PR 2 removes the global server `RwLock<Omnigraph>` (Step F). Without
admission control, one heavy actor would exhaust shared capacity
(Lance I/O threads, manifest churn, network) and starve other actors.
The WorkloadController bounds per-actor in-flight count + bytes and
provides a global rewrite-pool semaphore for compaction / index builds.
New file: `crates/omnigraph-server/src/workload.rs` (~250 LOC + 5 tests).
API:
- `WorkloadController::new(inflight_cap, byte_cap, rewrite_cap)` /
`from_env()` / `with_defaults()`.
- `try_admit(actor_id, est_bytes) -> Result<AdmissionGuard, RejectReason>`
acquires both an in-flight count permit and adds est_bytes to the
per-actor counter atomically; returns RejectReason on either gate.
- `try_admit_rewrite() -> Result<RewriteGuard, RejectReason>` for the
global rewrite pool (Step F maps RewriteGuard exhaustion to HTTP 503).
- `RejectReason::{InFlightCountExceeded, ByteBudgetExceeded,
GlobalRewriteExhausted}`.
Race-free admission via `tokio::sync::Semaphore::try_acquire_owned()`
for the count gate (master plan Finding 6: independent atomic
load+check+add lets two callers both pass a cap-N check; the Semaphore
gate is atomic). Bytes use `fetch_add` + decrement-on-rejection so the
cap is never exceeded even on rollback.
Defaults (override via env):
- OMNIGRAPH_PER_ACTOR_INFLIGHT_MAX=16
- OMNIGRAPH_PER_ACTOR_BYTES_MAX=4_294_967_296 (4 GiB)
- OMNIGRAPH_GLOBAL_REWRITE_MAX=4
Tests cover under-cap admission, byte-budget rollback, per-actor
isolation, global rewrite cap, and the load-bearing 32-concurrent-vs-
cap-16 race test (forces real contention via a broadcast release
channel so guards can't recycle permits task-by-task; pins the
master plan's race-free invariant).
Adds workspace dep `dashmap = "6"` for per-actor state.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
