpdate(func(tx *datastoreTx) error {

		serverEntries := tx.bucket(datastoreServerEntriesBucket)

		ipAddress := serverEntryFields.GetIPAddress()

		// Check not only that the entry exists, but is valid. This
		// will replace in the rare case where the data is corrupt.
		existingConfigurationVersion := -1
		existingData := serverEntries.get([]byte(ipAddress))
		if existingData != nil {
			var existingServerEntry *protocol.ServerEntry
			err := json.Unmarshal(existingData, &existingServerEntry)
			if err == nil {
				existingConfigurationVersion = existingServerEntry.ConfigurationVersion
			}
		}

		exists := existingConfigurationVersion > -1
		newer := exists && existingConfigurationVersion < serverEntryFields.GetConfigurationVersion()
		update := !exists || replaceIfExists || newer

		if !update {
			// Disabling this notice, for now, as it generates too much noise
			// in diagnostics with clients that always submit embedded servers
			// to the core on each run.
			// NoticeInfo("ignored update for server %s", serverEntry.IpAddress)
			return nil
		}

		data, err := json.Marshal(serverEntryFields)
		if err != nil {
			return common.ContextError(err)
		}
		err = serverEntries.put([]byte(ipAddress), data)
		if err != nil {
			return common.ContextError(err)
		}

		NoticeInfo("updated server %s", ipAddress)

		return nil
	})
	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// StoreServerEntries stores a list of server entries.
// There is an independent transaction for each entry insert/update.
func StoreServerEntries(
	config *Config,
	serverEntries []protocol.ServerEntryFields,
	replaceIfExists bool) error {

	for _, serverEntryFields := range serverEntries {
		err := StoreServerEntry(serverEntryFields, replaceIfExists)
		if err != nil {
			return common.ContextError(err)
		}
	}

	return nil
}

// StreamingStoreServerEntries stores a list of server entries.
// There is an independent transaction for each entry insert/update.
func StreamingStoreServerEntries(
	config *Config,
	serverEntries *protocol.StreamingServerEntryDecoder,
	replaceIfExists bool) error {

	// Note: both StreamingServerEntryDecoder.Next and StoreServerEntry
	// allocate temporary memory buffers for hex/JSON decoding/encoding,
	// so this isn't true constant-memory streaming (it depends on garbage
	// collection).

	n := 0
	for {
		serverEntry, err := serverEntries.Next()
		if err != nil {
			return common.ContextError(err)
		}

		if serverEntry == nil {
			// No more server entries
			break
		}

		err = StoreServerEntry(serverEntry, replaceIfExists)
		if err != nil {
			return common.ContextError(err)
		}

		n += 1
		if n == datastoreServerEntryFetchGCThreshold {
			DoGarbageCollection()
			n = 0
		}
	}

	return nil
}

// PromoteServerEntry sets the server affinity server entry ID to the
// specified server entry IP address.
func PromoteServerEntry(config *Config, ipAddress string) error {
	err := datastoreUpdate(func(tx *datastoreTx) error {

		serverEntryID := []byte(ipAddress)

		// Ensure the corresponding server entry exists before
		// setting server affinity.
		bucket := tx.bucket(datastoreServerEntriesBucket)
		data := bucket.get(serverEntryID)
		if data == nil {
			NoticeAlert(
				"PromoteServerEntry: ignoring unknown server entry: %s",
				ipAddress)
			return nil
		}

		bucket = tx.bucket(datastoreKeyValueBucket)
		err := bucket.put(datastoreAffinityServerEntryIDKey, serverEntryID)
		if err != nil {
			return err
		}

		// Store the current server entry filter (e.g, region, etc.) that
		// was in use when the entry was promoted. This is used to detect
		// when the top ranked server entry was promoted under a different
		// filter.

		currentFilter, err := makeServerEntryFilterValue(config)
		if err != nil {
			return err
		}

		return bucket.put(datastoreLastServerEntryFilterKey, currentFilter)
	})

	if err != nil {
		return common.ContextError(err)
	}
	return nil
}

func makeServerEntryFilterValue(config *Config) ([]byte, error) {

	// Currently, only a change of EgressRegion will "break" server affinity.
	// If the tunnel protocol filter changes, any existing affinity server
	// either passes the new filter, or it will be skipped anyway.

