invoice: allow creation of giant invoices.

lightning/bolts#877 talks about
removing this restriction (only Electrum actually enforced it on
receive), so start by allowing creation of giant invoices, though
we mark them as requiring mpp.

Changelog-Changed: JSON-RPC: `invoice` now allows creation of giant invoices (>= 2^32 msat)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

lightningd/invoice.c

@@ -834,6 +834,19 @@ invoice_complete(struct invoice_info *info,
 				    info->label->s);
 	}
 
+	/* If this requires a giant HTLC, most implementations cannot
+	 * send that much; will need to split. */
+	/* BOLT #2:
+	 * ### Adding an HTLC: `update_add_htlc`
+	 *...
+	 *   - for channels with `chain_hash` identifying the Bitcoin blockchain:
+	 *    - MUST set the four most significant bytes of `amount_msat` to 0.
+	 */
+	if (info->b11->msat
+	    && amount_msat_greater(*info->b11->msat, chainparams->max_payment)) {
+		warning_mpp = true;
+	}
+
 	/* Get details */
 	details = wallet_invoice_details(info, wallet, invoice);
 
@@ -1135,14 +1148,6 @@ static struct command_result *json_invoice(struct command *cmd,
 				    strlen(desc_val));
 	}
 
-	if (msatoshi_val
-	    && amount_msat_greater(*msatoshi_val, chainparams->max_payment)) {
-		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-				    "msatoshi cannot exceed %s",
-				    type_to_string(tmpctx, struct amount_msat,
-						   &chainparams->max_payment));
-	}
-
 	if (fallbacks) {
 		size_t i;
 		const jsmntok_t *t;

tests/test_invoices.py

@@ -54,9 +54,9 @@ def test_invoice(node_factory, chainparams):
     assert 'routes' not in b11
     assert 'warning_capacity' in inv
 
-    # Make sure no wumbo invoices
-    with pytest.raises(RpcError, match=r'msatoshi cannot exceed 4294967295msat'):
-        l2.rpc.invoice(4294967295 + 1, 'inv3', '?')
+    # Make sure wumbo invoices warn about mpp being needed.
+    inv = l2.rpc.invoice(4294967295 + 1, 'inv4', '?')
+    assert 'warning_mpp' in inv
     l2.rpc.invoice(4294967295, 'inv3', '?')
 
     # Test cltv option.