add limited pthread_setattr_default_np API to set stack size defaults

based on patch by Timo Teräs:

While generally this is a bad API, it is the only existing API to
affect c++ (std::thread) and c11 (thrd_create) thread stack size.
This patch allows applications only to increate stack and guard
page sizes.

include/pthread.h

@@ -215,6 +215,8 @@ int pthread_getaffinity_np(pthread_t, size_t, struct cpu_set_t *);
 int pthread_setaffinity_np(pthread_t, size_t, const struct cpu_set_t *);
 int pthread_getattr_np(pthread_t, pthread_attr_t *);
 int pthread_setname_np(pthread_t, const char *);
+int pthread_getattr_default_np(pthread_attr_t *);
+int pthread_setattr_default_np(const pthread_attr_t *);
 int pthread_tryjoin_np(pthread_t, void **);
 int pthread_timedjoin_np(pthread_t, void **, const struct timespec *);
 #endif

src/thread/pthread_create.c

@@ -163,6 +163,8 @@ static void *dummy_tsd[1] = { 0 };
 weak_alias(dummy_tsd, __pthread_tsd_main);
 
 volatile int __block_new_threads = 0;
+size_t __default_stacksize = DEFAULT_STACK_SIZE;
+size_t __default_guardsize = DEFAULT_GUARD_SIZE;
 
 static FILE *volatile dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);
@@ -186,10 +188,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 		| CLONE_THREAD | CLONE_SYSVSEM | CLONE_SETTLS
 		| CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID | CLONE_DETACHED;
 	int do_sched = 0;
-	pthread_attr_t attr = {
-		._a_stacksize = DEFAULT_STACK_SIZE,
-		._a_guardsize = DEFAULT_GUARD_SIZE,
-	};
+	pthread_attr_t attr = { 0 };
 
 	if (!libc.can_do_threads) return ENOSYS;
 	self = __pthread_self();
@@ -207,6 +206,11 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	if (attrp && !c11) attr = *attrp;
 
 	__acquire_ptc();
+	if (!attrp || c11) {
+		attr._a_stacksize = __default_stacksize;
+		attr._a_guardsize = __default_guardsize;
+	}
+
 	if (__block_new_threads) __wait(&__block_new_threads, 0, 1, 1);
 
 	if (attr._a_stackaddr) {

src/thread/pthread_setattr_default_np.c

@@ -0,0 +1,35 @@
+#include "pthread_impl.h"
+#include <string.h>
+
+extern size_t __default_stacksize;
+extern size_t __default_guardsize;
+
+int pthread_setattr_default_np(const pthread_attr_t *attrp)
+{
+	/* Reject anything in the attr object other than stack/guard size. */
+	pthread_attr_t tmp = *attrp, zero = { 0 };
+	tmp._a_stacksize = 0;
+	tmp._a_guardsize = 0;
+	if (memcmp(&tmp, &zero, sizeof tmp))
+		return EINVAL;
+
+	__inhibit_ptc();
+	if (attrp->_a_stacksize >= __default_stacksize)
+		__default_stacksize = attrp->_a_stacksize;
+	if (attrp->_a_guardsize >= __default_guardsize)
+		__default_guardsize = attrp->_a_guardsize;
+	__release_ptc();
+
+	return 0;
+}
+
+int pthread_getattr_default_np(pthread_attr_t *attrp)
+{
+	__acquire_ptc();
+	*attrp = (pthread_attr_t) {
+		._a_stacksize = __default_stacksize,
+		._a_guardsize = __default_guardsize,
+	};
+	__release_ptc();
+	return 0;
+}