	return []byte(config.EgressRegion), nil
}

func hasServerEntryFilterChanged(config *Config) (bool, error) {

	currentFilter, err := makeServerEntryFilterValue(config)
	if err != nil {
		return false, common.ContextError(err)
	}

	changed := false
	err = datastoreView(func(tx *datastoreTx) error {

		// previousFilter will be nil not found (not previously
		// set) which will never match any current filter.

		bucket := tx.bucket(datastoreKeyValueBucket)
		previousFilter := bucket.get(datastoreLastServerEntryFilterKey)
		if bytes.Compare(previousFilter, currentFilter) != 0 {
			changed = true
		}
		return nil
	})
	if err != nil {
		return false, common.ContextError(err)
	}

	return changed, nil
}

// ServerEntryIterator is used to iterate over
// stored server entries in rank order.
type ServerEntryIterator struct {
	config                       *Config
	applyServerAffinity          bool
	serverEntryIDs               [][]byte
	serverEntryIndex             int
	isTacticsServerEntryIterator bool
	isTargetServerEntryIterator  bool
	hasNextTargetServerEntry     bool
	targetServerEntry            *protocol.ServerEntry
}

// NewServerEntryIterator creates a new ServerEntryIterator.
//
// The boolean return value indicates whether to treat the first server(s)
// as affinity servers or not. When the server entry selection filter changes
// such as from a specific region to any region, or when there was no previous
// filter/iterator, the the first server(s) are arbitrary and should not be
// given affinity treatment.
//
// NewServerEntryIterator and any returned ServerEntryIterator are not
// designed for concurrent use as not all related datastore operations are
// performed in a single transaction.
//
func NewServerEntryIterator(config *Config) (bool, *ServerEntryIterator, error) {

	// When configured, this target server entry is the only candidate
	if config.TargetServerEntry != "" {
		return newTargetServerEntryIterator(config, false)
	}

	filterChanged, err := hasServerEntryFilterChanged(config)
	if err != nil {
		return false, nil, common.ContextError(err)
	}

	applyServerAffinity := !filterChanged

	iterator := &ServerEntryIterator{
		config:              config,
		applyServerAffinity: applyServerAffinity,
	}

	err = iterator.Reset()
	if err != nil {
		return false, nil, common.ContextError(err)
	}

	return applyServerAffinity, iterator, nil
}

func NewTacticsServerEntryIterator(config *Config) (*ServerEntryIterator, error) {

	// When configured, this target server entry is the only candidate
	if config.TargetServerEntry != "" {
		_, iterator, err := newTargetServerEntryIterator(config, true)
		return iterator, err
	}

	iterator := &ServerEntryIterator{
		isTacticsServerEntryIterator: true,
	}

	err := iterator.Reset()
	if err != nil {
		return nil, common.ContextError(err)
	}

	return iterator, nil
}

// newTargetServerEntryIterator is a helper for initializing the TargetServerEntry case
func newTargetServerEntryIterator(config *Config, isTactics bool) (bool, *ServerEntryIterator, error) {

	serverEntry, err := protocol.DecodeServerEntry(
		config.TargetServerEntry, common.GetCurrentTimestamp(), protocol.SERVER_ENTRY_SOURCE_TARGET)
	if err != nil {
		return false, nil, common.ContextError(err)
	}

	if isTactics {

		if len(serverEntry.GetSupportedTacticsProtocols()) == 0 {
			return false, nil, common.ContextError(errors.New("TargetServerEntry does not support tactics protocols"))
		}

	} else {

		if config.EgressRegion != "" && serverEntry.Region != config.EgressRegion {
			return false, nil, common.ContextError(errors.New("TargetServerEntry does not support EgressRegion"))
		}

		limitTunnelProtocols := config.clientParameters.Get().TunnelProtocols(parameters.LimitTunnelProtocols)
		if len(limitTunnelProtocols) > 0 {
			// At the ServerEntryIterator level, only limitTunnelProtocols is applied;
			// excludeIntensive is handled higher up.
			if len(serverEntry.GetSupportedProtocols(
				config.UseUpstreamProxy(), limitTunnelProtocols, false)) == 0 {
				return false, nil, common.ContextError(errors.New("TargetServerEntry does not support LimitTunnelProtocols"))
			}
		}
	}

	iterator := &ServerEntryIterator{
		isTacticsServerEntryIterator: isTactics,
		isTargetServerEntryIterator:  true,
		hasNextTargetServerEntry:     true,
		targetServerEntry:            serverEntry,
	}

