Copilot instructions

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2applyTo: "src/libraries/Microsoft.Extensions.Hosting*/**"
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5# Microsoft.Extensions.Hosting — Folder-Specific Guidance
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7## Host & Service Lifecycle (D15)
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9- `Host.StopAsync` must not throw when the cancellation token is canceled — it must allow hosted services and host lifetime to complete their shutdown logic
10- Shutdown signal handlers (`UseConsoleLifetime`, SIGTERM) must correctly dispose registrations and propagate to all hosted services on all platforms
11- If an app exposes readiness health checks, it should avoid listening for or accepting work until its own readiness conditions are met
12- `BackgroundService.ExecuteTask` may be null if the derived class did not call `base.StartAsync` — never assume it is set
13- `BackgroundService.ExecuteAsync` exceptions must be observed and logged — unobserved task exceptions silently crash the host
14- Graceful shutdown must handle `OperationCanceledException` from the stopping token without logging it as an error
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16## Hosted Service Ordering
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18- Hosted services start in registration order and stop in reverse order — document dependencies between services
19- Do not assume ordering across `IHostedService` implementations unless explicitly registered in sequence
20- Consider `ValidateOnStart` to catch configuration errors during startup instead of at first request
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22## Thread Safety & Static State (D6)
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24- Two hosts running in the same process must not interfere via static state
25- Static event handlers and registrations must be scoped to the host instance, not the process
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27## Cross-Platform Correctness (D19)
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29- Shutdown signal handling (SIGTERM, SIGINT, Ctrl+C) must work correctly on Windows, Linux, and macOS
30- Platform-specific behavior differences must be abstracted behind PAL layers or guarded with runtime checks
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32## Error Handling (D9)
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34- Exceptions from hosted services must be properly observed — unhandled exceptions in `ExecuteAsync` must not silently terminate the host
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36## Test Reliability (D11)
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38- Hosted service lifecycle tests must use generous timeouts (3+ minutes in stress/JIT stress pipelines)
39- Avoid tight timing assertions — prefer event-based synchronization over `Task.Delay`-based waits
40- Tests must not leak global state (environment variables, static fields) across test runs
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