[Issue 19313] New: Attaching external thread may result in segfault

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19313

          Issue ID: 19313
           Summary: Attaching external thread may result in segfault
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: mihails.strasuns at gmail.com

Split from https://issues.dlang.org/show_bug.cgi?id=19288 as on of sub-issues

Essence of the issue:

```
import core.sys.posix.pthread;
import core.memory;

extern(C)
void* entry_point(void*)
{
    // in real world case this may be called through `thread_attachThis`:
    GC.collect();
    return null;
}

void main()
{
    // ensure some garbage exists
    auto x = new int[1000];

    pthread_t thread;
    auto status = pthread_create(&thread, null, &entry_point, null);
    assert(status == 0);
    pthread_join(thread, null);
}
```

Same applies to other operating systems. Problems comes from the fact that GC
implementation is not ready for `Thread.getThis()` to be `null` and will crash
in several places if this happens. And because `thread_attachThis` has to call
`new Thread` to allocate new context, collection may happen before thread is
actually registered.

--