paymod: Generate type-safe accessor functions for modifier data

This should make it easy for JSON-RPC functions and modifiers to get the
associated data for a given modifier name. Useful if a modifier needs to
access its parent's modifier data, or in other functions that need to access
modifier data, e.g., when passing destination pointers into the `param()`
call.

plugins/libplugin-pay.c

@@ -146,6 +146,19 @@ void payment_continue(struct payment *p)
 	abort();
 }
 
+void *payment_mod_get_data(const struct payment *p,
+			   const struct payment_modifier *mod)
+{
+	for (size_t i = 0; p->modifiers[i] != NULL; i++)
+		if (p->modifiers[i] == mod)
+			return p->modifier_data[i];
+
+	/* If we ever get here it means that we asked for the data for a
+	 * non-existent modifier. This is a compile-time/wiring issue, so we
+	 * better check that modifiers match the data we ask for. */
+	abort();
+}
+
 static inline struct dummy_data *
 dummy_data_init(struct payment *p)
 {

plugins/libplugin-pay.h

@@ -120,6 +120,9 @@ struct payment_modifier {
 	void (*post_step_cb)(void *data, struct payment *p);
 };
 
+void *payment_mod_get_data(const struct payment *payment,
+			   const struct payment_modifier *mod);
+
 #define REGISTER_PAYMENT_MODIFIER(name, data_type, data_init_cb, step_cb)      \
 	struct payment_modifier name##_pay_mod = {                             \
 	    stringify(name),                                                   \
@@ -129,12 +132,22 @@ struct payment_modifier {
 			     void (*)(data_type, struct payment *), step_cb),  \
 	};
 
+/* The UNUSED marker is used to shut some compilers up. */
+#define REGISTER_PAYMENT_MODIFIER_HEADER(name, data_type)                      \
+	extern struct payment_modifier name##_pay_mod;                         \
+	UNUSED static inline data_type *payment_mod_##name##_get_data(         \
+	    const struct payment *p)                                           \
+	{                                                                      \
+		return payment_mod_get_data(p, &name##_pay_mod);               \
+	}
+
 struct dummy_data {
 	unsigned int *dummy_param;
 };
 
 /* List of globally available payment modifiers. */
 extern struct payment_modifier dummy_pay_mod;
+REGISTER_PAYMENT_MODIFIER_HEADER(retry, struct retry_mod_data);
 
 struct payment *payment_new(tal_t *ctx, struct command *cmd,
 			    struct payment *parent,