lua: fix decimal comparison with nil

Previously decimal comparison with nil failed with following error:
`expected decimal, number or string as 2 argument`.
Fix this. Throw a more verbose error in case of '>', '<', '>=', '<='
and fix equality check.

Follow-up #692

src/lua/decimal.c

@@ -69,6 +69,10 @@ ldecimal_##name(struct lua_State *L) {						\
 static int									\
 ldecimal_##name(struct lua_State *L) {						\
 	assert(lua_gettop(L) == 2);						\
+	if (lua_isnil(L, 1) || lua_isnil(L, 2)) {				\
+		luaL_error(L, "attempt to compare decimal with nil");		\
+		return 1;							\
+	}									\
 	decimal_t *lhs = lua_todecimal(L, 1);					\
 	decimal_t *rhs = lua_todecimal(L, 2);					\
 	lua_pushboolean(L, decimal_compare(lhs, rhs) cmp 0);			\
@@ -226,10 +230,23 @@ LDECIMAL_FUNC(exp, exp)
 LDECIMAL_FUNC(sqrt, sqrt)
 LDECIMAL_FUNC(abs, abs)
 
-LDECIMAL_CMPOP(eq, ==)
 LDECIMAL_CMPOP(lt, <)
 LDECIMAL_CMPOP(le, <=)
 
+static int
+ldecimal_eq(struct lua_State *L)
+{
+	assert(lua_gettop(L) == 2);
+	if (lua_isnil(L, 1) || lua_isnil(L, 2)) {
+		lua_pushboolean(L, false);
+		return 1;
+	}
+	decimal_t *lhs = lua_todecimal(L, 1);
+	decimal_t *rhs = lua_todecimal(L, 2);
+	lua_pushboolean(L, decimal_compare(lhs, rhs) == 0);
+	return 1;
+}
+
 static int
 ldecimal_minus(struct lua_State *L)
 {

test/app/decimal.result

@@ -223,6 +223,32 @@ b
  | - '0.1'
  | ...
 
+-- check comparsion with nil
+a == nil
+ | ---
+ | - false
+ | ...
+a ~= nil
+ | ---
+ | - true
+ | ...
+a > nil
+ | ---
+ | - error: '[string "return a > nil "]:1: attempt to compare decimal with nil'
+ | ...
+a < nil
+ | ---
+ | - error: '[string "return a < nil "]:1: attempt to compare decimal with nil'
+ | ...
+a >= nil
+ | ---
+ | - error: '[string "return a >= nil "]:1: attempt to compare decimal with nil'
+ | ...
+a <= nil
+ | ---
+ | - error: '[string "return a <= nil "]:1: attempt to compare decimal with nil'
+ | ...
+
 decimal.sqrt(a)
  | ---
  | - '3.1622776601683793319988935444327185337'

test/app/decimal.test.lua

@@ -62,6 +62,14 @@ a ~= b
 a
 b
 
+-- check comparsion with nil
+a == nil
+a ~= nil
+a > nil
+a < nil
+a >= nil
+a <= nil
+
 decimal.sqrt(a)
 decimal.ln(a)
 decimal.log10(a)