added tree interface (#1043)

* added tree interface

* Update forest/tree/tree.go

Co-authored-by: itaiad200 <itaiad200@gmail.com>

* correct after review

* correct after review - push again

* fix review comments

* had to make some identifiers global because of linter errors. will change once integrated with rest of tree package

* had to make some identifiers global because of linter errors. will change once integrated with rest of tree package

* simplied SaveTree interface by splitting it to two interfaces

Co-authored-by: itaiad200 <itaiad200@gmail.com>

forest/tree/tree.go

@@ -0,0 +1,57 @@
+package tree
+
+import (
+	// cache "github.com/treeverse/lakefs/forest/cache_map"
+	"github.com/treeverse/lakefs/graveler"
+	"github.com/treeverse/lakefs/graveler/committed/sstable"
+)
+
+type TreePart struct {
+	PartName sstable.ID   `json:"part_name"`
+	MaxKey   graveler.Key `json:"max_path"`
+}
+type TreeSlice struct {
+	TreeSlice []TreePart
+}
+type DummyMap map[string]string // place holder for cache_map package that will be merged soon
+
+type TreesRepo struct {
+	TreesMap   DummyMap
+	PartManger sstable.Manager
+}
+
+// InitTreeRepository creates the tree cache, and stores part Manager for operations of parts (currently implemented as sstables).
+// should be called at process init.
+// decisions on who calls it and how to get a treesRepository will be taken later
+type InitTreesRepository func(manager sstable.Manager) TreeRepo
+
+type TreeRepo interface {
+	// NewTreeWriter returns a writer that uses the part manager to create a new tree
+	NewTreeWriter(splitFactor int, // average number of keys that we want to stored in a part
+		// for more detail, look at "IsSplitKey"
+		closeAsync sstable.BatchWriterCloser, // component used to close part asynchronously, and wait for all part
+		// completions when tree writing completes
+	) TreeWriter
+	// NewScannerFromTreeID accepts a tree ID, and returns an iterator over the tree
+	NewIteratorFromTreeID(treeID graveler.TreeID, start graveler.Key) (graveler.ValueIterator, error)
+	// NewIteratorFromTreeParts accept a tree in memory, returns iterator over the tree
+	NewIteratorFromTreeSlice(treeSlice TreeSlice, start graveler.Key) (graveler.ValueIterator, error)
+	// GetPartManager give components of tree package to the configured part manager.
+	// will probably change later
+	// GetPartManger() sstable.Manager
+}
+
+type TreeWriter interface {
+	WriteEntry(record graveler.ValueRecord) error
+	// FlushIterToTree writes the content of an iterator to the tree.
+	FlushIterToTree(iter graveler.ValueIterator) error
+	// SaveTree stores the tree to tierFS. During tree writing, parts are closed asynchronously and copied by tierFS
+	// while writing continues. SaveTree waits until closing and copying all parts
+	SaveTree() (graveler.TreeID, error)
+	SaveTreeWithReusedParts(reuseTree TreeSlice, // A tree may be saved with additional parts that are "reused" from a base tree.
+	// these are parts that exist in a source tree, and are merged into the destination tree.
+	// an example of using it is in the apply process, which creates a new tree from a base tree and an input iterator.
+	// those parts of the base tree that were not modified by it input iterator will be merged into the resulting tree
+	// by passing them in the  reuseTree parameter.
+	) (graveler.TreeID, error)
+}