tart prune leaves broken OCI tag symlinks after deleting their digest

[TyceHerrman]

Description

tart prune can delete an OCI digest directory while leaving its tag symlink behind as a broken link.

This does not retain the VM's disk usage, but it leaves the OCI cache in an inconsistent state until the hidden garbage collector is run separately.

Steps to reproduce

1. Pull a tagged image:

   tart pull ghcr.io/cirruslabs/macos-tahoe-base:latest

2. Prune the cache:

   tart prune --entries caches --space-budget 0

3. Inspect the cache:

   $ du -sh ~/.tart/cache
     0B    ~/.tart/cache
   
   $ du -shA ~/.tart/cache
   3.5K    ~/.tart/cache
   
   $ find ~/.tart/cache -type l -print -exec readlink {} \;
   ~/.tart/cache/OCIs/ghcr.io/cirruslabs/macos-tahoe-base/latest
   ~/.tart/cache/OCIs/ghcr.io/cirruslabs/macos-tahoe-base/sha256:a8e1c8305758643f513fdccdd829c2243687c60791083dea42f73f0b7aeb435c

The digest target no longer exists, but the latest symlink remains.

Expected behavior

After pruning an OCI digest, Tart should remove any tag symlinks that now point to the deleted digest.

Actual behavior

The digest directory is deleted successfully, but its tag symlink remains broken.

The current workaround is a second command:

tart prune --gc

Root cause

VMStorageOCI.prunables() explicitly excludes symlinks and returns the digest VMDirectory objects:

https://github.com/openai/tart/blob/2.34.0/Sources/tart/VMStorageOCI.swift

The normal prune paths call Prunable.delete() directly:

https://github.com/openai/tart/blob/2.34.0/Sources/tart/Commands/Prune.swift

This bypasses VMStorageOCI.delete(_:), which is the deletion path that calls gc().

Additionally, when --gc is combined with a pruning criterion, Prune.run() invokes GC before deleting entries. Therefore this still leaves links broken by the subsequent prune:

tart prune --gc --entries caches --space-budget 0

Suggested fix

Run VMStorageOCI.gc() once after OCI cache pruning completes.

A regression test could:

1. Create a digest directory and tag symlink.
2. Prune the digest using both the age and space-budget paths.
3. Verify that the digest and newly broken tag symlink are both removed.
4. Verify that combining --gc with a pruning criterion also leaves no broken links.

Related history

• Cache Pruning #33 / Introduce "tart prune" command #164 introduced cache pruning.
• Implement garbage collection when deleting cached OCI VMs (w.r.t. symbolic links) #37 / Implement garbage collection when deleting cached OCI VMs #185 added garbage collection for broken OCI symlinks.
• Run garbage collection after each symlink and expose --gc in tart prune #191 added post-link garbage collection and the hidden tart prune --gc command.
• tart list questions #776 confirms that healthy tag symlinks are expected; this report concerns only broken ones.

Environment

Tart 2.34.0
macOS 27.0
Architecture: arm64