Add OstreeRepo option for an out-of-band cache dir

This allows you to have a writable cache dir even for a system-owned
repository.

Closes: #250
Approved by: cgwalters

src/libostree/ostree-repo.c

@@ -2573,6 +2573,36 @@ ostree_repo_set_disable_fsync (OstreeRepo    *self,
   self->disable_fsync = disable_fsync;
 }
 
+/**
+ * ostree_repo_set_cache_dir:
+ * @self: An #OstreeRepo
+ * @dfd: directory fd
+ * @path: subpath in @dfd
+ *
+ * Set a custom location for the cache directory used for e.g.
+ * per-remote summary caches. Setting this manually is useful when
+ * doing operations on a system repo as a user because you don't have
+ * write permissions in the repo, where the cache is normally stored.
+ */
+gboolean
+ostree_repo_set_cache_dir (OstreeRepo    *self,
+                           int            dfd,
+                           const char    *path,
+                           GCancellable  *cancellable,
+                           GError        **error)
+{
+  int fd;
+
+  if (!glnx_opendirat (dfd, path, TRUE, &fd, error))
+    return FALSE;
+
+  if (self->cache_dir_fd != -1)
+    close (self->cache_dir_fd);
+  self->cache_dir_fd = fd;
+
+  return TRUE;
+}
+
 /**
  * ostree_repo_get_disable_fsync:
  * @self: An #OstreeRepo