lib/pull: Add support for timestamp-check option, use in upgrader

For both flatpak and ostree-as-host, we really want to verify up front during
pulls that we're not being downgraded. Currently both flatpak and
`OstreeSysrootUpgrader` do this before deployments, but at that point we've
already downloaded all the data, which is annoying.

Closes: https://github.com/ostreedev/ostree/issues/687

Closes: #1055
Approved by: jlebon

src/libostree/ostree-core.c

@@ -2086,6 +2086,38 @@ ostree_commit_get_timestamp (GVariant  *commit_variant)
   return GUINT64_FROM_BE (ret);
 }
 
+/* Used in pull/deploy to validate we're not being downgraded */
+gboolean
+_ostree_compare_timestamps (const char   *current_rev,
+                            guint64       current_ts,
+                            const char   *new_rev,
+                            guint64       new_ts,
+                            GError      **error)
+{
+  /* Newer timestamp is OK */
+  if (new_ts > current_ts)
+    return TRUE;
+  /* If they're equal, ensure they're the same rev */
+  if (new_ts == current_ts || strcmp (current_rev, new_rev) == 0)
+    return TRUE;
+
+  /* Looks like a downgrade, format an error message */
+  g_autoptr(GDateTime) current_dt = g_date_time_new_from_unix_utc (current_ts);
+  g_autoptr(GDateTime) new_dt = g_date_time_new_from_unix_utc (new_ts);
+
+  if (current_dt == NULL || new_dt == NULL)
+    return glnx_throw (error, "Upgrade target revision '%s' timestamp (%" G_GINT64_FORMAT ") or current revision '%s' timestamp (%" G_GINT64_FORMAT ") is invalid",
+                       new_rev, new_ts,
+                       current_rev, current_ts);
+
+  g_autofree char *current_ts_str = g_date_time_format (current_dt, "%c");
+  g_autofree char *new_ts_str = g_date_time_format (new_dt, "%c");
+
+  return glnx_throw (error, "Upgrade target revision '%s' with timestamp '%s' is chronologically older than current revision '%s' with timestamp '%s'",
+                     new_rev, new_ts_str, current_rev, current_ts_str);
+}
+
+
 GVariant *
 _ostree_detached_metadata_append_gpg_sig (GVariant   *existing_metadata,
                                           GBytes     *signature_bytes)