	NoticeInfo("using TargetServerEntry: %s", serverEntry.IpAddress)

	return false, iterator, nil
}

// Reset a NewServerEntryIterator to the start of its cycle. The next
// call to Next will return the first server entry.
func (iterator *ServerEntryIterator) Reset() error {
	iterator.Close()

	if iterator.isTargetServerEntryIterator {
		iterator.hasNextTargetServerEntry = true
		return nil
	}

	// BoltDB implementation note:
	// We don't keep a transaction open for the duration of the iterator
	// because this would expose the following semantics to consumer code:
	//
	//     Read-only transactions and read-write transactions ... generally
	//     shouldn't be opened simultaneously in the same goroutine. This can
	//     cause a deadlock as the read-write transaction needs to periodically
	//     re-map the data file but it cannot do so while a read-only
	//     transaction is open.
	//     (https://github.com/boltdb/bolt)
	//
	// So the underlying serverEntriesBucket could change after the serverEntryIDs
	// list is built.

	var serverEntryIDs [][]byte

	err := datastoreView(func(tx *datastoreTx) error {

		bucket := tx.bucket(datastoreKeyValueBucket)

		serverEntryIDs = make([][]byte, 0)
		shuffleHead := 0

		var affinityServerEntryID []byte
		if iterator.applyServerAffinity {
			affinityServerEntryID = bucket.get(datastoreAffinityServerEntryIDKey)
			if affinityServerEntryID != nil {
				serverEntryIDs = append(serverEntryIDs, append([]byte(nil), affinityServerEntryID...))
				shuffleHead = 1
			}
		}

		bucket = tx.bucket(datastoreServerEntriesBucket)
		cursor := bucket.cursor()
		for key := cursor.firstKey(); key != nil; key = cursor.nextKey() {
			if affinityServerEntryID != nil {
				if bytes.Equal(affinityServerEntryID, key) {
					continue
				}
			}
			serverEntryIDs = append(serverEntryIDs, append([]byte(nil), key...))
		}
		cursor.close()

		for i := len(serverEntryIDs) - 1; i > shuffleHead-1; i-- {
			j := rand.Intn(i+1-shuffleHead) + shuffleHead
			serverEntryIDs[i], serverEntryIDs[j] = serverEntryIDs[j], serverEntryIDs[i]
		}

		return nil
	})
	if err != nil {
		return common.ContextError(err)
	}

	iterator.serverEntryIDs = serverEntryIDs
	iterator.serverEntryIndex = 0

	return nil
}

// Close cleans up resources associated with a ServerEntryIterator.
func (iterator *ServerEntryIterator) Close() {
	iterator.serverEntryIDs = nil
	iterator.serverEntryIndex = 0
}

// Next returns the next server entry, by rank, for a ServerEntryIterator.
// Returns nil with no error when there is no next item.
func (iterator *ServerEntryIterator) Next() (*protocol.ServerEntry, error) {

	var serverEntry *protocol.ServerEntry
	var err error

	defer func() {
		if err != nil {
			iterator.Close()
		}
	}()

	if iterator.isTargetServerEntryIterator {
		if iterator.hasNextTargetServerEntry {
			iterator.hasNextTargetServerEntry = false
			return MakeCompatibleServerEntry(iterator.targetServerEntry), nil
		}
		return nil, nil
	}

