Implement lazy iteration (ForEach) over collections

See #761

aptly had a concept of loading small amount of info per each object
into memory once collection is accessed for the first time.

This might have simplified some operations, but it doesn't scale well
with huge aptly databases.

This is just intermediate step towards better memory management -
list of objects is not loaded unless some method is called.
`ForEach` method (mainly used in cleanup) is reimplemented to
iterate over database without ever loading all the objects into memory.

Memory was even worse with previous approach, as for each item usually
`LoadComplete()` is called, which pulls even more data into memory
and item stays in memory till the end of the iteration as it is referenced
from `collection.list`.

For the subsequent PR: reimplement `ByUUID()` and probably other methods
to avoid loading all the items into memory, at least for all the collecitons
except for published repos. When published repository is being loaded, it
might pull source local repo which in turn would trigger loading for all the
local repos which is not acceptable.

deb/publish.go

@@ -852,28 +852,34 @@ type PublishedRepoCollection struct {
 
 // NewPublishedRepoCollection loads PublishedRepos from DB and makes up collection
 func NewPublishedRepoCollection(db database.Storage) *PublishedRepoCollection {
-	result := &PublishedRepoCollection{
+	return &PublishedRepoCollection{
 		RWMutex: &sync.RWMutex{},
 		db:      db,
 	}
+}
 
-	blobs := db.FetchByPrefix([]byte("U"))
-	result.list = make([]*PublishedRepo, 0, len(blobs))
+func (collection *PublishedRepoCollection) loadList() {
+	if collection.list != nil {
+		return
+	}
+
+	blobs := collection.db.FetchByPrefix([]byte("U"))
+	collection.list = make([]*PublishedRepo, 0, len(blobs))
 
 	for _, blob := range blobs {
 		r := &PublishedRepo{}
 		if err := r.Decode(blob); err != nil {
 			log.Printf("Error decoding published repo: %s\n", err)
 		} else {
-			result.list = append(result.list, r)
+			collection.list = append(collection.list, r)
 		}
 	}
-
-	return result
 }
 
 // Add appends new repo to collection and saves it
 func (collection *PublishedRepoCollection) Add(repo *PublishedRepo) error {
+	collection.loadList()
+
 	if collection.CheckDuplicate(repo) != nil {
 		return fmt.Errorf("published repo with storage/prefix/distribution %s/%s/%s already exists", repo.Storage, repo.Prefix, repo.Distribution)
 	}
@@ -889,6 +895,8 @@ func (collection *PublishedRepoCollection) Add(repo *PublishedRepo) error {
 
 // CheckDuplicate verifies that there's no published repo with the same name
 func (collection *PublishedRepoCollection) CheckDuplicate(repo *PublishedRepo) *PublishedRepo {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.Prefix == repo.Prefix && r.Distribution == repo.Distribution && r.Storage == repo.Storage {
 			return r
@@ -978,6 +986,8 @@ func (collection *PublishedRepoCollection) LoadComplete(repo *PublishedRepo, col
 
 // ByStoragePrefixDistribution looks up repository by storage, prefix & distribution
 func (collection *PublishedRepoCollection) ByStoragePrefixDistribution(storage, prefix, distribution string) (*PublishedRepo, error) {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.Prefix == prefix && r.Distribution == distribution && r.Storage == storage {
 			return r, nil
@@ -991,6 +1001,8 @@ func (collection *PublishedRepoCollection) ByStoragePrefixDistribution(storage,
 
 // ByUUID looks up repository by uuid
 func (collection *PublishedRepoCollection) ByUUID(uuid string) (*PublishedRepo, error) {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.UUID == uuid {
 			return r, nil
@@ -1001,6 +1013,8 @@ func (collection *PublishedRepoCollection) ByUUID(uuid string) (*PublishedRepo,
 
 // BySnapshot looks up repository by snapshot source
 func (collection *PublishedRepoCollection) BySnapshot(snapshot *Snapshot) []*PublishedRepo {
+	collection.loadList()
+
 	var result []*PublishedRepo
 	for _, r := range collection.list {
 		if r.SourceKind == SourceSnapshot {
@@ -1021,6 +1035,8 @@ func (collection *PublishedRepoCollection) BySnapshot(snapshot *Snapshot) []*Pub
 
 // ByLocalRepo looks up repository by local repo source
 func (collection *PublishedRepoCollection) ByLocalRepo(repo *LocalRepo) []*PublishedRepo {
+	collection.loadList()
+
 	var result []*PublishedRepo
 	for _, r := range collection.list {
 		if r.SourceKind == SourceLocalRepo {
@@ -1041,18 +1057,21 @@ func (collection *PublishedRepoCollection) ByLocalRepo(repo *LocalRepo) []*Publi
 
 // ForEach runs method for each repository
 func (collection *PublishedRepoCollection) ForEach(handler func(*PublishedRepo) error) error {
-	var err error
-	for _, r := range collection.list {
-		err = handler(r)
-		if err != nil {
-			return err
+	return collection.db.ProcessByPrefix([]byte("U"), func(key, blob []byte) error {
+		r := &PublishedRepo{}
+		if err := r.Decode(blob); err != nil {
+			log.Printf("Error decoding published repo: %s\n", err)
+			return nil
 		}
-	}
-	return err
+
+		return handler(r)
+	})
 }
 
 // Len returns number of remote repos
 func (collection *PublishedRepoCollection) Len() int {
+	collection.loadList()
+
 	return len(collection.list)
 }
 
@@ -1060,6 +1079,8 @@ func (collection *PublishedRepoCollection) Len() int {
 func (collection *PublishedRepoCollection) CleanupPrefixComponentFiles(prefix string, components []string,
 	publishedStorage aptly.PublishedStorage, collectionFactory *CollectionFactory, progress aptly.Progress) error {
 
+	collection.loadList()
+
 	var err error
 	referencedFiles := map[string][]string{}
 
@@ -1141,6 +1162,9 @@ func (collection *PublishedRepoCollection) CleanupPrefixComponentFiles(prefix st
 func (collection *PublishedRepoCollection) Remove(publishedStorageProvider aptly.PublishedStorageProvider,
 	storage, prefix, distribution string, collectionFactory *CollectionFactory, progress aptly.Progress,
 	force, skipCleanup bool) error {
+
+	collection.loadList()
+
 	repo, err := collection.ByStoragePrefixDistribution(storage, prefix, distribution)
 	if err != nil {
 		return err