Return ENOTSUP from pthread_create when threads are not available.

See WebAssembly/wasi-libc#716

Author: sbc100

system/lib/pthread/library_pthread_stub.c

@@ -92,7 +92,11 @@ int pthread_barrier_destroy(pthread_barrier_t* mutex) { return 0; }
 int pthread_barrier_wait(pthread_barrier_t* mutex) { return 0; }
 
 int __pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg) {
-  return EAGAIN;
+  // ENOTSUP, while not mentioned in the pthread_create docs, does better
+  // describe the situation.
+  // See https://github.com/WebAssembly/wasi-libc/pull/716 for discussion
+  // on this error code vs, for example, EAGAIN.
+  return ENOTSUP;
 }
 
 weak_alias(__pthread_create, emscripten_builtin_pthread_create);

test/other/test_pthread_stub.c

@@ -22,12 +22,12 @@ void* start_pthread(void* arg) {
 }
 
 #define CHECK(X, EXPECTED) rtn = X; if (rtn != EXPECTED) printf(#X " returned %s\n", strerror(rtn)); assert(rtn == EXPECTED)
-#define CHECK_FAIL(X) CHECK(X, EAGAIN)
+#define CHECK_FAIL(X) CHECK(X, ENOTSUP)
 #define CHECK_SUCCESS(X) CHECK(X, 0)
 
 #define CHECK_C11(X, expected) rtn = X; if (rtn != expected) printf(#X " returned %d\n", rtn); assert(rtn == expected)
 #define CHECK_C11_SUCCESS(X) CHECK_C11(X, thrd_success)
-#define CHECK_C11_FAIL(X) CHECK_C11(X, thrd_nomem)
+#define CHECK_C11_FAIL(X) CHECK_C11(X, thrd_error)
 
 void test_c11_threads() {
   printf("test_c11_threads\n");