storage: Add object type hint parameter to ObjectStorage.Get. (#69)

Some storage backends can optimize object lookup if they get
the object type that is expected. So we the signature of the Get
method is now Get(Hash, ObjectType).

Added generic tests for storage backends.

2 files changed, 23 insertions, 9 deletions

diff --git a/core/object.go b/core/object.go
index 9c9a74e..7e021cb 100644
--- a/core/object.go
+++ b/core/object.go
@@ -38,17 +38,22 @@ type Object interface {
 	Writer() (ObjectWriter, error)
 }
 
-// ObjectType internal object type's
+// ObjectType internal object type
+// Integer values from 0 to 7 map to those exposed by git.
+// AnyObject is used to represent any from 0 to 7.
 type ObjectType int8
 
 const (
-	InvalidObject  ObjectType = 0
-	CommitObject   ObjectType = 1
-	TreeObject     ObjectType = 2
-	BlobObject     ObjectType = 3
-	TagObject      ObjectType = 4
+	InvalidObject ObjectType = 0
+	CommitObject  ObjectType = 1
+	TreeObject    ObjectType = 2
+	BlobObject    ObjectType = 3
+	TagObject     ObjectType = 4
+	// 5 reserved for future expansion
 	OFSDeltaObject ObjectType = 6
 	REFDeltaObject ObjectType = 7
+
+	AnyObject ObjectType = -127
 )
 
 func (t ObjectType) String() string {
@@ -132,7 +137,7 @@ func (iter *ObjectLookupIter) Next() (Object, error) {
 		return nil, io.EOF
 	}
 	hash := iter.series[iter.pos]
-	obj, err := iter.storage.Get(hash)
+	obj, err := iter.storage.Get(hash, AnyObject)
 	if err == nil {
 		iter.pos++
 	}
@@ -146,7 +151,7 @@ func (iter *ObjectLookupIter) ForEach(cb func(Object) error) error {
 	defer iter.Close()
 
 	for _, hash := range iter.series {
-		obj, err := iter.storage.Get(hash)
+		obj, err := iter.storage.Get(hash, AnyObject)
 		if err != nil {
 			return err
 		}
diff --git a/core/storage.go b/core/storage.go
index f3225d8..f3ec52b 100644
--- a/core/storage.go
+++ b/core/storage.go
@@ -9,7 +9,16 @@ var ErrStop = errors.New("stop iter")
 type ObjectStorage interface {
 	NewObject() Object
 	Set(Object) (Hash, error)
-	Get(Hash) (Object, error)
+	// Get an object by hash with the given ObjectType.
+	//
+	// Implementors should return (nil, core.ErrObjectNotFound) if an object
+	// doesn't exist with both the given hash and object type.
+	//
+	// Valid ObjectType values are CommitObject, BlobObject, TagObject, TreeObject
+	// and AnyObject.
+	//
+	// If AnyObject is given, the object must be looked up regardless of its type.
+	Get(Hash, ObjectType) (Object, error)
 	Iter(ObjectType) (ObjectIter, error)
 }