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fix(tests): widen waitForever CPU test window to eliminate parallel-suite flakiness #107

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fix(tests): widen waitForever CPU test window to eliminate parallel-suite flakiness #107

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22 changes: 13 additions & 9 deletions Tests/Container-Compose-StaticTests/WaitForeverCpuTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -32,18 +32,22 @@ struct WaitForeverCpuTests {
/// Regression for #27: `waitForever()` must suspend, not busy-loop.
///
/// Method: take a `getrusage(RUSAGE_SELF)` snapshot, spawn a child Task
/// that calls `waitForever()`, sleep for 200ms wall-clock, take a second
/// that calls `waitForever()`, sleep for 500ms wall-clock, take a second
/// snapshot, and compare user-CPU consumed.
///
/// On the bug (`for await _ in AsyncStream<Void>(unfolding: {})`), the
/// child task pins one core, so user CPU consumed during the 200ms window
/// is around 200,000 µs (one full core). With the fix
/// child task pins one core, so user CPU consumed during the 500ms window
/// is around 500,000 µs (one full core). With the fix
/// (`withUnsafeContinuation { _ in }`), the child task suspends and
/// consumes essentially nothing.
///
/// Threshold of 50,000 µs gives ~4× headroom over a noisy CI baseline
/// (test-runner overhead, parallel tasks) while still reliably catching
/// a single core's worth of busy-loop work.
/// The measurement is process-wide, and swift-testing runs other suites
/// in parallel in the same process, so the baseline includes their CPU
/// (observed up to ~70,000 µs for the full static suite). That noise is
/// front-loaded — the rest of the suite finishes within the first
/// ~100ms — while a busy-loop scales with the window. The threshold of
/// half a core (250,000 µs over 500ms) therefore sits ~3× above suite
/// noise and ~2× below a pinned core.
///
/// Side effect: this test leaks one suspended task per invocation
/// (`waitForever` is `-> Never` and the suspended task can't be cancelled
Expand All @@ -61,12 +65,12 @@ struct WaitForeverCpuTests {
await composeUp.waitForever()
}

try await Task.sleep(nanoseconds: 200_000_000) // 200ms
try await Task.sleep(nanoseconds: 500_000_000) // 500ms

let after = userCpuMicroseconds()
let consumed = after - before

#expect(consumed < 50_000,
"waitForever consumed \(consumed) µs of user CPU in 200ms — likely busy-looping (regression for #27)")
#expect(consumed < 250_000,
"waitForever consumed \(consumed) µs of user CPU in 500ms — likely busy-looping (regression for #27)")
}
}
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