	// There are no region/protocol indexes for the server entries bucket.
	// Loop until we have the next server entry that matches the iterator
	// filter requirements.
	for {
		if iterator.serverEntryIndex >= len(iterator.serverEntryIDs) {
			// There is no next item
			return nil, nil
		}

		serverEntryID := iterator.serverEntryIDs[iterator.serverEntryIndex]
		iterator.serverEntryIndex += 1

		var data []byte

		err = datastoreView(func(tx *datastoreTx) error {
			bucket := tx.bucket(datastoreServerEntriesBucket)
			value := bucket.get(serverEntryID)
			if value != nil {
				// Must make a copy as slice is only valid within transaction.
				data = make([]byte, len(value))
				copy(data, value)
			}
			return nil
		})
		if err != nil {
			return nil, common.ContextError(err)
		}

		if data == nil {
			// In case of data corruption or a bug causing this condition,
			// do not stop iterating.
			NoticeAlert("ServerEntryIterator.Next: unexpected missing server entry: %s", string(serverEntryID))
			continue
		}

		err = json.Unmarshal(data, &serverEntry)
		if err != nil {
			// In case of data corruption or a bug causing this condition,
			// do not stop iterating.
			NoticeAlert("ServerEntryIterator.Next: %s", common.ContextError(err))
			continue
		}

		if iterator.serverEntryIndex%datastoreServerEntryFetchGCThreshold == 0 {
			DoGarbageCollection()
		}

		// Check filter requirements

		if iterator.isTacticsServerEntryIterator {

			// Tactics doesn't filter by egress region.
			if len(serverEntry.GetSupportedTacticsProtocols()) > 0 {
				break
			}

		} else {

			if iterator.config.EgressRegion == "" ||
				serverEntry.Region == iterator.config.EgressRegion {
				break
			}
		}
	}

	return MakeCompatibleServerEntry(serverEntry), nil
}

// MakeCompatibleServerEntry provides backwards compatibility with old server entries
// which have a single meekFrontingDomain and not a meekFrontingAddresses array.
// By copying this one meekFrontingDomain into meekFrontingAddresses, this client effectively
// uses that single value as legacy clients do.
func MakeCompatibleServerEntry(serverEntry *protocol.ServerEntry) *protocol.ServerEntry {
	if len(serverEntry.MeekFrontingAddresses) == 0 && serverEntry.MeekFrontingDomain != "" {
		serverEntry.MeekFrontingAddresses =
			append(serverEntry.MeekFrontingAddresses, serverEntry.MeekFrontingDomain)
	}

	return serverEntry
}

func scanServerEntries(scanner func(*protocol.ServerEntry)) error {
	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreServerEntriesBucket)
		cursor := bucket.cursor()
		n := 0
		for key, value := cursor.first(); key != nil; key, value = cursor.next() {
			var serverEntry *protocol.ServerEntry
			err := json.Unmarshal(value, &serverEntry)
			if err != nil {
				// In case of data corruption or a bug causing this condition,
				// do not stop iterating.
				NoticeAlert("scanServerEntries: %s", common.ContextError(err))
				continue
			}
			scanner(serverEntry)

			n += 1
			if n == datastoreServerEntryFetchGCThreshold {
				DoGarbageCollection()
				n = 0
			}
		}
		cursor.close()
		return nil
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// CountServerEntries returns a count of stored server entries.
func CountServerEntries() int {
	count := 0
	err := scanServerEntries(func(_ *protocol.ServerEntry) {
		count += 1
	})

	if err != nil {
		NoticeAlert("CountServerEntries failed: %s", err)
		return 0
	}

	return count
}

// CountServerEntriesWithLimits returns a count of stored server entries for
// the specified region and tunnel protocol limits.
func CountServerEntriesWithLimits(
	useUpstreamProxy bool, region string, limitState *limitTunnelProtocolsState) (int, int) {

	// When CountServerEntriesWithLimits is called only
	// limitTunnelProtocolState is fixed; excludeIntensive is transitory.
	excludeIntensive := false

	initialCount := 0
	count := 0
	err := scanServerEntries(func(serverEntry *protocol.ServerEntry) {
		if region == "" || serverEntry.Region == region {

			if limitState.isInitialCandidate(excludeIntensive, serverEntry) {
				initialCount += 1
			}

			if limitState.isCandidate(excludeIntensive, serverEntry) {
				count += 1
			}

		}
	})

	if err != nil {
		NoticeAlert("CountServerEntriesWithLimits failed: %s", err)
		return 0, 0
	}

	return initialCount, count
}

// ReportAvailableRegions prints a notice with the available egress regions.
// When limitState has initial protocols, the available regions are limited
// to those available for the initial protocols; or if limitState has general
// limited protocols, the available regions are similarly limited.
func ReportAvailableRegions(config *Config, limitState *limitTunnelProtocolsState) {

