replace bad cancellation cleanup abi with a sane one

the old abi was intended to duplicate glibc's abi at the expense of
being ugly and slow, but it turns out glib was not even using that abi
except on non-gcc-compatible compilers (which it doesn't even support)
and was instead using an exceptions-in-c/unwind-based approach whose
abi we could not duplicate anyway without nasty dwarf2/unwind
integration.

the new abi is copied from a very old glibc abi, which seems to still
be supported/present in current glibc. it avoids all unwinding,
whether by sjlj or exceptions, and merely maintains a linked list of
cleanup functions to be called from the context of pthread_exit. i've
made some care to ensure that longjmp out of a cleanup function should
work, even though it is not required to.

this change breaks abi compatibility with programs which were using
pthread cancellation, which is unfortunate, but that's why i'm making
the change now rather than later. considering that most pthread
features have not been usable until recently anyway, i don't see it as
a major issue at this point.

src/thread/cancellation.c

@@ -1,23 +1,20 @@
 #include "pthread_impl.h"
 
-static void dummy(struct __ptcb *cb)
-{
-}
-weak_alias(dummy, __pthread_do_unwind);
-weak_alias(dummy, __pthread_do_register);
-weak_alias(dummy, __pthread_do_unregister);
+void __do_cleanup_push();
+void __do_cleanup_pop();
 
-void __pthread_unwind_next(struct __ptcb *cb)
+void _pthread_cleanup_push(struct __ptcb *cb, void (*f)(void *), void *x)
 {
-	__pthread_do_unwind(cb);
+	__do_cleanup_push(cb, f, x);
 }
 
-void __pthread_register_cancel(struct __ptcb *cb)
+void _pthread_cleanup_pop(struct __ptcb *cb, int run)
 {
-	__pthread_do_register(cb);
+	__do_cleanup_pop(cb, run);
 }
 
-void __pthread_unregister_cancel(struct __ptcb *cb)
+static void dummy()
 {
-	__pthread_do_unregister(cb);
 }
+weak_alias(dummy, __do_cleanup_push);
+weak_alias(dummy, __do_cleanup_pop);

src/thread/i386/cancellation.s

@@ -1,34 +0,0 @@
-.text
-.global __pthread_register_cancel
-.type   __pthread_register_cancel,@function
-__pthread_register_cancel:
-	pushl %eax
-	call __pthread_do_register
-	popl %eax
-	ret
-
-.global __pthread_unregister_cancel
-.type   __pthread_unregister_cancel,@function
-__pthread_unregister_cancel:
-	pushl %eax
-	call __pthread_do_unregister
-	popl %eax
-	ret
-
-.global __pthread_unwind_next
-.type   __pthread_unwind_next,@function
-__pthread_unwind_next:
-	pushl %eax
-	call __pthread_do_unwind
-	popl %eax
-__pthread_do_unwind:
-__pthread_do_register:
-__pthread_do_unregister:
-	ret
-
-.weak __pthread_do_unwind
-.weak __pthread_do_register
-.weak __pthread_do_unregister
-.type __pthread_do_unwind,@function
-.type __pthread_do_register,@function
-.type __pthread_do_unregister,@function

src/thread/pthread_create.c

@@ -8,14 +8,18 @@ weak_alias(dummy_0, __synccall_lock);
 weak_alias(dummy_0, __synccall_unlock);
 weak_alias(dummy_0, __pthread_tsd_run_dtors);
 
-void __pthread_do_unwind(struct __ptcb *cb)
+void pthread_exit(void *result)
 {
 	pthread_t self = pthread_self();
 	int n;
 
-	if (cb->__next) {
-		self->cancelbuf = cb->__next->__next;
-		longjmp((void *)cb->__next->__jb, 1);
+	self->result = result;
+
+	while (self->cancelbuf) {
+		void (*f)(void *) = self->cancelbuf->__f;
+		void *x = self->cancelbuf->__x;
+		self->cancelbuf = self->cancelbuf->__next;
+		f(x);
 	}
 
 	__pthread_tsd_run_dtors();
@@ -39,17 +43,19 @@ void __pthread_do_unwind(struct __ptcb *cb)
 	__syscall(SYS_exit, 0);
 }
 
-void __pthread_do_register(struct __ptcb *cb)
+void __do_cleanup_push(struct __ptcb *cb, void (*f)(void *), void *x)
 {
 	struct pthread *self = pthread_self();
+	cb->__f = f;
+	cb->__x = x;
 	cb->__next = self->cancelbuf;
 	self->cancelbuf = cb;
 }
 
-void __pthread_do_unregister(struct __ptcb *cb)
+void __do_cleanup_pop(struct __ptcb *cb, int run)
 {
-	struct pthread *self = __pthread_self();
-	self->cancelbuf = self->cancelbuf->__next;
+	__pthread_self()->cancelbuf = cb->__next;
+	if (run) cb->__f(cb->__x);
 }
 
 static int start(void *p)
@@ -134,11 +140,3 @@ int pthread_create(pthread_t *res, const pthread_attr_t *attr, void *(*entry)(vo
 	*res = new;
 	return 0;
 }
-
-void pthread_exit(void *result)
-{
-	struct pthread *self = pthread_self();
-	struct __ptcb cb = { .__next = self->cancelbuf };
-	self->result = result;
-	__pthread_do_unwind(&cb);
-}