Fatal error "FileDescriptor 3 was not closed" on Windows

[stefanspringer1]

Describe the bug

A simple call to run results in "Fatal error: FileDescriptor 3 was not closed" on Windows.

To Reproduce

The following main program:

let _ = try await run(.path(#"C:\tmp\Hello.exe"#))

calls the program "Hello.exe" which has the following code:

print("hello")

Expected behavior

Expected is that Hello.exe runs, prints "hello" and no error occurs.

Environment:

• Windows 11
• Swift version 6.1.2 (swift-6.1.2-RELEASE), Target: x86_64-unknown-windows-msvc
• swift-subprocess 80bd50f from 2025-06-13

Additional context

It might have something to do with #72.

[stefanspringer1]

Aha, of course the program does not wait. This explains it.

[stefanspringer1]

Hmm... The following code works on macOS (with another path to the executable), but on Windows it outputs the "Fatal error: FileDescriptor 3 was not closed" above.

let semaphore = DispatchSemaphore(value: 0)
DispatchQueue.global(qos: .userInitiated).async {
    Task {
        print("calling...")
        let result = try await run(.path(#"C:\tmp\Hello.exe"#))
        print(result.standardOutput)
        semaphore.signal()
        print("done calling")
    }
}
print("Waiting for call to finish...")
semaphore.wait()
print("DONE ALL")

So there does seem to a problem.

[kcieplak]

I am as well seeing the same failure.

13:23:09  Build of product 'spi-metrics-collector' complete! (25.42s)
13:23:10  Subprocess/Configuration.swift:636: Fatal error: FileDescriptor 3 was not closed
13:23:10  Current stack trace:
13:23:10  0    spi-metrics-collector.exe          0x00007ff739ee638c <unavailable> + 1270668
13:23:10  1    KERNEL32.DLL                       0x00007ffbd6e68700 BaseThreadInitThunk + 64
13:23:10  2    ntdll.dll                          0x00007ffbd7938280 RtlUserThreadStart + 68

The code is simply doing a Subprocess.run and trying to get the status and stdout.

let versionResult = try await Subprocess.run(.name("swift-build"), arguments: ["--version"])
        if versionResult.terminationStatus == .exited(0) {
            swiftVersion = versionResult.standardOutput!
                .replacingOccurrences(of: "Swift Package Manager - Swift ", with: "")
                .trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)
        }