chore: introduce memorystorev2 (#193)

This adds a `memorystorev2` package containing an alternative Memory
Store implementation designed specifically for FDv2 usage.

It is similar to the existing memory store, with obsolete methods
removed and new methods added - but the core `upsert/init` logic remains
the same.

internal/memorystorev2/memory_store.go

@@ -0,0 +1,170 @@
+// Package memorystorev2 contains an implementation for a transactional memory store suitable
+// for the FDv2 architecture.
+package memorystorev2
+
+import (
+	"sync"
+
+	"github.com/launchdarkly/go-sdk-common/v3/ldlog"
+	"github.com/launchdarkly/go-server-sdk/v7/subsystems/ldstoretypes"
+)
+
+// Store provides an abstraction that makes flag and segment data available to other components.
+// It accepts updates in batches - for instance, flag A was upserted while segment B was deleted -
+// such that the contents of the store are consistent with a single payload version at any given time.
+//
+// The terminology used is "basis" and "deltas". First, the store's basis is set. This is this initial
+// data, upon which subsequent deltas will be applied. Whenever the basis is set, any existing data
+// is discarded.
+//
+// Deltas are then applied to the store. A single delta update transforms the contents of the store
+// atomically.
+type Store struct {
+	data        map[ldstoretypes.DataKind]map[string]ldstoretypes.ItemDescriptor
+	initialized bool
+	sync.RWMutex
+	loggers ldlog.Loggers
+}
+
+// New creates a new Store. The Store is uninitialized until SetBasis is called.
+func New(loggers ldlog.Loggers) *Store {
+	return &Store{
+		data:        make(map[ldstoretypes.DataKind]map[string]ldstoretypes.ItemDescriptor),
+		initialized: false,
+		loggers:     loggers,
+	}
+}
+
+// SetBasis sets the basis of the Store. Any existing data is discarded.
+// When the basis is set, the store becomes initialized.
+func (s *Store) SetBasis(allData []ldstoretypes.Collection) {
+	s.Lock()
+	defer s.Unlock()
+
+	s.data = make(map[ldstoretypes.DataKind]map[string]ldstoretypes.ItemDescriptor)
+
+	for _, coll := range allData {
+		items := make(map[string]ldstoretypes.ItemDescriptor)
+		for _, item := range coll.Items {
+			items[item.Key] = item.Item
+		}
+		s.data[coll.Kind] = items
+	}
+
+	s.initialized = true
+}
+
+// ApplyDelta applies a delta update to the store. ApplyDelta should not be called until
+// SetBasis has been called at least once. The return value indicates, for each DataKind
+// present in the delta, whether the item in the delta was actually updated or not.
+//
+// An item is updated only if the version of the item in the delta is greater than the version
+// in the store, or it wasn't already present.
+func (s *Store) ApplyDelta(allData []ldstoretypes.Collection) map[ldstoretypes.DataKind]map[string]bool {
+	updatedMap := make(map[ldstoretypes.DataKind]map[string]bool)
+
+	s.Lock()
+	defer s.Unlock()
+
+	for _, coll := range allData {
+		for _, item := range coll.Items {
+			updated := s.upsert(coll.Kind, item.Key, item.Item)
+			if updatedMap[coll.Kind] == nil {
+				updatedMap[coll.Kind] = make(map[string]bool)
+			}
+			updatedMap[coll.Kind][item.Key] = updated
+		}
+	}
+
+	return updatedMap
+}
+
+// Get retrieves an item of the specified kind from the store. If the item is not found, then
+// ItemDescriptor{}.NotFound() is returned with a nil error.
+func (s *Store) Get(kind ldstoretypes.DataKind, key string) (ldstoretypes.ItemDescriptor, error) {
+	s.RLock()
+
+	var item ldstoretypes.ItemDescriptor
+	coll, ok := s.data[kind]
+	if ok {
+		item, ok = coll[key]
+	}
+
+	s.RUnlock()
+
+	if ok {
+		return item, nil
+	}
+	if s.loggers.IsDebugEnabled() {
+		s.loggers.Debugf(`Key %s not found in "%s"`, key, kind)
+	}
+	return ldstoretypes.ItemDescriptor{}.NotFound(), nil
+}
+
+// GetAll retrieves all items of the specified kind from the store.
+func (s *Store) GetAll(kind ldstoretypes.DataKind) ([]ldstoretypes.KeyedItemDescriptor, error) {
+	s.RLock()
+	defer s.RUnlock()
+	return s.getAll(kind), nil
+}
+
+func (s *Store) getAll(kind ldstoretypes.DataKind) []ldstoretypes.KeyedItemDescriptor {
+	var itemsOut []ldstoretypes.KeyedItemDescriptor
+	if itemsMap, ok := s.data[kind]; ok {
+		if len(itemsMap) > 0 {
+			itemsOut = make([]ldstoretypes.KeyedItemDescriptor, 0, len(itemsMap))
+			for key, item := range itemsMap {
+				itemsOut = append(itemsOut, ldstoretypes.KeyedItemDescriptor{Key: key, Item: item})
+			}
+		}
+	}
+	return itemsOut
+}
+
+// GetAllKinds retrieves all items of all kinds from the store. This is different from calling
+// GetAll for each kind because it provides a consistent view of the entire store at a single point in time.
+func (s *Store) GetAllKinds() []ldstoretypes.Collection {
+	s.RLock()
+	defer s.RUnlock()
+
+	allData := make([]ldstoretypes.Collection, 0, len(s.data))
+	for kind := range s.data {
+		itemsOut := s.getAll(kind)
+		allData = append(allData, ldstoretypes.Collection{Kind: kind, Items: itemsOut})
+	}
+
+	return allData
+}
+
+func (s *Store) upsert(
+	kind ldstoretypes.DataKind,
+	key string,
+	newItem ldstoretypes.ItemDescriptor) bool {
+	var coll map[string]ldstoretypes.ItemDescriptor
+	var ok bool
+	shouldUpdate := true
+	updated := false
+	if coll, ok = s.data[kind]; ok {
+		if item, ok := coll[key]; ok {
+			if item.Version >= newItem.Version {
+				shouldUpdate = false
+			}
+		}
+	} else {
+		s.data[kind] = map[string]ldstoretypes.ItemDescriptor{key: newItem}
+		shouldUpdate = false // because we already initialized the map with the new item
+		updated = true
+	}
+	if shouldUpdate {
+		coll[key] = newItem
+		updated = true
+	}
+	return updated
+}
+
+// IsInitialized returns true if the store has ever been initialized with a basis.
+func (s *Store) IsInitialized() bool {
+	s.RLock()
+	defer s.RUnlock()
+	return s.initialized
+}