[SR-15474] Unawaited async let crashes when it goes out of scope

[swift-ci]

Previous ID	SR-15474
Radar	rdar://problem/85310493
Original Reporter	hiimtmac (JIRA User)
Type	Bug

Environment

Darwin hiimtmac.localdomain 21.1.0 Darwin Kernel Version 21.1.0: Wed Oct 13 17:33:23 PDT 2021; root:xnu-8019.41.5~1/RELEASE_X86_64 x86_64

Xcode 13.1
Build version 13A1030d

Swift version 5.5.1 (swift-5.5.1-RELEASE)
Target: x86_64-unknown-linux-gnu

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug, Concurrency, Linux, Runtime
Assignee	@etcwilde
Priority	Medium

md5: 8dd3461f2b203191394482dd01384ec5

Issue Description:

Allowing an async let to go out of scope without ever await ing it causes a crash.

The crash does not happen in LLDB unless the offending async let is within an await ed context.

The following code crashes on Linux, but not on macOS:

import Foundation

@main
struct SwiftBug {
    static func main() async throws {
        print("starting")
        
        try await iWillDie()

        print("done")
    }
    
    static func iWillDie() async throws {
        async let ones = doAsync()
        async let twos = doAsync()
        
        print(try await ones)
        if false {
            print(try await twos)
        }
    }
    
    static func doAsync() async throws -> [Int] {
        [1]
    }
}

I've created a repo that reproduces the crash: https://github.com/hiimtmac/swift-bug

LLDB trace:

root@531888c25919:/# lldb .build/debug/Run
(lldb) target create ".build/debug/Run"
Current executable set to '/.build/debug/Run' (x86_64).
(lldb) run
Process 19 launched: '/.build/debug/Run' (x86_64)
starting
[1]
Process 19 stopped
* thread #​4, name = 'Run', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
    frame #&#8203;0: 0x00007ffff75f2e50 libswiftCore.so`swift::metadataimpl::ValueWitnesses<swift::metadataimpl::SwiftRetainableBox>::destroy(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*)
libswiftCore.so`swift::metadataimpl::ValueWitnesses<swift::metadataimpl::SwiftRetainableBox>::destroy:
->  0x7ffff75f2e50 <+0>: movq   (%rdi), %rdi
    0x7ffff75f2e53 <+3>: jmp    0x7ffff75efe30            ; swift_release
    0x7ffff75f2e58:      nopl   (%rax,%rax)
libswiftCore.so`swift::metadataimpl::ValueWitnesses<swift::metadataimpl::SwiftRetainableBox>::initializeWithCopy:
    0x7ffff75f2e60 <+0>: pushq  %r14
Target 0: (Run) stopped.
(lldb) bt
* thread #&#8203;4, name = 'Run', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
  * frame #&#8203;0: 0x00007ffff75f2e50 libswiftCore.so`swift::metadataimpl::ValueWitnesses<swift::metadataimpl::SwiftRetainableBox>::destroy(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*)
    frame #&#8203;1: 0x00007ffff77b9db6 libswift_Concurrency.so`_asyncLet_finish_continuation(swift::AsyncContext*, void*) + 86
    frame #&#8203;2: 0x00007ffff77b86eb libswift_Concurrency.so`swift::runJobInEstablishedExecutorContext(swift::Job*) + 187
    frame #&#8203;3: 0x00007ffff77b8ce8 libswift_Concurrency.so`swift_job_run + 72
    frame #&#8203;4: 0x00007ffff701d435 libdispatch.so`_dispatch_continuation_pop + 357
    frame #&#8203;5: 0x00007ffff701d1e3 libdispatch.so`_dispatch_async_redirect_invoke + 211
    frame #&#8203;6: 0x00007ffff7029142 libdispatch.so`_dispatch_worker_thread + 514
    frame #&#8203;7: 0x00007ffff709a609 libpthread.so.0`start_thread + 217
    frame #&#8203;8: 0x00007ffff6ed9293 libc.so.6`__clone + 67
(lldb) 

[ktoso]

@swift-ci create

[ktoso]

Hmmm... I thought such issues were fixed; It definitely does not crash on today's recent focal nightly builds.

[etcwilde]

Hello, I am not seeing this bug anymore.

The above code is emitting the following output when built with the current top-of-tree:
```
starting
[1]
done
```

Please let us know if you're still seeing this issue.