	// When ReportAvailableRegions is called only limitTunnelProtocolState is
	// fixed; excludeIntensive is transitory.
	excludeIntensive := false

	regions := make(map[string]bool)
	err := scanServerEntries(func(serverEntry *protocol.ServerEntry) {

		isCandidate := false
		if limitState.hasInitialProtocols() {
			isCandidate = limitState.isInitialCandidate(excludeIntensive, serverEntry)
		} else {
			isCandidate = limitState.isCandidate(excludeIntensive, serverEntry)
		}

		if isCandidate {
			regions[serverEntry.Region] = true
		}
	})

	if err != nil {
		NoticeAlert("ReportAvailableRegions failed: %s", err)
		return
	}

	regionList := make([]string, 0, len(regions))
	for region := range regions {
		// Some server entries do not have a region, but it makes no sense to return
		// an empty string as an "available region".
		if region != "" {
			regionList = append(regionList, region)
		}
	}

	NoticeAvailableEgressRegions(regionList)
}

// SetSplitTunnelRoutes updates the cached routes data for
// the given region. The associated etag is also stored and
// used to make efficient web requests for updates to the data.
func SetSplitTunnelRoutes(region, etag string, data []byte) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSplitTunnelRouteETagsBucket)
		err := bucket.put([]byte(region), []byte(etag))

		bucket = tx.bucket(datastoreSplitTunnelRouteDataBucket)
		err = bucket.put([]byte(region), data)
		return err
	})

	if err != nil {
		return common.ContextError(err)
	}
	return nil
}

// GetSplitTunnelRoutesETag retrieves the etag for cached routes
// data for the specified region. If not found, it returns an empty string value.
func GetSplitTunnelRoutesETag(region string) (string, error) {

	var etag string

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSplitTunnelRouteETagsBucket)
		etag = string(bucket.get([]byte(region)))
		return nil
	})

	if err != nil {
		return "", common.ContextError(err)
	}
	return etag, nil
}

// GetSplitTunnelRoutesData retrieves the cached routes data
// for the specified region. If not found, it returns a nil value.
func GetSplitTunnelRoutesData(region string) ([]byte, error) {

	var data []byte

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSplitTunnelRouteDataBucket)
		value := bucket.get([]byte(region))
		if value != nil {
			// Must make a copy as slice is only valid within transaction.
			data = make([]byte, len(value))
			copy(data, value)
		}
		return nil
	})

	if err != nil {
		return nil, common.ContextError(err)
	}
	return data, nil
}

// SetUrlETag stores an ETag for the specfied URL.
// Note: input URL is treated as a string, and is not
// encoded or decoded or otherwise canonicalized.
func SetUrlETag(url, etag string) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreUrlETagsBucket)
		err := bucket.put([]byte(url), []byte(etag))
		return err
	})

	if err != nil {
		return common.ContextError(err)
	}
	return nil
}

// GetUrlETag retrieves a previously stored an ETag for the
// specfied URL. If not found, it returns an empty string value.
func GetUrlETag(url string) (string, error) {

	var etag string

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreUrlETagsBucket)
		etag = string(bucket.get([]byte(url)))
		return nil
	})

	if err != nil {
		return "", common.ContextError(err)
	}
	return etag, nil
}

// SetKeyValue stores a key/value pair.
func SetKeyValue(key, value string) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreKeyValueBucket)
		err := bucket.put([]byte(key), []byte(value))
		return err
	})

	if err != nil {
		return common.ContextError(err)
	}
	return nil
}

// GetKeyValue retrieves the value for a given key. If not found,
// it returns an empty string value.
func GetKeyValue(key string) (string, error) {

	var value string

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreKeyValueBucket)
		value = string(bucket.get([]byte(key)))
		return nil
	})

	if err != nil {
		return "", common.ContextError(err)
	}
	return value, nil
}

// Persistent stat records in the persistentStatStateUnreported
// state are available for take out.
//
// Records in the persistentStatStateReporting have been taken
// out and are pending either deletion (for a successful request)
// or change to StateUnreported (for a failed request).
//
// All persistent stat records are reverted to StateUnreported
// when the datastore is initialized at start up.

