Make more improvements to threading support

- fix rare thread exit race condition on openbsd
- pthread_getattr_np() now supplies detached status
- child threads may now pthread_join() the main thread
- introduce sigandset(), sigorset(), and sigisemptyset()
- introduce pthread_cleanup_push() and pthread_cleanup_pop()

libc/thread/pthread_getattr_np.c

@@ -16,12 +16,51 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/atomic.h"
 #include "libc/str/str.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
 
+/**
+ * Gets thread attributes.
+ *
+ * These attributes are copied from the ones supplied when
+ * pthread_create() was called. However this function supplies
+ * additional runtime information too:
+ *
+ * 1. The detached state. You can use pthread_attr_getdetachstate() on
+ *    the `attr` result to see, for example, if a thread detached itself
+ *    or some other thread detached it, after it was spawned.
+ *
+ * 2. The thread's stack. You can use pthread_attr_getstack() to see the
+ *    address and size of the stack that was allocated by cosmo for your
+ *    thread. This is useful for knowing where the stack is. It can also
+ *    be useful If you explicitly configured a stack too, since we might
+ *    have needed to slightly tune the address and size to meet platform
+ *    requirements. This function returns information that reflects that
+ *
+ * 3. You can view changes pthread_create() may have made to the stack
+ *    guard size by calling pthread_attr_getguardsize() on `attr`
+ *
+ * @param attr is output argument that receives attributes, which should
+ *     find its way to pthread_attr_destroy() when it's done being used
+ * @return 0 on success, or errno on error
+ * @raise ENOMEM is listed as a possible result by LSB 5.0
+ */
 int pthread_getattr_np(pthread_t thread, pthread_attr_t *attr) {
   struct PosixThread *pt = (struct PosixThread *)thread;
   memcpy(attr, &pt->attr, sizeof(pt->attr));
+  switch (atomic_load_explicit(&pt->status, memory_order_relaxed)) {
+    case kPosixThreadJoinable:
+    case kPosixThreadTerminated:
+      attr->__detachstate = PTHREAD_CREATE_JOINABLE;
+      break;
+    case kPosixThreadDetached:
+    case kPosixThreadZombie:
+      attr->__detachstate = PTHREAD_CREATE_DETACHED;
+      break;
+    default:
+      unreachable;
+  }
   return 0;
 }