Call pthread_detach to make sure the resources get reclaimed

We don't call pthread_join on the threads we create, so apparently
unless we call pthread_detach we just don't get the resources back
(ex stack memory).

src/core/threading.cpp

@@ -351,6 +351,7 @@ intptr_t start_thread(void* (*start_func)(Box*, Box*, Box*), Box* arg1, Box* arg
     RELEASE_ASSERT(code == 0, "");
     if (VERBOSITY() >= 2)
         printf("pthread thread_id: 0x%lx\n", thread_id);
+    pthread_detach(thread_id);
 
     static_assert(sizeof(pthread_t) <= sizeof(intptr_t), "");
     return thread_id;

test/tests/thread_spawning.py

+# allow-warning: converting unicode literal to str
+# fail-if: '-x' in EXTRA_JIT_ARGS
+
+# Make sure we can spawn a bunch of threads
+
+import threading
+
+cv = threading.Condition()
+
+MAX_WORKERS = 2
+nworkers = 0
+
+def worker():
+    global nworkers
+    with cv:
+        nworkers -= 1
+
+        cv.notifyAll()
+
+threads = []
+for i in xrange(400):
+    with cv:
+        while nworkers >= MAX_WORKERS:
+            cv.wait()
+
+    t = threading.Thread(target=worker)
+    t.start()
+    threads.append(t)
+
+for t in threads:
+    t.join()