feat(sync): let a producer report whether a publish can still land

[lxsaah]

Why

A facade built on aimdb-sync has to answer "can I still publish?" for its own callers, and today it cannot.

fork::generation and fork::forked_since are crate-private, and asking the AimDbHandle is not open to an FFI door that keeps it behind a lock — answering while a shutdown holds that lock is exactly how a caller's interpreter lock deadlocks against its own teardown. That is the lock-ordering rule the Python door established.

So the question had no supported route. aimdb-weather-mesh's StationHandle::is_closed reached for aimdb_sync::fork::* instead, and stopped compiling when #235 made the module private.

What

One method on SyncProducer:

pub fn check(&self) -> SyncResult<()> {
    self.runtime()?.check()
}

Two lines, because #235 had already put every route in behind a single check. Ok(()), or RuntimeShutdown / ForkedChild.

It answers from the one component reachable without the handle's lock — which is also why a publish never queues behind a shutdown. And it is the check set() already performs, not a second one beside it, so it cannot report open while a publish would be refused.

On exposing fork instead

Not done, deliberately. What this commits to in semver is the question, not the mechanism. Publishing the generation pair would pin stamp-and-compare — the objection 2bcc5a3 raised when it withdrew them mid-PR.

That commit deferred the decision for want of a real caller to design against. The caller exists now, and its requirement is specific: cheap, and never the mutex. check() is designed against it. The note on fork::generation claiming no such layer exists is corrected to point at the supported route.

Tests

• check_reports_what_a_publish_would_find — usable while attached, RuntimeShutdown after detach, and the same verdict set() reaches.
• The forked-child test now also asserts check() agrees with the two publishes beside it. That agreement is the property a facade depends on.

Verification

• aimdb-sync: 58 tests green (was 57), plus --no-default-features and --features data-contracts.
• cargo fmt --check and clippy clean on all three feature configurations.
• End-to-end through the C++ door in aimdb-weather-mesh (make spike-cpp), all four checks of the fork round green.

Merge order

aimdb-dev/aimdb-weather-mesh has a companion PR that consumes this. This one lands first — that one does not compile without it.

🤖 Generated with Claude Code