PSBT: tal_wally updates and docs

Default wally_tal_ctx to NULL, add extra asserts and tal_checks, and documentation explaning the usage of tal_wally_start/end.

Changelog-None

Author: ddustin

common/setup.c

@@ -11,6 +11,7 @@
 static void *cln_wally_tal(size_t size)
 {
 	assert(wally_tal_ctx);
+	assert(tal_check(wally_tal_ctx, "cln_wally_tal ctx check"));
 	return tal_arr_label(wally_tal_ctx, u8, size, "cln_wally_tal");
 }
 

common/utils.c

@@ -10,7 +10,7 @@
 #include <errno.h>
 #include <locale.h>
 
-const tal_t *wally_tal_ctx;
+const tal_t *wally_tal_ctx = NULL;
 secp256k1_context *secp256k1_ctx;
 const tal_t *tmpctx;
 
@@ -35,8 +35,14 @@ void tal_wally_start(void)
 void tal_wally_end(const tal_t *parent)
 {
 	tal_t *p;
+	if (!wally_tal_ctx) {
+		/* This makes valgrind show us backtraces! */
+		*(u8 *)wally_tal_ctx = '\0';
+		abort();
+	}
 	while ((p = tal_first(wally_tal_ctx)) != NULL) {
-		/* Refuse to make a loop! */
+		/* Refuse to make a loop!
+		 * Did you mean to use tal_wally_end_onto? */
 		assert(p != parent);
 		/* Don't steal backtrace from wally_tal_ctx! */
 		if (tal_name(p) && streq(tal_name(p), "backtrace")) {
@@ -45,6 +51,7 @@ void tal_wally_end(const tal_t *parent)
 		}
 		tal_steal(parent, p);
 	}
+	assert(tal_check(wally_tal_ctx, "tal_wally_end ctx check"));
 	wally_tal_ctx = tal_free(wally_tal_ctx);
 }
 

common/utils.h

@@ -112,12 +112,32 @@ void clean_tmpctx(void);
 /* Call this before any libwally function which allocates. */
 void tal_wally_start(void);
 
-/* Then call this to reparent everything onto this parent */
+/* Then call this to reparent everything onto this parent.
+ *
+ * This method should *not* be used for creation of psbt objects,
+ * tal_wally_end_onto should be used instead!
+ *
+ * Typical usage is to end on the PSBT being modified.
+ */
 void tal_wally_end(const tal_t *parent);
 
 /* ... or this if you want to reparent onto something which is
  * allocated by libwally here.  Fixes up this from_wally obj to have a
- * proper tal_name, too! */
+ * proper tal_name, too!
+ *
+ * This results in a tal hierarchy like so:
+ * parent -> from_wally -> [everything wally allocated]
+ *
+ * This is typically used when allocating a new psbt and ensuring all the psbt's
+ * member objects are children of the PSBT itself.
+ *
+ * For example:
+ * struct wally_psbt *clone;
+ * tal_wally_start();
+ * if (wally_psbt_clone_alloc(psbt, 0, &clone) != WALLY_OK)
+ * 	abort();
+ * tal_wally_end_onto(ctx, clone, struct wally_psbt);
+ */
 #define tal_wally_end_onto(parent, from_wally, type)                           \
 	tal_wally_end_onto_(                                                   \
 	    (parent), (from_wally),                                            \