tal: allow notifiers on NULL.

A destructor on NULL doesn't make sense, but notifiers (eg. new children)
do.  We fix up a mistake in run-notifier (comparing ctx with itself) and
loose typing in tal.c's tal_add_notifier_ too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/tal.c

@@ -191,14 +191,14 @@ static void *from_tal_hdr(const struct tal_hdr *hdr)
 	return (void *)(hdr + 1);
 }
 
-#ifdef TAL_DEBUG
-static void *from_tal_hdr_or_null(struct tal_hdr *hdr)
+static void *from_tal_hdr_or_null(const struct tal_hdr *hdr)
 {
 	if (hdr == &null_parent.hdr)
 		return NULL;
 	return from_tal_hdr(hdr);
 }
 
+#ifdef TAL_DEBUG
 static struct tal_hdr *debug_tal(struct tal_hdr *tal)
 {
 	tal_check(from_tal_hdr_or_null(tal), "TAL_DEBUG ");
@@ -234,7 +234,7 @@ static void notify(const struct tal_hdr *ctx,
 				else
 					n->u.destroy(from_tal_hdr(ctx));
 			} else
-				n->u.notifyfn(from_tal_hdr(ctx), type,
+				n->u.notifyfn(from_tal_hdr_or_null(ctx), type,
 					      (void *)info);
 		}
 	}
@@ -528,7 +528,7 @@ bool tal_add_destructor2_(const tal_t *ctx, void (*destroy)(void *me, void *arg)
 bool tal_add_notifier_(const tal_t *ctx, enum tal_notify_type types,
 		       void (*callback)(tal_t *, enum tal_notify_type, void *))
 {
-	tal_t *t = debug_tal(to_tal_hdr(ctx));
+	struct tal_hdr *t = debug_tal(to_tal_hdr_or_null(ctx));
 	struct notifier *n;
 
 	assert(types);
@@ -556,7 +556,7 @@ bool tal_del_notifier_(const tal_t *ctx,
 		       void (*callback)(tal_t *, enum tal_notify_type, void *),
 		       bool match_extra_arg, void *extra_arg)
 {
-	struct tal_hdr *t = debug_tal(to_tal_hdr(ctx));
+	struct tal_hdr *t = debug_tal(to_tal_hdr_or_null(ctx));
 	enum tal_notify_type types;
 
         types = del_notifier_property(t, callback, match_extra_arg, extra_arg);

ccan/tal/tal.h

@@ -227,7 +227,7 @@ enum tal_notify_type {
 
 /**
  * tal_add_notifier - add a callback function when this context changes.
- * @ptr: The tal allocated object.
+ * @ptr: The tal allocated object, or NULL.
  * @types: Bitwise OR of the types the callback is interested in.
  * @callback: the function to call.
  *

ccan/tal/test/run-notifier.c

@@ -5,7 +5,7 @@
 static enum tal_notify_type expect;
 static void *expect_info;
 static char *ctx;
-static unsigned int notified1, notified2;
+static unsigned int notified1, notified2, notified_null;
 
 /* Make sure we always move on resize. */
 static void *my_realloc(void *old, size_t size)
@@ -20,9 +20,9 @@ static void *my_realloc(void *old, size_t size)
 	return new;
 }
 
-static void notify1(char *p UNNEEDED, enum tal_notify_type notify, void *info)
+static void notify1(char *p, enum tal_notify_type notify, void *info)
 {
-	ok1(ctx == ctx);
+	ok1(p == ctx);
 	ok1(notify == expect);
 	if (expect_info == &expect_info)
 		expect_info = info;
@@ -38,6 +38,17 @@ static void notify2(char *ctx UNNEEDED,
 	notified2++;
 }
 
+static void notify_null(void *p, enum tal_notify_type notify, void *info)
+{
+	ok1(p == NULL);
+	ok1(notify == expect);
+	if (expect_info == &expect_info)
+		expect_info = info;
+	else
+		ok1(info == expect_info);
+	notified_null++;
+}
+
 static bool seen_move, seen_resize;
 static void resize_notifier(char *p, enum tal_notify_type notify, void *info)
 {
@@ -62,7 +73,7 @@ int main(void)
 {
 	char *child, *new_ctx;
 
-	plan_tests(56);
+	plan_tests(65);
 
 	ctx = tal(NULL, char);
 	ok1(tal_add_notifier(ctx, 511, notify1));
@@ -119,6 +130,20 @@ int main(void)
 	ok1(notified1 == 7);
 	ok1(notified2 == 1);
 
+	/* Notifiers on NULL work, too. */
+	ok1(tal_add_notifier(NULL, TAL_NOTIFY_ADD_CHILD|TAL_NOTIFY_DEL_CHILD,
+			     notify_null));
+	expect = TAL_NOTIFY_ADD_CHILD;
+	expect_info = &expect_info;
+	child = tal(NULL, char);
+	ok1(notified_null == 1);
+
+	expect = TAL_NOTIFY_DEL_CHILD;
+	expect_info = child;
+	tal_free(child);
+	ok1(notified_null == 2);
+	ok1(tal_del_notifier(NULL, notify_null));
+
 	tal_set_backend(NULL, my_realloc, NULL, NULL);
 	ctx = new_ctx = tal(NULL, char);
 	ok1(tal_add_notifier(new_ctx, 511, resize_notifier));