tal/autoptr: new module.

Helps with the common case of wanting to NULL out a pointer when the object
freed.  We could also track it if resized, but that's TODO.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/autoptr/LICENSE

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+../../../licenses/BSD-MIT

ccan/tal/autoptr/_info

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+#include "config.h"
+#include <stdio.h>
+#include <string.h>
+
+/**
+ * tal/autoptr - automatic updates of pointers to tal objects.
+ *
+ * This code updates pointers when the pointed-to object is freed.
+ *
+ * Maintainer: Rusty Russell <rusty@rustcorp.com.au>
+ * License: BSD-MIT
+ * Example:
+ *	#include <ccan/tal/autoptr/autoptr.h>
+ *	#include <assert.h>
+ *
+ *	static void *p;
+ *
+ *	int main(void)
+ *	{
+ *		char *c = tal(NULL, char);
+ *
+ *		// Sets p to point to c.
+ *		autonull_set_ptr(NULL, &p, c);
+ *		assert(p == c);
+ *
+ *		// Automatically clears p.
+ *		tal_free(c);
+ *		assert(p == NULL);
+ *		return 0;
+ *	}
+ */
+int main(int argc, char *argv[])
+{
+	/* Expect exactly one argument */
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		printf("ccan/tal\n");
+		return 0;
+	}
+
+	return 1;
+}

ccan/tal/autoptr/autoptr.c

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+/* MIT (BSD) license - see LICENSE file for details */
+#include <ccan/tal/autoptr/autoptr.h>
+
+struct autonull {
+	void **pp;
+};
+
+static void autonull_remove(struct autonull *a);
+static void autonull_null_out(tal_t *p UNNEEDED, struct autonull *a)
+{
+	void **pp = a->pp;
+	tal_del_destructor(a, autonull_remove);
+	tal_free(a);
+	*pp = NULL;
+}
+
+static void autonull_remove(struct autonull *a)
+{
+	/* Don't NULL us out now. */
+	tal_del_destructor2(*a->pp, autonull_null_out, a);
+}
+
+struct autonull *autonull_set_ptr_(const tal_t *ctx, void *pp, const tal_t *p)
+{
+	struct autonull *a = tal(ctx, struct autonull);
+	a->pp = (void **)pp;
+	*a->pp = (void *)p;
+
+	/* If p is freed, NULL out a->pp */
+	tal_add_destructor2(*a->pp, autonull_null_out, a);
+
+	/* If they free autonull, it removes other destructor. */
+	tal_add_destructor(a, autonull_remove);
+	return a;
+}

ccan/tal/autoptr/autoptr.h

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+/* MIT (BSD) license - see LICENSE file for details */
+#ifndef CCAN_TAL_AUTOPTR_H
+#define CCAN_TAL_AUTOPTR_H
+#include <ccan/tal/tal.h>
+
+struct autonull;
+
+/**
+ * autonull_set_ptr - set a pointer, NULL it when pointer tal_free'd.
+ * @ctx: the tal context which owns this autonull.
+ * @pp: pointer to tal pointer
+ * @p: the tal pointer to set *@pp to.
+ *
+ * *@pp is set to @p.  When @p is tal_free'd directly or indirectly, *@pp will
+ * be set to NULL.  Or, if the returned object is freed, the callback is
+ * deactivated.
+ *
+ * Example:
+ *	struct parent {
+ *		struct child *c;
+ *	};
+ *	struct child {
+ *		const char *name;
+ *	};
+ *
+ *	int main(void)
+ *	{
+ *		struct parent *p = tal(NULL, struct parent);
+ *		struct child *c = tal(p, struct child);
+ *		c->name = "Child";
+ *
+ *		autonull_set_ptr(p, &p->c, c);
+ *		assert(p->c == c);
+ *
+ *		// Automatically clears p->c.
+ *		tal_free(c);
+ *		assert(p->c == NULL);
+ *		return 0;
+ *	}
+ *
+ */
+#define autonull_set_ptr(ctx, pp, p)		\
+	autonull_set_ptr_((ctx), (pp) + 0*sizeof(*(pp) = (p)), (p))
+
+struct autonull *autonull_set_ptr_(const tal_t *ctx, void *pp, const tal_t *p);
+
+#endif /* CCAN_TAL_AUTOPTR_H */

ccan/tal/autoptr/test/run.c

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+#include <ccan/tal/autoptr/autoptr.h>
+/* Include the C files directly. */
+#include <ccan/tal/autoptr/autoptr.c>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	char *p1, *p2, *p3;
+	struct autonull *a;
+
+	/* This is how many tests you plan to run */
+	plan_tests(8);
+
+	p1 = tal(NULL, char);
+
+	// Sets p1 to point to p2.
+	autonull_set_ptr(NULL, &p2, p1);
+	ok1(p2 == p1);
+	tal_free(p1);
+	ok1(p2 == NULL);
+
+	// Using p1 as the parent is the same. */
+	p1 = tal(NULL, char);
+	autonull_set_ptr(p1, &p2, p1);
+	ok1(p2 == p1);
+	tal_free(p1);
+	ok1(p2 == NULL);
+
+	// Freeing autodata deactivates it.
+	p1 = tal(NULL, char);
+	a = autonull_set_ptr(NULL, &p2, p1);
+	ok1(p2 == p1);
+	tal_free(a);
+	tal_free(p1);
+	ok1(p2 == p1);
+
+	// Making p3 the parent means freeing p3 deactivates it.
+	p3 = tal(NULL, char);
+	p1 = tal(NULL, char);
+	autonull_set_ptr(p3, &p2, p1);
+	ok1(p2 == p1);
+	tal_free(p3);
+	tal_free(p1);
+	ok1(p2 == p1);
+
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}