var persistentStatStateUnreported = []byte("0")
var persistentStatStateReporting = []byte("1")

var persistentStatTypes = []string{
	datastorePersistentStatTypeRemoteServerList,
}

// StorePersistentStat adds a new persistent stat record, which
// is set to StateUnreported and is an immediate candidate for
// reporting.
//
// The stat is a JSON byte array containing fields as
// required by the Psiphon server API. It's assumed that the
// JSON value contains enough unique information for the value to
// function as a key in the key/value datastore. This assumption
// is currently satisfied by the fields sessionId + tunnelNumber
// for tunnel stats, and URL + ETag for remote server list stats.
func StorePersistentStat(statType string, stat []byte) error {

	if !common.Contains(persistentStatTypes, statType) {
		return common.ContextError(fmt.Errorf("invalid persistent stat type: %s", statType))
	}

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket([]byte(statType))
		err := bucket.put(stat, persistentStatStateUnreported)
		return err
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// CountUnreportedPersistentStats returns the number of persistent
// stat records in StateUnreported.
func CountUnreportedPersistentStats() int {

	unreported := 0

	err := datastoreView(func(tx *datastoreTx) error {

		for _, statType := range persistentStatTypes {

			bucket := tx.bucket([]byte(statType))
			cursor := bucket.cursor()
			for key, value := cursor.first(); key != nil; key, value = cursor.next() {
				if 0 == bytes.Compare(value, persistentStatStateUnreported) {
					unreported++
					break
				}
			}
			cursor.close()
		}
		return nil
	})

	if err != nil {
		NoticeAlert("CountUnreportedPersistentStats failed: %s", err)
		return 0
	}

	return unreported
}

// TakeOutUnreportedPersistentStats returns up to maxCount persistent
// stats records that are in StateUnreported. The records are set to
// StateReporting. If the records are successfully reported, clear them
// with ClearReportedPersistentStats. If the records are not successfully
// reported, restore them with PutBackUnreportedPersistentStats.
func TakeOutUnreportedPersistentStats(maxCount int) (map[string][][]byte, error) {

	stats := make(map[string][][]byte)

	err := datastoreUpdate(func(tx *datastoreTx) error {

		count := 0

		for _, statType := range persistentStatTypes {

			bucket := tx.bucket([]byte(statType))
			cursor := bucket.cursor()
			for key, value := cursor.first(); key != nil; key, value = cursor.next() {

				if count >= maxCount {
					break
				}

				// Perform a test JSON unmarshaling. In case of data corruption or a bug,
				// skip the record.
				var jsonData interface{}
				err := json.Unmarshal(key, &jsonData)
				if err != nil {
					NoticeAlert(
						"Invalid key in TakeOutUnreportedPersistentStats: %s: %s",
						string(key), err)
					continue
				}

				if 0 == bytes.Compare(value, persistentStatStateUnreported) {
					// Must make a copy as slice is only valid within transaction.
					data := make([]byte, len(key))
					copy(data, key)

					if stats[statType] == nil {
						stats[statType] = make([][]byte, 0)
					}

					stats[statType] = append(stats[statType], data)
					count += 1
				}
			}
			cursor.close()

			for _, key := range stats[statType] {
				err := bucket.put(key, persistentStatStateReporting)
				if err != nil {
					return err
				}
			}

		}
		return nil
	})

	if err != nil {
		return nil, common.ContextError(err)
	}

	return stats, nil
}

// PutBackUnreportedPersistentStats restores a list of persistent
// stat records to StateUnreported.
func PutBackUnreportedPersistentStats(stats map[string][][]byte) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {

		for _, statType := range persistentStatTypes {

			bucket := tx.bucket([]byte(statType))
			for _, key := range stats[statType] {
				err := bucket.put(key, persistentStatStateUnreported)
				if err != nil {
					return err
				}
			}
		}

		return nil
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// ClearReportedPersistentStats deletes a list of persistent
// stat records that were successfully reported.
func ClearReportedPersistentStats(stats map[string][][]byte) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {

