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662cf67ff8
PR 3 of the MR-668 multi-graph server work. Pure data structure — no
routing changes yet (that's PR 4a).
New file: `crates/omnigraph-server/src/registry.rs`
- `GraphHandle { key: GraphKey, uri: String, engine: Arc<Omnigraph>,
policy: Option<Arc<PolicyEngine>> }` — the per-graph state that the
routing middleware (PR 4a) will inject as a request extension.
- `RegistrySnapshot { graphs: HashMap<GraphKey, Arc<GraphHandle>> }` —
immutable snapshot; replaced atomically via `ArcSwap`.
- `GraphRegistry { snapshot: ArcSwap<_>, mutate: Mutex<()> }` — lock-free
reads, mutex-serialized mutations.
- `RegistryLookup { Ready(Arc<GraphHandle>) | Gone }` — two-valued, no
`Tombstoned` variant since DELETE is deferred in v0.7.0 scope.
- `InsertError { DuplicateKey | DuplicateUri }` — both rejection cases
for create-graph (maps to HTTP 409 in PR 7).
- Methods: `new`, `from_handles` (bulk startup-time init), `get`, `list`,
`len`, `insert`.
Race semantics pinned by three multi-thread tests:
- `concurrent_insert_same_key_exactly_one_succeeds` — N=8 spawned
inserts with the same key; exactly 1 returns Ok, 7 return DuplicateKey.
- `concurrent_insert_distinct_keys_all_succeed` — N=8 spawned inserts
with distinct keys; all succeed.
- `concurrent_reads_during_inserts_see_consistent_snapshots` — reader
loop concurrent with sequential writes; every listed handle's key
resolves via `get()` (no torn state).
Why no tombstones field: `DELETE /graphs/{id}` is deferred to bound
the scope of v0.7.0. Without a delete endpoint, there's no use for
tombstones — every key in the registry is `Ready`, and every key
not in the registry is `Gone`. When DELETE lands later, the
`Tombstoned` variant + `tombstones: HashSet<GraphKey>` slot in
additively without breaking caller signatures (the `Gone` variant
remains the "not currently active" case).
Why `tokio::sync::Mutex`: insert is async because PR 7's flow holds
this mutex across the atomic YAML rewrite step (file I/O). std::Mutex
would footgun across .await.
Dependency additions: `arc-swap = { workspace = true }`,
`thiserror = { workspace = true }` (used by InsertError).
Tests: 12 new (12 passing). 74 server lib tests total green
(62 from PR 1 + 12 new). Clippy clean on server crate.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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