doc: be explicit (with tests!) on when we don't know amount_msat for listsendpays.

Sangbida and I traced back through ancient history: when the pay plugin was introduced
in 0.9.0 (2019!) it already used the amount_msat parameter (then called `msatoshi`),
so this case effectively "never happens".

But we added a test for it just in case.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

contrib/msggen/msggen/schema.json

@@ -21942,7 +21942,7 @@
                       "amount_msat": {
                         "type": "msat",
                         "description": [
-                          "The amount of millisatoshi we intended to send to the destination."
+                          "The amount of millisatoshi we intended to send to the destination.  This can only be missing in the case of someone manually calling sendonion without the `amount_msat` parameter (which no plugin currently does)."
                         ]
                       },
                       "amount_sent_msat": {
@@ -24449,7 +24449,7 @@
                 "amount_msat": {
                   "type": "msat",
                   "description": [
-                    "The amount delivered to destination (if known)."
+                    "The amount delivered to destination (if known).  This is not known in the case where sendonion(7) was used to manually initiate a payment without the `amount_msat` parameter."
                   ]
                 },
                 "destination": {

doc/schemas/listpays.json

@@ -192,7 +192,7 @@
                   "amount_msat": {
                     "type": "msat",
                     "description": [
-                      "The amount of millisatoshi we intended to send to the destination."
+                      "The amount of millisatoshi we intended to send to the destination.  This can only be missing in the case of someone manually calling sendonion without the `amount_msat` parameter (which no plugin currently does)."
                     ]
                   },
                   "amount_sent_msat": {

doc/schemas/listsendpays.json

@@ -144,7 +144,7 @@
             "amount_msat": {
               "type": "msat",
               "description": [
-                "The amount delivered to destination (if known)."
+                "The amount delivered to destination (if known).  This is not known in the case where sendonion(7) was used to manually initiate a payment without the `amount_msat` parameter."
               ]
             },
             "destination": {

tests/test_pay.py

@@ -2995,16 +2995,20 @@ def test_sendonion_rpc(node_factory):
     invs = l4.rpc.listinvoices(label="lbl")['invoices']
     assert(len(invs) == 1 and invs[0]['status'] == 'paid')
 
-    pays = l1.rpc.listsendpays()['payments']
-    assert(len(pays) == 1 and pays[0]['status'] == 'complete'
-           and pays[0]['payment_hash'] == inv['payment_hash'])
+    pay = only_one(l1.rpc.listsendpays()['payments'])
+    assert (pay['status'] == 'complete'
+            and pay['payment_hash'] == inv['payment_hash'])
+
+    # listsendpays promised that amount would be missing if sendonion didn't
+    # specify
+    assert 'amount_msat' not in pay
 
     # And now for a failing payment, using a payment_hash that doesn't match an
     # invoice
     payment_hash = "00" * 32
     onion = l1.rpc.createonion(hops=hops, assocdata=payment_hash)
     l1.rpc.sendonion(onion=onion['onion'], first_hop=first_hop,
-                     payment_hash=payment_hash)
+                     payment_hash=payment_hash, amount_msat=amt)
 
     try:
         l1.rpc.waitsendpay(payment_hash=payment_hash)
@@ -3013,13 +3017,16 @@ def test_sendonion_rpc(node_factory):
         assert(e.error['code'] == 202)
         assert(e.error['message'] == "Malformed error reply")
 
-    pays = l1.rpc.listsendpays(payment_hash=payment_hash)['payments']
-    assert(len(pays) == 1 and pays[0]['status'] == 'failed'
-           and pays[0]['payment_hash'] == payment_hash)
-    assert('erroronion' in pays[0])
+    pay = only_one(l1.rpc.listsendpays(payment_hash=payment_hash)['payments'])
+    assert(pay['status'] == 'failed'
+           and pay['payment_hash'] == payment_hash)
+    assert 'erroronion' in pay
+
+    # Since we told sendonion the destination amount, listsendpays will know:
+    assert pay['amount_msat'] == amt
 
     # Fail onion is msg + padding = 256 + 2*2 byte lengths + 32 byte HMAC
-    assert(len(pays[0]['erroronion']) == (256 + 32 + 2 + 2) * 2)
+    assert(len(pay['erroronion']) == (256 + 32 + 2 + 2) * 2)
 
     # Let's try that again, this time we give it the shared_secrets so it
     # should be able to decode the error.