libostree: Add some additional metadata to the summary file

• Commit timestamps, so it’s easy to work out whether a given commit is
   newer than the one we have locally
 • Summary file timestamp, so it’s easy to work out whether the summary
   file is more up to date than another summary file
 • Summary file expiry time, so clients can work out when they should
   expect the summary file to next be updated, and hence can query for
   it at roughly the right time

The expiry time requires input from the user, so is currently never set
automatically. Programs using libostree can set it if they wish.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #826
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -43,6 +43,14 @@ G_BEGIN_DECLS
  * */
 #define _OSTREE_MAX_OUTSTANDING_WRITE_REQUESTS 16
 
+/* Well-known keys for the additional metadata field in a summary file. */
+#define OSTREE_SUMMARY_LAST_MODIFIED "ostree.summary.last-modified"
+#define OSTREE_SUMMARY_EXPIRES "ostree.summary.expires"
+
+/* Well-known keys for the additional metadata field in a commit in a ref entry
+ * in a summary file. */
+#define OSTREE_COMMIT_TIMESTAMP "ostree.commit.timestamp"
+
 typedef enum {
   OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
 } OstreeRepoTestErrorFlags;