multi: add reset closure to kvdb.View

This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.

channeldb/waitingproof.go

@@ -42,13 +42,14 @@ type WaitingProofStore struct {
 // NewWaitingProofStore creates new instance of proofs storage.
 func NewWaitingProofStore(db *DB) (*WaitingProofStore, error) {
 	s := &WaitingProofStore{
-		db:    db,
-		cache: make(map[WaitingProofKey]struct{}),
+		db: db,
 	}
 
 	if err := s.ForAll(func(proof *WaitingProof) error {
 		s.cache[proof.Key()] = struct{}{}
 		return nil
+	}, func() {
+		s.cache = make(map[WaitingProofKey]struct{})
 	}); err != nil && err != ErrWaitingProofNotFound {
 		return nil, err
 	}
@@ -122,7 +123,9 @@ func (s *WaitingProofStore) Remove(key WaitingProofKey) error {
 
 // ForAll iterates thought all waiting proofs and passing the waiting proof
 // in the given callback.
-func (s *WaitingProofStore) ForAll(cb func(*WaitingProof) error) error {
+func (s *WaitingProofStore) ForAll(cb func(*WaitingProof) error,
+	reset func()) error {
+
 	return kvdb.View(s.db, func(tx kvdb.RTx) error {
 		bucket := tx.ReadBucket(waitingProofsBucketKey)
 		if bucket == nil {
@@ -144,12 +147,12 @@ func (s *WaitingProofStore) ForAll(cb func(*WaitingProof) error) error {
 
 			return cb(proof)
 		})
-	})
+	}, reset)
 }
 
 // Get returns the object which corresponds to the given index.
 func (s *WaitingProofStore) Get(key WaitingProofKey) (*WaitingProof, error) {
-	proof := &WaitingProof{}
+	var proof *WaitingProof
 
 	s.mu.RLock()
 	defer s.mu.RUnlock()
@@ -172,6 +175,8 @@ func (s *WaitingProofStore) Get(key WaitingProofKey) (*WaitingProof, error) {
 
 		r := bytes.NewReader(v)
 		return proof.Decode(r)
+	}, func() {
+		proof = &WaitingProof{}
 	})
 
 	return proof, err