pkg/storage: introduce storage.Backend Watch method (#1135)

Currently, we're doing "sync" in databroker server. If we're going to
support multiple databroker servers instance, this mechanism won't work.

This commit moves the "sync" to storage backend, by adding new Watch
method. The Watch method will return a channel for the caller. Everytime
something happens inside the storage, we notify the caller by sending a
message to this channel.

pkg/storage/inmemory/inmemory.go

@@ -14,6 +14,7 @@ import (
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/types/known/anypb"
 
+	"github.com/pomerium/pomerium/internal/signal"
 	"github.com/pomerium/pomerium/pkg/grpc/databroker"
 	"github.com/pomerium/pomerium/pkg/storage"
 )
@@ -49,14 +50,17 @@ type DB struct {
 	byID       *btree.BTree
 	byVersion  *btree.BTree
 	deletedIDs []string
+	onchange   *signal.Signal
 }
 
 // NewDB creates a new in-memory database for the given record type.
 func NewDB(recordType string, btreeDegree int) *DB {
+	s := signal.New()
 	return &DB{
 		recordType: recordType,
 		byID:       btree.New(btreeDegree),
 		byVersion:  btree.New(btreeDegree),
+		onchange:   s,
 	}
 }
 
@@ -81,6 +85,7 @@ func (db *DB) ClearDeleted(_ context.Context, cutoff time.Time) {
 
 // Delete marks a record as deleted.
 func (db *DB) Delete(_ context.Context, id string) error {
+	defer db.onchange.Broadcast()
 	db.replaceOrInsert(id, func(record *databroker.Record) {
 		record.DeletedAt = ptypes.TimestampNow()
 		db.deletedIDs = append(db.deletedIDs, id)
@@ -122,12 +127,25 @@ func (db *DB) List(_ context.Context, sinceVersion string) ([]*databroker.Record
 
 // Put replaces or inserts a record in the db.
 func (db *DB) Put(_ context.Context, id string, data *anypb.Any) error {
+	defer db.onchange.Broadcast()
 	db.replaceOrInsert(id, func(record *databroker.Record) {
 		record.Data = data
 	})
 	return nil
 }
 
+// Watch returns the underlying signal.Signal binding channel to the caller.
+// Then the caller can listen to the channel for detecting changes.
+func (db *DB) Watch(ctx context.Context) chan struct{} {
+	ch := db.onchange.Bind()
+	go func() {
+		<-ctx.Done()
+		close(ch)
+		db.onchange.Unbind(ch)
+	}()
+	return ch
+}
+
 func (db *DB) replaceOrInsert(id string, f func(record *databroker.Record)) {
 	db.mu.Lock()
 	defer db.mu.Unlock()