Maybe looking at whatever CRT/call sequence mingw uses could help,

They almost certainly call accept(), which is probably what is being called now.

But I don't think changing the accept() call is the right way to address problems in PerlIO. The former currently works, it's the latter that has problems.

even if it moves Perl towards bringing its own OS to the table

Perl already has far too many layers, one atop the other. Each added by a different person with a different agenda, and all intermingling in a way that is a nightmare to unravel. They never seem to throw anything away, just gloss over with a shiny new coat of code. Adding more layers isn't the right approach.

[accept()] currently works, it's [PerlIO] that has problems.

I don't see how PerlIO can be a problem. It's a system to provide IO layers to handles. Individual functions in Perl's IO library might have bugs, but that doesn't mean PerlIO is a problem.

In fact, this replicates the problem outside of PerlIO (I think):

#! perl -lw
use strict;
use threads;
use IO::Socket;

my $lsn = new IO::Socket::INET(
    Listen => 5, LocalPort => '12345'
) or die "Failed to open listening port: $!\n";

async{
    print 'accept starting';
    my $c = $lsn->accept;
    print 'accept returned';
}->detach;

sleep 2;

print "-d starting";
-d $lsn;    # Freezes
print "-d returned";
[download]

The problem does not lie in the CRT. This does everything that my sample in the OP and your sample do, using the CRT directly, and there are no signs of any locking:

<Reveal this spoiler or all in this thread>

Output:

C:\test>828831.exe
828831.c(115):_fstat( lsn, &s ) == -1
        failed with error 9 (Bad file descriptor)
828831.c(116):_tell( lsn )
        failed with error 9 (Bad file descriptor)
828831.c(117):_dup( lsn ) == -1
        failed with error 9 (Bad file descriptor)
Connect from: 120
Thread started for client 120
Connect from: 128
Thread started for client 128
Thread completed 4895 echos for client 128
Thread completed 11575 echos for client 120
[download]

The locking is occuring within Perl itself, but damned if I can work out where. It must be in Win32 specific code given that other OSs aren't suffering the same problem. But given that OS conditional code is spread throughout the entire code base, it'll take someone clever than me to find it.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"I'd rather go naked than blow up my ass"