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## Problem - Previously, an async mutex was held for the duration of `ComputeHook::notify`. This served multiple purposes: - Ensure updates to a given tenant are sent in the proper order - Prevent concurrent calls into neon_local endpoint updates in test environments (neon_local is not safe to call concurrently) - Protect the inner ComputeHook::state hashmap that is used to calculate when to send notifications. This worked, but had the major downside that while we're waiting for a compute hook request to the control plane to succeed, we can't notify about any other tenants. Notifications block progress of live migrations, so this is a problem. ## Summary of changes - Protect `ComputeHook::state` with a sync lock instead of an async lock - Use a separate async lock ( `ComputeHook::neon_local_lock` ) for preventing concurrent calls into neon_local, and only take this in the neon_local code path. - Add per-tenant async locks in ShardedComputeHookTenant, and use these to ensure that only one remote notification can be sent at once per tenant. If several shards update concurrently, their updates will be coalesced. - Add an explicit semaphore that limits concurrency of calls into the cloud control plane.