make pthread_attr_init honor defaults set by pthread_setattr_default_np

this fixes a major gap in the intended functionality of
pthread_setattr_default_np. if application/library code creating a
thread does not pass a null attribute pointer to pthread_create, but
sets up an attribute object to change other properties while leaving
the stack alone, the created thread will get a stack with size
DEFAULT_STACK_SIZE. this makes pthread_setattr_default_np useless for
working around stack overflow issues in such applications, and leaves
a major risk of regression if previously-working code switches from
using a null attribute pointer to an attribute object.

this change aligns the behavior more closely with the glibc
pthread_setattr_default_np functionality too, albeit via a different
mechanism. glibc encodes "default" specially in the attribute object
and reads the actual default at thread creation time. with this
commit, we now copy the current default into the attribute object at
pthread_attr_init time, so that applications that query the properties
of the attribute object will see the right values.

3 files changed, 11 insertions, 4 deletions

diff --git a/src/thread/default_attr.c b/src/thread/default_attr.c
new file mode 100644
index 00000000..46fe98ee
--- /dev/null
+++ b/src/thread/default_attr.c
@@ -0,0 +1,4 @@
+#include "pthread_impl.h"
+
+size_t __default_stacksize = DEFAULT_STACK_SIZE;
+size_t __default_guardsize = DEFAULT_GUARD_SIZE;
diff --git a/src/thread/pthread_attr_init.c b/src/thread/pthread_attr_init.c
index 8f6e3374..a962b460 100644
--- a/src/thread/pthread_attr_init.c
+++ b/src/thread/pthread_attr_init.c
@@ -1,9 +1,12 @@
 #include "pthread_impl.h"
 
+extern size_t __default_stacksize;
+extern size_t __default_guardsize;
+
 int pthread_attr_init(pthread_attr_t *a)
 {
 	*a = (pthread_attr_t){0};
-	a->_a_stacksize = DEFAULT_STACK_SIZE;
-	a->_a_guardsize = DEFAULT_GUARD_SIZE;
+	a->_a_stacksize = __default_stacksize;
+	a->_a_guardsize = __default_guardsize;
 	return 0;
 }
diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index 2c066cff..2df2e9f9 100644
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -165,8 +165,8 @@ 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;
+extern size_t __default_stacksize;
+extern size_t __default_guardsize;
 
 static FILE *volatile dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);