Fill correct stack info in pthread_getattr_np stub

See use case here:
https://github.com/python/cpython/pull/130398/files#diff-c22186367cbe20233e843261998dc027ae5f1f8c0d2e778abfa454ae74cc59deR365-R380

I guess for stack protection, this only enables introspecting whether we run out
of the spill stack. I'm not sure whether to expect running out of spill stack or
running out of true stack to be more common. But at least this unbreaks the
code.

Author: hoodmane

system/lib/pthread/library_pthread_stub.c

@@ -277,6 +277,9 @@ int pthread_condattr_setpshared(pthread_condattr_t *attr, int shared) {
 }
 
 int pthread_getattr_np(pthread_t thread, pthread_attr_t *attr) {
+  attr->_a_stackaddr = emscripten_stack_get_base();
+  attr->_a_stacksize = emscripten_stack_get_base() - emscripten_stack_get_end();
+  attr->_a_guardsize = __default_guardsize;
   return 0;
 }
 

test/other/test_stub_pthread_getattr_np.c

@@ -0,0 +1,17 @@
+#define _GNU_SOURCE
+#include "pthread.h"
+#include "stdio.h"
+#include "assert.h"
+
+int main() {
+    pthread_attr_t attr;
+    size_t stack_size, guard_size;
+    void *stack_addr;
+
+    assert(pthread_getattr_np(pthread_self(), &attr) == 0);
+    assert(pthread_attr_getguardsize(&attr, &guard_size) == 0);
+    assert(pthread_attr_getstack(&attr, &stack_addr, &stack_size) == 0);
+    assert(pthread_attr_destroy(&attr) == 0);
+    printf("stack_addr: %p, stack_size: %zu, guard_size: %zu\n", stack_addr, stack_size, guard_size);
+    return 0;
+}

test/test_other.py

@@ -15777,3 +15777,6 @@ def test_rollup(self):
 
   def test_rlimit(self):
     self.do_other_test('test_rlimit.c', emcc_args=['-O1'])
+
+  def test_stub_pthread_getattr_np(self):
+    self.do_runf(test_file('other/test_stub_pthread_getattr_np.c'), 'stack_addr: 0, stack_size: 65536, guard_size: 8192')