Implement streaming functions for local backend

backend/local.go

@@ -3,6 +3,7 @@ package backend
 import (
 	"errors"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -223,6 +224,54 @@ func (b *Local) Create(t Type, data []byte) (ID, error) {
 	return id, nil
 }
 
+// CreateFrom reads content from rd and stores it as type t. Returned is the
+// storage ID. If the blob is already present, returns ErrAlreadyPresent and
+// the blob's ID.
+func (b *Local) CreateFrom(t Type, rd io.Reader) (ID, error) {
+	// TODO: make sure that tempfile is removed upon error
+
+	// check hash while writing
+	hr := NewHashingReader(rd, newHash())
+
+	// create tempfile in backend
+	file, err := b.tempFile()
+	if err != nil {
+		return nil, err
+	}
+
+	// write data to tempfile
+	_, err = io.Copy(file, hr)
+	if err != nil {
+		return nil, err
+	}
+
+	err = file.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	// get ID
+	id := ID(hr.Sum(nil))
+
+	// check for duplicate ID
+	res, err := b.Test(t, id)
+	if err != nil {
+		return nil, arrar.Annotate(err, "test for presence")
+	}
+
+	if res {
+		return id, ErrAlreadyPresent
+	}
+
+	// rename file
+	err = b.renameFile(file, t, id)
+	if err != nil {
+		return nil, err
+	}
+
+	return id, nil
+}
+
 // Construct path for given Type and ID.
 func (b *Local) filename(t Type, id ID) string {
 	return filepath.Join(b.dirname(t, id), id.String())
@@ -256,6 +305,23 @@ func (b *Local) Get(t Type, id ID) ([]byte, error) {
 	return buf, nil
 }
 
+// GetReader returns a reader that yields the content stored under the given
+// ID. The content is not verified. The reader should be closed after draining
+// it.
+func (b *Local) GetReader(t Type, id ID) (io.ReadCloser, error) {
+	if id == nil {
+		return nil, errors.New("unable to load nil ID")
+	}
+
+	// try to open file
+	file, err := os.Open(b.filename(t, id))
+	if err != nil {
+		return nil, err
+	}
+
+	return file, nil
+}
+
 // Test returns true if a blob of the given type and ID exists in the backend.
 func (b *Local) Test(t Type, id ID) (bool, error) {
 	// try to open file