refactor(core): use Latch for MCP startup gate - #43570
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What
Each MCP server entry's
startupsignal is a pure open-once gate: it is only ever completed withExit.void— deliberately, even on registration failure or replacement interruption, so readers cannot hang ("Settle startup even when registration fails..." comment). ADeferred<void>leaves "never delivers a failure" as a convention enforced by three completion sites;Latchmakes it structural — the type cannot fail or interrupt waiters.How
packages/core/src/mcp/index.ts:ServerEntry.startupis nowLatch.Latch, created withLatch.makeUnsafe()(starts closed).entry.startup.open/entry.startup.openUnsafe(); theensuring(Effect.sync(() => Deferred.doneUnsafe(..., Exit.void)))wrapper becomes plainensuring(entry.startup.open).entry.startup.await.No behavior change. Found by a repo-wide audit of
Deferred<void>-as-gate patterns; sibling PRs convert the other pure-gate sites.Testing
packages/core:bun typecheck,bun test test/mcp.test.ts test/mcp-instructions.test.ts test/mcp-oauth.test.ts test/mcp-import-boundary.test.ts(38 pass), and the fullbun run testsuite (0 fail across 1908 tests).