Hey there @emontnemery, mind taking a look at this pull request as its been labeled with a integration (cast) you are listed as a codeowner for? Thanks!
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Verified google cast was still discovered and tested with cast.show_lovelace_view

@bdraco I think this is basically fine. Two points though:

• cast will now require HA's zeroconf to be enabled or it will fail to work.
  Does zeroconf need to be added as a dependency of cast?
  Will there be an error message if cast is enabled but zeroconf is disabled?
• The cast integration is not only relying on zeroconf for device discovery, but also to keep track of IP address changes and speaker groups. Can the shared zeroconf instance be "messed" with by other components, e.g. restarted, cache flushed etc?

@bdraco I think this is basically fine. Two points though:

• cast will now require HA's zeroconf to be enabled or it will fail to work.
  Does zeroconf need to be added as a dependency of cast?

Yes 🤦 on my part, I added it to after_dependencies

Will there be an error message if cast is enabled but zeroconf is disabled?

There will now :)

• The cast integration is not only relying on zeroconf for device discovery, but also to keep track of IP address changes and speaker groups. Can the shared zeroconf instance be "messed" with by other components, e.g. restarted, cache flushed etc?

When doing the initial investigation I identified the following integrations using Zeroconf:

homekit - safe if close is guarded
zeroconf - safe
cast - safe if close is guarded
pyvizio - unchecked as its relaying on home assistant's discovery (don't plan on adding to the share)
libpurecool - unchecked as its relaying on home assistant's discovery (don't plan on adding to the share)
aioesphomeapi - momentary use only and than destroyed, not worth sharing
netdisco - safe if close is guarded (may add to the share, but might not be worth the effort at this point)

The big risk is having the Zeroconf instance closed. We have guarded against that in zeroconf/__init__.py:class HaZeroconf by blocking the close until Home Assistant stops.

Right now the only integrations using it are zeroconf and homekit and I know both of these are safe. I'm also working on an a PR for python-zeroconf to try to solve the problem where each browser is running its own thread as its ever growing performance issue as we add new integrations to zeroconf discovery.

@emontnemery Thanks for reviewing.