thread/Thread: add debug attribute "inside_handle"

This attribute shall be used only for IsInside() to make this safe
against a race condition described in #188:

> There is no requirement on the implementation that the ID of the
> created thread be available before the newly created thread starts
> executing.

http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_create.html):

This means that on some pthread implementations (e.g. Haiku), the
assert(thread.IsInside()) could fail.

Closes #188

NEWS

@@ -1,4 +1,5 @@
 ver 0.20.16 (not yet released)
+* fix crash in debug build on Haiku and other operating systems
 
 ver 0.20.15 (2018/01/05)
 * queue: fix crash after seek failure

src/thread/Thread.cxx

@@ -86,6 +86,10 @@ Thread::ThreadProc(void *ctx)
 {
 	Thread &thread = *(Thread *)ctx;
 
+#ifndef NDEBUG
+	thread.inside_handle = pthread_self();
+#endif
+
 	thread.Run();
 
 	return nullptr;

src/thread/Thread.hxx

@@ -41,6 +41,16 @@ class Thread {
 	DWORD id;
 #else
 	pthread_t handle = pthread_t();
+
+#ifndef NDEBUG
+	/**
+	 * This handle is only used by IsInside(), and is set by the
+	 * thread function.  Since #handle is set by pthread_create()
+	 * which is racy, we need this attribute for early checks
+	 * inside the thread function.
+	 */
+	pthread_t inside_handle = pthread_t();
+#endif
 #endif
 
 public:
@@ -79,7 +89,7 @@ public:
 		   default-constructed values" (comment from
 		   libstdc++) - and if both libstdc++ and libc++ get
 		   away with this, we can do it as well */
-		return pthread_self() == handle;
+		return pthread_self() == inside_handle;
 #endif
 	}
 #endif