[Issue 20680] New: core.thread.Thread leaks OS handle when not joined

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20680

          Issue ID: 20680
           Summary: core.thread.Thread leaks OS handle when not joined
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: chalucha at gmail.com

When thread is just created and forgotten about - which is pretty common case
for worker threads (ie with std.concurrency.spawn) it leaks the OS handle (at
least on windows) as CloseHandle is not called in Thread destructor.

My guess is that it's because of these lines:
* https://github.com/dlang/druntime/blob/master/src/core/thread/osthread.d#L304
* https://github.com/dlang/druntime/blob/master/src/core/thread/osthread.d#L697

The last line yields true so CloseHandle isn't called in Thread destructor.

See the test case below and watch what happens with process handles ie in
sysinternals procexp or handles utilities.


```D
import core.thread;
import core.time;
import std;

extern(C) __gshared string[] rt_options = [ "gcopt=parallel:0" ];

void main() {
        writeln("press enter to start");
        readln();

        Thread[10] threads;
        foreach (i; 0..10) threads[i] = new Thread(&threadFn).start();

        Thread.sleep(1.seconds); // make sure they are done

        foreach (i; 0..10) {
            // threads[i].join(); # if joined, OS handle is closed as expected
            destroy(threads[i]); # enforce destructors to be called
        }

        writeln("threads disposed, press enter to exit");
        readln();
}

void threadFn() { }
```

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