libplugin: Introduce payment_abort to terminate a payment

The main responsibility of this new function is to mark a payment
process as terminated and set a reasonable error message, that will be
displayed to the caller. We also skip the remaining modifiers since
they might end up clobbering the message.

plugins/libplugin-pay.c

@@ -61,6 +61,7 @@ struct payment *payment_new(tal_t *ctx, struct command *cmd,
 	p->invstring = NULL;
 	p->routetxt = NULL;
 	p->max_htlcs = UINT32_MAX;
+	p->aborterror = NULL;
 
 	/* Copy over the relevant pieces of information. */
 	if (parent != NULL) {
@@ -1884,6 +1885,14 @@ static void payment_finished(struct payment *p)
 
 			json_add_string(ret, "status", "complete");
 
+			if (command_finished(cmd, ret)) {/* Ignore result. */}
+			return;
+		} else if (p->aborterror != NULL) {
+			/* We set an explicit toplevel error message,
+			 * so let's report that. */
+			ret = jsonrpc_stream_fail(cmd, PAY_STOPPED_RETRYING,
+						  p->aborterror);
+			payment_json_add_attempts(ret, "attempts", p);
 			if (command_finished(cmd, ret)) {/* Ignore result. */}
 			return;
 		} else if (result.failure == NULL || result.failure->failcode < NODE) {
@@ -2028,6 +2037,32 @@ void payment_continue(struct payment *p)
 	abort();
 }
 
+void payment_abort(struct payment *p, const char *fmt, ...) {
+	va_list ap;
+	struct payment *root = payment_root(p);
+	payment_set_step(p, PAYMENT_STEP_FAILED);
+	p->end_time = time_now();
+
+	va_start(ap, fmt);
+	p->failreason = tal_vfmt(p, fmt, ap);
+	va_end(ap);
+
+	root->abort = true;
+
+	/* Only set the abort error if it's not yet set, otherwise we
+	 * might end up clobbering the earliest and decisive failure
+	 * with less relevant ones. */
+	if (root->aborterror == NULL)
+		root->aborterror = tal_dup_talarr(root, char, p->failreason);
+
+	paymod_log(p, LOG_INFORM, "%s", p->failreason);
+
+	/* Do not use payment_continue, because that'd continue
+	 * applying the modifiers before calling
+	 * payment_finished(). */
+	payment_finished(p);
+}
+
 void payment_fail(struct payment *p, const char *fmt, ...)
 {
 	va_list ap;

plugins/libplugin-pay.h

@@ -292,6 +292,10 @@ struct payment {
 	 * and payment_lower_max_htlcs functions.
 	 */
 	u32 max_htlcs;
+
+	/* A human readable error message that is used as a top-level
+	 * explanation if a payment is aborted. */
+	char *aborterror;
 };
 
 struct payment_modifier {
@@ -436,6 +440,13 @@ void payment_set_step(struct payment *p, enum payment_step newstep);
 /* Fails a partial payment and continues with the core flow. */
 void payment_fail(struct payment *p, const char *fmt, ...) PRINTF_FMT(2,3);
 
+/* Fails a payment process by setting the root payment to
+ * aborted. This will cause all subpayments to terminate as soon as
+ * they can, and sets the root failreason so we have a sensible error
+ * message. The failreason is overwritten if it is already set, since
+ * we probably know better what happened in the modifier.. */
+void payment_abort(struct payment *p, const char *fmt, ...) PRINTF_FMT(2,3);
+
 struct payment *payment_root(struct payment *p);
 struct payment_tree_result payment_collect_result(struct payment *p);