thread_pthread.c: allocate sigaltstack before pthread_create

A new (not-initialized-yet) pthread attempts to allocate sigaltstack by
using xmalloc.  It may cause GC, but because the thread is not
initialized yet, ruby_native_thread_p() returns false, which leads to
"[FATAL] failed to allocate memory" and exit.

In fact, we can observe the error message in the log of OpenBSD CI:
https://rubyci.org/logs/rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20200306T083005Z.log.html.gz

This changeset allocates sigaltstack before pthread is created.

1 files changed, 4 insertions, 1 deletions

diff --git a/thread_pthread.c b/thread_pthread.c
index 29284ff0f9..b9ac63f298 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -950,7 +950,7 @@ static void *
 thread_start_func_1(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
-    RB_ALTSTACK_INIT(void *altstack);
+    RB_ALTSTACK_INIT(void *altstack, th->altstack);
 #if USE_THREAD_CACHE
   thread_start:
 #endif
@@ -1099,6 +1099,9 @@ native_thread_create(rb_thread_t *th)
         const size_t stack_size = th->vm->default_params.thread_machine_stack_size + th->vm->default_params.thread_vm_stack_size;
 	const size_t space = space_size(stack_size);
 
+#ifdef USE_SIGALTSTACK
+        th->altstack = rb_allocate_sigaltstack();
+#endif
         th->ec->machine.stack_maxsize = stack_size - space;
 
 	CHECK_ERR(pthread_attr_init(&attr));