storage: add a storage abstraction

There are several places where we want to expose an abstraction for
storage for the object database.  Add read-write and read-only storage
abstractions and a backend abstraction that encapsulates one or more of
these abstractions as interfaces that can be implemented.

Provide a method to determine whether the data read from the storage is
compressed, since pack data will be uncompressed for us, but most other
storage will not be.

Author: bk2204

storage/backend.go

@@ -0,0 +1,10 @@
+package storage
+
+// Backend is an encapsulation of a set of read-only and read-write interfaces
+// for reading and writing objects.
+type Backend interface {
+	// Storage returns a read source and optionally a write source.
+	// Generally, the write location, if present, should also be a read
+	// location.
+	Storage() (Storage, WritableStorage)
+}

storage/storage.go

@@ -0,0 +1,30 @@
+package storage
+
+import "io"
+
+// Storage implements an interface for reading, but not writing, objects in an
+// object database.
+type Storage interface {
+	// Open returns a handle on an existing object keyed by the given object
+	// ID.  It returns an error if that file does not already exist.
+	Open(oid []byte) (f io.ReadCloser, err error)
+
+	// Close closes the filesystem, after which no more operations are
+	// allowed.
+	Close() error
+
+	// Compressed indicates whether data read from this storage source will
+	// be zlib-compressed.
+	IsCompressed() bool
+}
+
+// WritableStorage implements an interface for reading and writing objects in
+// an object database.
+type WritableStorage interface {
+	Storage
+
+	// Store copies the data given in "r" to the unique object path given by
+	// "oid". It returns an error if that file already exists (acting as if
+	// the `os.O_EXCL` mode is given in a bitmask to os.Open).
+	Store(oid []byte, r io.Reader) (n int64, err error)
+}