From: Junio C Hamano Date: Fri, 13 Feb 2015 21:53:08 GMT Subject: Re: [PATCH 8/8] reflog_expire(): lock symbolic refs themselves, not their referent Message-ID: In-Reply-To: <54DE5A72.6030106@alum.mit.edu> Michael Haggerty writes: > Now back to the real world. Currently, if R is changed *through* a > symbolic reference S, then the reflogs for both R and S are updated, but > not the reflogs for any other symbolic references that might point at R. > If R is changed directly, then no symref's reflogs are affected, except > for the special case that HEAD's reflog is changed if it points directly > at R. This limitation is a hack to avoid having to walk symrefs > backwards to find any symrefs that might be pointing at R. Yup. > It might actually not be extremely expensive to follow symrefs > backwards. Symbolic references cannot be packed, so we would only have > to scan the loose references; we could ignore packed refs. But it would > still be a lot more expensive than just updating one file. I don't know > that it's worth it, given that symbolic references are used so sparingly. I personally do not think it is worth it. I further think that it would be perfectly OK to do one of the following: - We only maintain reflogs for $GIT_DIR/HEAD; no other symrefs get their own reflog, and we only check $GIT_DIR/HEAD when updating refs/heads/* and no other refs for direct reference (i.e. HEAD -> refs/something/else -> refs/heads/master symref chain is ignored). - In addition to the above, we also maintain reflogs for $GIT_DIR/refs/remotes/*/HEAD but support only when they directly point into a remote tracking branch in the same hierarchy. $GIT_DIR/refs/remotes/foo/HEAD that points at $GIT_DIR/refs/remotes/bar/master is ignored and will get an undefined behaviour. > I think that the rule about locks as expressed above can be carried over > the the real world: > > We should hold the locks on exactly those references (symbolic > or regular) whose reflogs we plan to change. We should acquire all > of the locks before making any changes. Sure.