content: break up into multiple files

Break up the content store prototype into a few logical files. We have a
file for the store, the writer and helpers.

Also, the writer has been modified to remove write and exec permissions
on blobs in the store.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

content/helpers.go

@@ -0,0 +1,53 @@
+package content
+
+import (
+	"io"
+	"os"
+
+	"github.com/docker/distribution/digest"
+	"github.com/pkg/errors"
+)
+
+// OpenBlob opens the blob for reading identified by dgst.
+//
+// The opened blob may also implement seek. Callers can detect with io.Seeker.
+func OpenBlob(cs *ContentStore, dgst digest.Digest) (io.ReadCloser, error) {
+	path, err := cs.GetPath(dgst)
+	if err != nil {
+		return nil, err
+	}
+
+	fp, err := os.Open(path)
+	return fp, err
+}
+
+// WriteBlob writes data with the expected digest into the content store. If
+// expected already exists, the method returns immediately and the reader will
+// not be consumed.
+//
+// This is useful when the digest and size are known beforehand.
+//
+// Copy is buffered, so no need to wrap reader in buffered io.
+func WriteBlob(cs *ContentStore, r io.Reader, size int64, expected digest.Digest) error {
+	cw, err := cs.Begin(expected.Hex())
+	if err != nil {
+		return err
+	}
+	buf := bufPool.Get().([]byte)
+	defer bufPool.Put(buf)
+
+	nn, err := io.CopyBuffer(cw, r, buf)
+	if err != nil {
+		return err
+	}
+
+	if nn != size {
+		return errors.Errorf("failed size verification: %v != %v", nn, size)
+	}
+
+	if err := cw.Commit(size, expected); err != nil {
+		return err
+	}
+
+	return nil
+}