plumbing: add HasEncodedObject method to Storer

This allows the user to check whether an object exists, without
reading all the object data from storage.

Issue: KBFS-2445

Author: strib

plumbing/storer/object_test.go

@@ -133,6 +133,15 @@ func (o *MockObjectStorage) SetEncodedObject(obj plumbing.EncodedObject) (plumbi
 	return plumbing.ZeroHash, nil
 }
 
+func (o *MockObjectStorage) HasEncodedObject(h plumbing.Hash) error {
+	for _, o := range o.db {
+		if o.Hash() == h {
+			return nil
+		}
+	}
+	return plumbing.ErrObjectNotFound
+}
+
 func (o *MockObjectStorage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (plumbing.EncodedObject, error) {
 	for _, o := range o.db {
 		if o.Hash() == h {

storage/filesystem/object.go

@@ -126,6 +126,32 @@ func (s *ObjectStorage) SetEncodedObject(o plumbing.EncodedObject) (plumbing.Has
 	return o.Hash(), err
 }
 
+// HasEncodedObject returns nil if the object exists, without actually
+// reading the object data from storage.
+func (s *ObjectStorage) HasEncodedObject(h plumbing.Hash) (err error) {
+	// Check unpacked objects
+	f, err := s.dir.Object(h)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return err
+		}
+		// Fall through to check packed objects.
+	} else {
+		defer ioutil.CheckClose(f, &err)
+		return nil
+	}
+
+	// Check packed objects.
+	if err := s.requireIndex(); err != nil {
+		return err
+	}
+	_, _, offset := s.findObjectInPackfile(h)
+	if offset == -1 {
+		return plumbing.ErrObjectNotFound
+	}
+	return nil
+}
+
 // EncodedObject returns the object with the given hash, by searching for it in
 // the packfile and the git object directories.
 func (s *ObjectStorage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (plumbing.EncodedObject, error) {

storage/memory/storage.go

@@ -115,6 +115,14 @@ func (o *ObjectStorage) SetEncodedObject(obj plumbing.EncodedObject) (plumbing.H
 	return h, nil
 }
 
+func (o *ObjectStorage) HasEncodedObject(h plumbing.Hash) (err error) {
+	_, ok := o.Objects[h]
+	if !ok {
+		return plumbing.ErrObjectNotFound
+	}
+	return nil
+}
+
 func (o *ObjectStorage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (plumbing.EncodedObject, error) {
 	obj, ok := o.Objects[h]
 	if !ok || (plumbing.AnyObject != t && obj.Type() != t) {