`await`ing `async let` function from lazy initialized object doesn't execute concurrently in an `XCTest` test case

[sja26]

Description

Executing inside of an XCTest class' test case method, that calls await on multiple consecutive async lets of which the async function resides in a lazy initialized variable, doesn't go on to run each function concurrently. Instead, only the first async function is called, the next async function doesn't get called concurrently. Targeting macOS or iOS Simulator yields the same result.

Steps to reproduce

1. Clone example repo containing Swift Package
2. Change directory to SwiftStructuredConcurrencyLazyBug
3. Run swift test

Expected behavior

Executing inside of an XCTest class' test case method, that calls await on multiple consecutive async lets of which the async function resides in a lazy initialized variable, should run each function concurrently.

Environment

• swift-driver version: 1.62.15 Apple Swift version 5.7.2 (swiftlang-5.7.2.135.5 clang-1400.0.29.51)
  Target: x86_64-apple-macosx13.0
• Xcode 14.2 Build version 14C18
• Deployment target: macOS 13, iOS 16

Additional context

I can't reproduce this behaviour outside of an XCTest class. See Swift Playground SwiftStructuredConcurrencyLazy in example repo.

[pjechris]

Ran into the same issue today. When using async let the async function is actually not started straight away but only when awaiting it. That is when writing:

async let test = myAsyncFunc()

let result = await test

And using step-by-step debugger, I enter myAsyncFunc body only when await test is called.

But I also see this behaviour inside app, not only XCTest. I don't know if my way of testing is accurate but if it is the case then this might be a big bug.

[ktoso]

AFAIR simulator is currently single threaded which is the reason for such behavior, but you say you're encountering the same on macos?

[pjechris]

@ktoso sorry I was not clear enough: on an iOS app on simulator. I'll try to redo some tests on real devices in case of.