Use _Noreturn (C11; GNU89) properly

A function that returns with no value is a different thing from a
function that doesn't return at all.  Those are two orthogonal
concepts, commonly confused.

pthread_create(3) expects a pointer to a start routine that has a
very precise prototype:

    void *(*start_routine)(void *);

However, other thread functions, such as kernel ones, expect:

    void (*start_routine)(void *);

Providing a different one is incorrect, and has only been working
because the ABIs happen to produce a compatible function.

We should use '_Noreturn void', since it's the natural type, and
then provide a '_Noreturn void *' wrapper for pthread functions.

For consistency, replace most cases of __NORETURN or
__attribute__((noreturn)) by _Noreturn.  _Noreturn is understood
by -std=gnu89, so it should be safe to use everywhere.

Ref: https://github.com/openzfs/zfs/pull/13110#discussion_r808450136
Ref: https://software.codidact.com/posts/285972
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Closes #13120

lib/libzpool/Makefile.am

@@ -24,11 +24,6 @@ AM_CFLAGS += $(ZLIB_CFLAGS)
 
 AM_CFLAGS += -DLIB_ZPOOL_BUILD
 
-# For the void (*)(void *) -> void *(*)(void *) cast in zk_thread_create()
-# See https://github.com/openzfs/zfs/pull/13110#discussion_r808450136
-kernel.$(OBJEXT): CFLAGS += $(NO_CAST_FUNCTION_TYPE)
-kernel.l$(OBJEXT): CFLAGS += $(NO_CAST_FUNCTION_TYPE)
-
 lib_LTLIBRARIES = libzpool.la
 
 USER_C = \

lib/libzpool/kernel.c

@@ -75,12 +75,28 @@ struct proc p0;
 #define	TS_STACK_MIN	MAX(PTHREAD_STACK_MIN, 32768)
 #define	TS_STACK_MAX	(256 * 1024)
 
+struct zk_thread_wrapper {
+	void (*func)(void *);
+	void *arg;
+};
+
+static void *
+zk_thread_wrapper(void *arg)
+{
+	struct zk_thread_wrapper ztw;
+	memcpy(&ztw, arg, sizeof (ztw));
+	free(arg);
+	ztw.func(ztw.arg);
+	return (NULL);
+}
+
 kthread_t *
 zk_thread_create(void (*func)(void *), void *arg, size_t stksize, int state)
 {
 	pthread_attr_t attr;
 	pthread_t tid;
 	char *stkstr;
+	struct zk_thread_wrapper *ztw;
 	int detachstate = PTHREAD_CREATE_DETACHED;
 
 	VERIFY0(pthread_attr_init(&attr));
@@ -117,7 +133,10 @@ zk_thread_create(void (*func)(void *), void *arg, size_t stksize, int state)
 	VERIFY0(pthread_attr_setstacksize(&attr, stksize));
 	VERIFY0(pthread_attr_setguardsize(&attr, PAGESIZE));
 
-	VERIFY0(pthread_create(&tid, &attr, (void *(*)(void *))func, arg));
+	VERIFY(ztw = malloc(sizeof (*ztw)));
+	ztw->func = func;
+	ztw->arg = arg;
+	VERIFY0(pthread_create(&tid, &attr, zk_thread_wrapper, ztw));
 	VERIFY0(pthread_attr_destroy(&attr));
 
 	return ((void *)(uintptr_t)tid);

lib/libzpool/taskq.c

@@ -211,7 +211,7 @@ taskq_wait_outstanding(taskq_t *tq, taskqid_t id)
 	taskq_wait(tq);
 }
 
-static void
+static _Noreturn void
 taskq_thread(void *arg)
 {
 	taskq_t *tq = arg;