pay: Print the HTLC result as soon as it's known

[cdecker]

We used to not print what happened with an HTLC in the pay
plugin. This meant that to follow the HTLCs we'd have to map the pay
HTLCs to the lightningd HTLCs, and then trace that. BY having pay
print the outcome as it sees it, we can make that tracking much
simpler, even allowing for tooling to do it for us.

Changelog-None This is a log-only change

[Lagrang3]

I think one of the paymod_log to inform of a failed HTLC is repeated.
To log for the HTLC failure it would be enough to do so once after the
if(p->result->state==PAYMENT_COMPLETE){} block.

[cdecker]

Each one of the paymod_log is directly followed by a return. I don't see how any of them could be duplicated?

[Lagrang3]

Why not this?:

diff --git a/plugins/libplugin-pay.c b/plugins/libplugin-pay.c
index 17254703f..49a682f61 100644
--- a/plugins/libplugin-pay.c
+++ b/plugins/libplugin-pay.c
@@ -1639,9 +1639,19 @@ payment_waitsendpay_finished(struct command *cmd,
        payment_result_infer(p->route, p->result);
 
        if (p->result->state == PAYMENT_COMPLETE) {
+               paymod_log(p, LOG_INFORM,
+                          "HTLC result state=success, sent=%s, deliver=%s",
+                          fmt_amount_msat(tmpctx, p->result->amount_sent),
+                          fmt_amount_msat(tmpctx, p->final_amount));
                payment_set_step(p, PAYMENT_STEP_SUCCESS);
                return payment_continue(p);
        }
+       paymod_log(
+           p, LOG_INFORM,
+           "HTLC result state=failed, sent=%s, todeliver=%s, code=%d (%s)",
+           fmt_amount_msat(tmpctx, p->result->amount_sent),
+           fmt_amount_msat(tmpctx, p->final_amount), p->result->code,
+           p->result->failcodename);
 
        payment_chanhints_unapply_route(p);