		for _, statType := range persistentStatTypes {

			bucket := tx.bucket([]byte(statType))
			for _, key := range stats[statType] {
				err := bucket.delete(key)
				if err != nil {
					return err
				}
			}
		}

		return nil
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// resetAllPersistentStatsToUnreported sets all persistent stat
// records to StateUnreported. This reset is called when the
// datastore is initialized at start up, as we do not know if
// persistent records in StateReporting were reported or not.
func resetAllPersistentStatsToUnreported() error {

	err := datastoreUpdate(func(tx *datastoreTx) error {

		for _, statType := range persistentStatTypes {

			bucket := tx.bucket([]byte(statType))
			resetKeys := make([][]byte, 0)
			cursor := bucket.cursor()
			for key := cursor.firstKey(); key != nil; key = cursor.nextKey() {
				resetKeys = append(resetKeys, key)
			}
			cursor.close()
			// TODO: data mutation is done outside cursor. Is this
			// strictly necessary in this case? As is, this means
			// all stats need to be loaded into memory at once.
			// https://godoc.org/github.com/boltdb/bolt#Cursor
			for _, key := range resetKeys {
				err := bucket.put(key, persistentStatStateUnreported)
				if err != nil {
					return err
				}
			}
		}

		return nil
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// CountSLOKs returns the total number of SLOK records.
func CountSLOKs() int {

	count := 0

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSLOKsBucket)
		cursor := bucket.cursor()
		for key := cursor.firstKey(); key != nil; key = cursor.nextKey() {
			count++
		}
		cursor.close()
		return nil
	})

	if err != nil {
		NoticeAlert("CountSLOKs failed: %s", err)
		return 0
	}

	return count
}

// DeleteSLOKs deletes all SLOK records.
func DeleteSLOKs() error {

	err := datastoreUpdate(func(tx *datastoreTx) error {
		return tx.clearBucket(datastoreSLOKsBucket)
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

// SetSLOK stores a SLOK key, referenced by its ID. The bool
// return value indicates whether the SLOK was already stored.
func SetSLOK(id, key []byte) (bool, error) {

	var duplicate bool

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSLOKsBucket)
		duplicate = bucket.get(id) != nil
		err := bucket.put([]byte(id), []byte(key))
		return err
	})

	if err != nil {
		return false, common.ContextError(err)
	}

	return duplicate, nil
}

// GetSLOK returns a SLOK key for the specified ID. The return
// value is nil if the SLOK is not found.
func GetSLOK(id []byte) ([]byte, error) {

	var key []byte

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(datastoreSLOKsBucket)
		key = bucket.get(id)
		return nil
	})

	if err != nil {
		return nil, common.ContextError(err)
	}

	return key, nil
}

// TacticsStorer implements tactics.Storer.
type TacticsStorer struct {
}

func (t *TacticsStorer) SetTacticsRecord(networkID string, record []byte) error {
	return setBucketValue(datastoreTacticsBucket, []byte(networkID), record)
}

func (t *TacticsStorer) GetTacticsRecord(networkID string) ([]byte, error) {
	return getBucketValue(datastoreTacticsBucket, []byte(networkID))
}

func (t *TacticsStorer) SetSpeedTestSamplesRecord(networkID string, record []byte) error {
	return setBucketValue(datastoreSpeedTestSamplesBucket, []byte(networkID), record)
}

func (t *TacticsStorer) GetSpeedTestSamplesRecord(networkID string) ([]byte, error) {
	return getBucketValue(datastoreSpeedTestSamplesBucket, []byte(networkID))
}

// GetTacticsStorer creates a TacticsStorer.
func GetTacticsStorer() *TacticsStorer {
	return &TacticsStorer{}
}

func setBucketValue(bucket, key, value []byte) error {

	err := datastoreUpdate(func(tx *datastoreTx) error {
		bucket := tx.bucket(bucket)
		err := bucket.put(key, value)
		return err
	})

	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

func getBucketValue(bucket, key []byte) ([]byte, error) {

	var value []byte

	err := datastoreView(func(tx *datastoreTx) error {
		bucket := tx.bucket(bucket)
		value = bucket.get(key)
		return nil
	})

	if err != nil {
		return nil, common.ContextError(err)
	}

	return value, nil
}