Implement custom thread numbering for POSIX

For every UNIX-derived (Android, Linux, macOS, iOS) flavor, a global counter is atomically incremented on thread start. That id is kept as thread-local storage.

Therefore, thread ids are sequential numbers, trivially comparable. This improves the previous state of things, in which `pthread_t` were casted to `Thread::ID` and unportabily compared. Also big, ugly thread ids appeared.

drivers/unix/thread_posix.cpp

@@ -36,8 +36,18 @@
 #include <pthread_np.h>
 #endif
 
+#include "core/safe_refcount.h"
 #include "os/memory.h"
 
+static pthread_key_t _create_thread_id_key() {
+	pthread_key_t key;
+	pthread_key_create(&key, NULL);
+	return key;
+}
+
+pthread_key_t ThreadPosix::thread_id_key = _create_thread_id_key();
+Thread::ID ThreadPosix::next_thread_id = 0;
+
 Thread::ID ThreadPosix::get_id() const {
 
 	return id;
@@ -51,7 +61,8 @@ Thread *ThreadPosix::create_thread_posix() {
 void *ThreadPosix::thread_callback(void *userdata) {
 
 	ThreadPosix *t = reinterpret_cast<ThreadPosix *>(userdata);
-	t->id = (ID)pthread_self();
+	t->id = atomic_increment(&next_thread_id);
+	pthread_setspecific(thread_id_key, (void *)t->id);
 
 	ScriptServer::thread_enter(); //scripts may need to attach a stack
 
@@ -77,7 +88,7 @@ Thread *ThreadPosix::create_func_posix(ThreadCreateCallback p_callback, void *p_
 }
 Thread::ID ThreadPosix::get_thread_id_func_posix() {
 
-	return (ID)pthread_self();
+	return (ID)pthread_getspecific(thread_id_key);
 }
 void ThreadPosix::wait_to_finish_func_posix(Thread *p_thread) {
 

drivers/unix/thread_posix.h

@@ -42,6 +42,9 @@
 
 class ThreadPosix : public Thread {
 
+	static pthread_key_t thread_id_key;
+	static ID next_thread_id;
+
 	pthread_t pthread;
 	pthread_attr_t pthread_attr;
 	ThreadCreateCallback callback;

platform/android/thread_jandroid.cpp

@@ -29,9 +29,19 @@
 /*************************************************************************/
 #include "thread_jandroid.h"
 
+#include "core/safe_refcount.h"
 #include "os/memory.h"
 #include "script_language.h"
 
+static pthread_key_t _create_thread_id_key() {
+	pthread_key_t key;
+	pthread_key_create(&key, NULL);
+	return key;
+}
+
+pthread_key_t ThreadAndroid::thread_id_key = _create_thread_id_key();
+Thread::ID ThreadAndroid::next_thread_id = 0;
+
 Thread::ID ThreadAndroid::get_id() const {
 
 	return id;
@@ -47,7 +57,8 @@ void *ThreadAndroid::thread_callback(void *userdata) {
 	ThreadAndroid *t = reinterpret_cast<ThreadAndroid *>(userdata);
 	setup_thread();
 	ScriptServer::thread_enter(); //scripts may need to attach a stack
-	t->id = (ID)pthread_self();
+	t->id = atomic_increment(&next_thread_id);
+	pthread_setspecific(thread_id_key, (void *)t->id);
 	t->callback(t->user);
 	ScriptServer::thread_exit();
 	return NULL;
@@ -68,7 +79,7 @@ Thread *ThreadAndroid::create_func_jandroid(ThreadCreateCallback p_callback, voi
 
 Thread::ID ThreadAndroid::get_thread_id_func_jandroid() {
 
-	return (ID)pthread_self();
+	return (ID)pthread_getspecific(thread_id_key);
 }
 
 void ThreadAndroid::wait_to_finish_func_jandroid(Thread *p_thread) {

platform/android/thread_jandroid.h

@@ -41,6 +41,9 @@
 
 class ThreadAndroid : public Thread {
 
+	static pthread_key_t thread_id_key;
+	static ID next_thread_id;
+
 	pthread_t pthread;
 	pthread_attr_t pthread_attr;
 	ThreadCreateCallback callback;