Non-robust lock files in containers can lead to repo corruption

I tracked down a source of Git corrupting repositories to lock file design not being robust when containers / PID namespaces are present.

In my case, the corruption stemmed from premature release of the `git gc` lock in the gc.pid file. But since the lock file code for that file is in gc.c, there could be other lock files in Git affected by the same design limitation as well.

The lock design of gc.pid stores the current hostname and PID of the locking process in the file. If another process comes along and its hostname matches the stored hostname, it checks to see if the listed PID exists. If the PID is missing, it assumes the lock is stale and releases the lock.

A limitation with this approach is it isn't robust in the presence of containers / PID namespaces. In containers, it is common for the hostname to match the container host's hostname. Or the hostname will be static string. In Kubernetes, all containers within a pod share the same hostname. Containers (almost always) run in separate PID namespaces, so PIDs from outside the container aren't visible to the container itself. This means that if e.g. 2 `git gc` processes are running with the same hostname in separate containers / PID namespaces, Git could prematurely release the lock file because it thinks the "other" PID is dead and repo corruption could ensue due to the 2 `git gc` processes racing with each other.

The on-disk format of lock files obviously needs to be backwards compatible with older clients. One backwards compatible solution is to append something to the hostname to disambiguate containers / PID namespaces. Mercurial appends the current PID namespace identifier to the hostname [1] and my experience is that this is sufficient to mitigate the issue. It is possible more robust solutions are achievable.

Gregory

[1] https://www.mercurial-scm.org/repo/hg/rev/1f151a33af8e