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This isn't an issue with podman, which will only ever use one directory. But CRI-O generally uses two directories, and we want to make sure that changes to the fallback directory are not clobbering hooks configured in the override directory. More background in [1]. I've split the handling into a single-directory block and a multiple-directory block so we don't waste time polling the filesystem for single-directory removals. I'm using the single-directory block for the the zero-directory case as well. Managers with zero directories should not be receiving fsnotify events, so I don't think it really matters which block handles them. If we want to handle this case robustly (because we're concerned about something in the hook package adjusted the private .directories property on the fly?), then we'll probably want to add an explicit zero-directory block in future work. [1]: https://github.com/kubernetes-incubator/cri-o/pull/1470 Signed-off-by: W. Trevor King <wking@tremily.us> Closes: #757 Approved by: rhatdan
OCI Hooks Configuration
For POSIX platforms, the OCI runtime configuration supports hooks for configuring custom actions related to the life cycle of the container.
The way you enable the hooks above is by editing the OCI runtime configuration before running the OCI runtime (e.g. [runc][runc]).
CRI-O and podman create create the OCI configuration for you, and this documentation allows developers to configure them to set their intended hooks.
One problem with hooks is that the runtime actually stalls execution of the container before running the hooks and stalls completion of the container, until all hooks complete.
This can cause some performance issues.
Also a lot of hooks just check if certain configuration is set and then exit early, without doing anything.
For example the oci-systemd-hook only executes if the command is init or systemd, otherwise it just exits.
This means if we automatically enabled all hooks, every container would have to execute oci-systemd-hook, even if they don't run systemd inside of the container.
Performance would also suffer if we exectuted each hook at each stage (pre-start, post-start, and post-stop).
The hooks configuration is documented in [oci-hooks.5][docs/oci-hooks.5.md].