Shared content across namespaces

Update content ingests to use content from another namespace.
Ingests must be committed to make content available and the
client will see the sharing as an ingest which has already
been fully written to, but not completed.

Updated the database version to change the ingest record in
the database from a link key to an object with a link and
expected value. This expected value is used to indicate that
the content already exists and an underlying writer may
not yet exist.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/helpers.go

@@ -62,7 +62,7 @@ func WriteBlob(ctx context.Context, cs Ingester, ref string, r io.Reader, size i
 }
 
 // Copy copies data with the expected digest from the reader into the
-// provided content store writer.
+// provided content store writer. This copy commits the writer.
 //
 // This is useful when the digest and size are known beforehand. When
 // the size or digest is unknown, these values may be empty.
@@ -97,6 +97,22 @@ func Copy(ctx context.Context, cw Writer, r io.Reader, size int64, expected dige
 	return nil
 }
 
+// CopyReaderAt copies to a writer from a given reader at for the given
+// number of bytes. This copy does not commit the writer.
+func CopyReaderAt(cw Writer, ra ReaderAt, n int64) error {
+	ws, err := cw.Status()
+	if err != nil {
+		return err
+	}
+
+	buf := bufPool.Get().(*[]byte)
+	defer bufPool.Put(buf)
+
+	_, err = io.CopyBuffer(cw, io.NewSectionReader(ra, ws.Offset, n), *buf)
+
+	return err
+}
+
 // seekReader attempts to seek the reader to the given offset, either by
 // resolving `io.Seeker`, by detecting `io.ReaderAt`, or discarding
 // up to the given offset.