It's a been a while since this thread has been updated, so I figure I should share what I've learned.
First: here's a script which reproduces the bug reliably and includes steps to revert: https://gist.github.com/dmeiselman/ecb35ffcb675c38e6eeb14292948c2e3
Don't run this on your production cluster.
Here's my understanding of this bug and it's current state:
Some operation sets the snapshot_of metadata field of a valid snapshot to NULL (We can do this intentionally by making a VM, taking a snapshot of it, then deleting the parent disk.)
If you perform a revert of this invalid snapshot, revert's "Updating the snapshot_of fields" step looks for VDIs whose snapshot_of matches the snapshot's source disk, so it can relink them to the new disk. If the source is NULL, the search becomes "every VDI with NULL", which matches ISOs, base images, ordinary VM disks. Congrats, every real disk and ISO in every SR now looks like a snapshot.
Now these disks have a value set for snapshot_of which means XOA hides the VDI in the web UI.
The fix is to delete or clone the super-parent, and all of its snapshots. sr_scan will correct the metadata for all the VMs and ISOs and you're back where you started.
This happens in line 401 of https://github.com/xapi-project/xen-api/blob/v26.1.19/ocaml/xapi/xapi_vm_snapshot.ml#L389-L408
PR 7290 https://github.com/xapi-project/xen-api/pull/7290 has a fix, which looks like it's coming in xapi v26.1.21. (It's worth reading the Pull Request, as it has a more graceful description of the issue, it really helped me wrap my head around it.)
