Open cap-eviction sessions at the build-slot cap - #1872
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The cap-eviction scenario fails on nearly every main run since it landed:
openSession (session-4): no mcp-session-id header. The scenario and the session-build semaphore merged within minutes of each other and were never tested together — the scenario's last green run predates the semaphore.Mechanism, from the failed shard's boot.log: the scenario opened cold sessions 8 at a time, but only 4 build slots exist, so the surplus inits sat queued inside the Durable Object's
blockConcurrencyWhilewindow. On a CI runner where four concurrent builds already saturate the CPU, queue wait plus build time crossed workerd's budget —blockConcurrencyWhile() ... waited for too long, the platform reset the DO, andinitializereturned without a session id.The burst width is incidental to what the scenario pins (cap eviction through a real cross-DO request, which only needs the total session count to cross the cap), so it now opens sessions at the build-slot cap. No assertions changed.
The queue-inside-
blockConcurrencyWhileinteraction itself is real but bounded in production: 10s max queue wait plus a 1-2s build fits the 30s budget. It only overflows when builds themselves crawl, which is the starved-CI condition.Verified: the scenario passes locally with the change.