lib/core: add ostree_checksum_file_at API

This is like `ostree_checksum_file` but fd-relative. This will be used
by https://github.com/ostreedev/ostree/pull/1258.

AFAICT, we actually didn't have any tests that check the `checksum` CLI.
Add a basic one here to test the old code as well as the new code.

Closes: #1263
Approved by: cgwalters

src/libostree/ostree-core.h

@@ -21,6 +21,7 @@
 
 #pragma once
 
+#include <sys/stat.h>
 #include <gio/gio.h>
 #include <ostree-types.h>
 
@@ -420,6 +421,26 @@ gboolean ostree_checksum_file (GFile             *f,
                                GCancellable      *cancellable,
                                GError           **error);
 
+/**
+ * OstreeChecksumFlags:
+ *
+ * Since: 2017.13
+ */
+typedef enum {
+  OSTREE_CHECKSUM_FLAGS_NONE = 0,
+  OSTREE_CHECKSUM_FLAGS_IGNORE_XATTRS = (1 << 0),
+} OstreeChecksumFlags;
+
+_OSTREE_PUBLIC
+gboolean ostree_checksum_file_at (int               dfd,
+                                  const char       *path,
+                                  struct stat      *stbuf,
+                                  OstreeObjectType  objtype,
+                                  OstreeChecksumFlags flags,
+                                  char            **out_checksum,
+                                  GCancellable     *cancellable,
+                                  GError          **error);
+
 _OSTREE_PUBLIC
 void ostree_checksum_file_async (GFile                 *f,
                                  OstreeObjectType       objtype,