Tear down deltaproxy sub-proxies on shutdown (#70221)

[ggiesen]

What does this PR do?

Makes a deltaproxy tear down its sub-proxies when it stops.

What issues does this PR fix?

Fixes #70221

Root cause

post_master_init builds the sub-proxies into self.deltaproxy_objs, and nothing in the shutdown path ever touches that dict again. salt.minion.ProxyMinion had no destroy() of its own, so it inherited Minion.destroy(), which tears down only the instance it is called on. Each sub-proxy is a full ProxyMinion with its own req_channel, schedule, beacons and periodic callbacks, so all of those were abandoned on every stop or restart.

Previous Behavior

A SIGTERM to a deltaproxy hosting three sub-proxies produced exactly one teardown:

destroy dpcontrol

New Behavior

destroy minion1
destroy minion2
destroy minion3
destroy dpcontrol

Merge requirements satisfied?

• Docs
• Changelog
• Tests written/updated

Tests written?

Yes, three in tests/pytests/unit/test_minion.py: sub-proxies are torn down and the control proxy still tears itself down afterwards; one sub-proxy raising does not stop the others or the control proxy; and a single proxy with no deltaproxy_objs is unaffected. The first two fail against unmodified 3008.x.

Verified against a live deltaproxy on 3008.2 with three sub-proxies, where destroy() calls went from 1 to 4 in the order above.

One note on reproducing: this is only observable once the proxy actually reaches its teardown. On an unpatched 3008.2 the daemon exits before the scheduled graceful shutdown runs at all, which is #70217 / #70218, so that fix needs to be in place first or nothing is torn down regardless. The two are independent changes.

A related question, deliberately not addressed here

While tracing this I noticed the proxymodule's own shutdown() is never invoked on daemon stop for any proxy, not just sub-proxies -- the only callers anywhere in salt/ are salt/modules/junos.py and salt/modules/status.py. So a proxymodule that opens a device session has no teardown hook salt itself calls when the daemon stops.

That may be deliberate, and changing it would alter behaviour for every proxy deployment rather than just deltaproxy, so this PR does not touch it. Raised in #70221 in case it is not intentional, and happy to follow up separately if you want it changed.

Commits signed with GPG?

No