Scripting: better Lua number -> string conversion in luaRedisGenericCommand().

The lua_to*string() family of functions use a non optimal format
specifier when converting integers to strings. This has both the problem
of the number being converted in exponential notation, which we don't
use as a Redis return value when floating point numbers are involed,
and, moreover, there is a loss of precision since the default format
specifier is not able to represent numbers that must be represented
exactly in the IEEE 754 number mantissa.

The new code handles it as a special case using a saner conversion.

This fixes issue #1118.

src/scripting.c

@@ -233,12 +233,22 @@ int luaRedisGenericCommand(lua_State *lua, int raise_error) {
         char *obj_s;
         size_t obj_len;
 
-        obj_s = (char*)lua_tolstring(lua,j+1,&obj_len);
-        if (obj_s == NULL) break; /* Not a string. */
+        if (lua_isnumber(lua,j+1)) {
+            /* We can't use lua_tolstring() for number -> string conversion
+             * since Lua uses a format specifier that loses precision. */
+            char dbuf[64];
+            lua_Number num = lua_tonumber(lua,j+1);
+
+            obj_len = snprintf(dbuf,sizeof(dbuf),"%.17g",(double)num);
+            obj_s = dbuf;
+        } else {
+            obj_s = (char*)lua_tolstring(lua,j+1,&obj_len);
+            if (obj_s == NULL) break; /* Not a string. */
+        }
 
         /* Try to use a cached object. */
-        if (j < LUA_CMD_OBJCACHE_SIZE && cached_objects[j] && 
-            cached_objects_len[j] >= obj_len) 
+        if (j < LUA_CMD_OBJCACHE_SIZE && cached_objects[j] &&
+            cached_objects_len[j] >= obj_len)
         {
             char *s = cached_objects[j]->ptr;
             struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));