Initial basic static delta code drop

This has a very basic level of functionality (deltas can be generated,
and applied offline).  There is only some stubbed out pull code to
fetch them via HTTP.

But, better to commit this now and improve it from a known starting
point, rather than have it languish in a branch.

src/libostree/ostree-core.c

@@ -1208,13 +1208,41 @@ ostree_checksum_from_bytes_v (GVariant *csum_v)
  * ostree_checksum_bytes_peek:
  * @bytes: #GVariant of type ay
  *
- * Returns: (transfer none): Binary checksum data in @bytes; do not free
+ * Returns: (transfer none) (array fixed-size=32) (element-type guint8): Binary checksum data in @bytes; do not free.  If @bytes does not have the correct length, return %NULL.
  */
 const guchar *
 ostree_checksum_bytes_peek (GVariant *bytes)
 {
   gsize n_elts;
-  return g_variant_get_fixed_array (bytes, &n_elts, 1);
+  const guchar *ret;
+  ret = g_variant_get_fixed_array (bytes, &n_elts, 1);
+  if (G_UNLIKELY (n_elts != 32))
+    return NULL;
+  return ret;
+}
+
+/**
+ * ostree_checksum_bytes_peek_validate:
+ * @bytes: #GVariant of type ay
+ * @error: Errror
+ *
+ * Like ostree_checksum_bytes_peek(), but also throws @error.
+ *
+ * Returns: (transfer none) (array fixed-size=32) (element-type guint8): Binary checksum data
+ */
+const guchar *
+ostree_checksum_bytes_peek_validate (GVariant  *bytes,
+                                     GError   **error)
+{
+  const guchar *ret = ostree_checksum_bytes_peek (bytes);
+  if (G_UNLIKELY (!ret))
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Invalid checksum of length %" G_GUINT64_FORMAT
+                   " expected 32", (guint64) g_variant_n_children (bytes));
+      return NULL;
+    }
+  return ret;
 }
 
 /*
@@ -1293,8 +1321,23 @@ _ostree_get_relative_object_path (const char         *checksum,
   return g_string_free (path, FALSE);
 }
 
+char *
+_ostree_get_relative_static_delta_path (const char        *from,
+                                        const char        *to)
+{
+  return g_strdup_printf ("deltas/%s-%s/meta", from, to);
+}
+
+char *
+_ostree_get_relative_static_delta_part_path (const char        *from,
+                                             const char        *to,
+                                             guint              i)
+{
+  return g_strdup_printf ("deltas/%s-%s/%u", from, to, i);
+}
+
 /*
- * ostree_file_header_parse:
+ * file_header_parse:
  * @metadata: A metadata variant of type %OSTREE_FILE_HEADER_GVARIANT_FORMAT
  * @out_file_info: (out): Parsed file information
  * @out_xattrs: (out): Parsed extended attribute set
@@ -1449,15 +1492,7 @@ gboolean
 ostree_validate_structureof_csum_v (GVariant  *checksum,
                                     GError   **error)
 {
-  gsize n_children = g_variant_n_children (checksum);
-  if (n_children != 32)
-    {
-      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
-                   "Invalid checksum of length %" G_GUINT64_FORMAT
-                   " expected 32", (guint64) n_children);
-      return FALSE;
-    }
-  return TRUE;
+  return ostree_checksum_bytes_peek_validate (checksum, error) != NULL;
 }
 
 /**