invoice: don't allow zero-value invoices.

You can't pay them anyway, and at least one person used 0 instead of "any".

Closes: #3808
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: JSON-RPC: `invoice` no longer accepts zero amounts (did you mean "any"?)

doc/lightning-invoice.7.md

@@ -17,7 +17,7 @@ lightning daemon can use to pay this invoice. This token includes a
 invoice, if any exists.
 
 The *msatoshi* parameter can be the string "any", which creates an
-invoice that can be paid with any amount. Otherwise it is in
+invoice that can be paid with any amount. Otherwise it is a positive value in
 millisatoshi precision; it can be a whole number, or a whole number
 ending in *msat* or *sat*, or a number with three decimal places ending
 in *sat*, or a number with 1 to 11 decimal places ending in *btc*.

lightningd/invoice.c

@@ -826,22 +826,24 @@ static struct route_info **unpack_routes(const tal_t *ctx,
 }
 #endif /* DEVELOPER */
 
-static struct command_result *param_msat_or_any(struct command *cmd,
-						const char *name,
-						const char *buffer,
-						const jsmntok_t *tok,
-						struct amount_msat **msat)
+static struct command_result *param_positive_msat_or_any(struct command *cmd,
+							 const char *name,
+							 const char *buffer,
+							 const jsmntok_t *tok,
+							 struct amount_msat **msat)
 {
 	if (json_tok_streq(buffer, tok, "any")) {
 		*msat = NULL;
 		return NULL;
 	}
 	*msat = tal(cmd, struct amount_msat);
-	if (parse_amount_msat(*msat, buffer + tok->start, tok->end - tok->start))
+	if (parse_amount_msat(*msat, buffer + tok->start, tok->end - tok->start)
+	    && !amount_msat_eq(**msat, AMOUNT_MSAT(0)))
 		return NULL;
 
 	return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-			    "'%s' should be millisatoshis or 'any', not '%.*s'",
+			    "'%s' should be positive millisatoshis or 'any',"
+			    " not '%.*s'",
 			    name,
 			    tok->end - tok->start,
 			    buffer + tok->start);
@@ -963,7 +965,7 @@ static struct command_result *json_invoice(struct command *cmd,
 	info->cmd = cmd;
 
 	if (!param(cmd, buffer, params,
-		   p_req("msatoshi", param_msat_or_any, &msatoshi_val),
+		   p_req("msatoshi", param_positive_msat_or_any, &msatoshi_val),
 		   p_req("label", param_label, &info->label),
 		   p_req("description", param_escaped_string, &desc_val),
 		   p_opt_def("expiry", param_time, &expiry, 3600*24*7),

tests/test_invoices.py

@@ -59,6 +59,26 @@ def test_invoice(node_factory, chainparams):
     l2.rpc.invoice(4294967295, 'inv3', '?')
 
 
+def test_invoice_zeroval(node_factory):
+    """A zero value invoice is unpayable, did you mean 'any'?"""
+    l1 = node_factory.get_node()
+
+    with pytest.raises(RpcError, match=r"positive .* not '0'"):
+        l1.rpc.invoice(0, 'inv', '?')
+
+    with pytest.raises(RpcError, match=r"positive .* not '0msat'"):
+        l1.rpc.invoice('0msat', 'inv', '?')
+
+    with pytest.raises(RpcError, match=r"positive .* not '0sat'"):
+        l1.rpc.invoice('0sat', 'inv', '?')
+
+    with pytest.raises(RpcError, match=r"positive .* not '0.00000000btc'"):
+        l1.rpc.invoice('0.00000000btc', 'inv', '?')
+
+    with pytest.raises(RpcError, match=r"positive .* not '0.00000000000btc'"):
+        l1.rpc.invoice('0.00000000000btc', 'inv', '?')
+
+
 def test_invoice_weirdstring(node_factory):
     l1 = node_factory.get_node()