Scripting: redis.NIL to return nil bulk replies.

Lua arrays can't contain nil elements (see
http://www.lua.org/pil/19.1.html for more information), so Lua scripts
were not able to return a multi-bulk reply containing nil bulk
elements inside.

This commit introduces a special conversion: a table with just
a "nilbulk" field set to a boolean value is converted by Redis as a nil
bulk reply, but at the same time for Lua this type is not a "nil" so can
be used inside Lua arrays.

This type is also assigned to redis.NIL, so the following two forms
are equivalent and will be able to return a nil bulk reply as second
element of a three elements array:

    EVAL "return {1,redis.NIL,3}" 0
    EVAL "return {1,{nilbulk=true},3}" 0

The result in redis-cli will be:

    1) (integer) 1
    2) (nil)
    3) (integer) 3

src/scripting.c

@@ -516,6 +516,14 @@ void scriptingInit(void) {
     lua_pushcfunction(lua, luaRedisSha1hexCommand);
     lua_settable(lua, -3);
 
+    /* redis.NIL */
+    lua_pushstring(lua, "NIL");
+    lua_newtable(lua);
+    lua_pushstring(lua, "nilbulk");
+    lua_pushboolean(lua, 1);
+    lua_settable(lua, -3);
+    lua_settable(lua, -3);
+
     /* Finally set the table as 'redis' global var. */
     lua_setglobal(lua,"redis");
 
@@ -610,9 +618,30 @@ void luaReplyToRedisReply(redisClient *c, lua_State *lua) {
         addReplyLongLong(c,(long long)lua_tonumber(lua,-1));
         break;
     case LUA_TTABLE:
-        /* We need to check if it is an array, an error, or a status reply.
-         * Error are returned as a single element table with 'err' field.
-         * Status replies are returned as single elment table with 'ok' field */
+        /* The table can be an array or it may be in a special format that
+         * Lua uses to return special Redis protocol data types.
+         *
+         * 1) Errors are retuned as a single element table with 'err' field.
+         * 2) Status reply are returned as a single element table with 'ok'
+         *    field.
+         * 3) A Redis nil bulk reply is returned as a single element table
+         *    with 'nilbulk' field set to true.
+         *
+         * All the rest is considered just an array and is translated into
+         * a Redis multi bulk reply. */
+
+        /* Nil bulk reply */
+        lua_pushstring(lua,"nilbulk");
+        lua_gettable(lua,-2);
+        t = lua_type(lua,-1);
+        if (t == LUA_TBOOLEAN) {
+            addReply(c,shared.nullbulk);
+            lua_pop(lua,2);
+            return;
+        }
+        lua_pop(lua,1);
+
+        /* Error reply */
         lua_pushstring(lua,"err");
         lua_gettable(lua,-2);
         t = lua_type(lua,-1);
@@ -624,8 +653,9 @@ void luaReplyToRedisReply(redisClient *c, lua_State *lua) {
             lua_pop(lua,2);
             return;
         }
-
         lua_pop(lua,1);
+
+        /* Status reply */
         lua_pushstring(lua,"ok");
         lua_gettable(lua,-2);
         t = lua_type(lua,-1);
@@ -636,6 +666,7 @@ void luaReplyToRedisReply(redisClient *c, lua_State *lua) {
             sdsfree(ok);
             lua_pop(lua,1);
         } else {
+            /* Multi bulk reply. */
             void *replylen = addDeferredMultiBulkLength(c);
             int j = 1, mbulklen = 0;
 

tests/unit/scripting.tcl

@@ -30,6 +30,10 @@ start_server {tags {"scripting"}} {
         set _ $e
     } {this is an error}
 
+    test {EVAL - Lua nil reply -> Redis protocol type conversion} {
+        r eval {return {1,redis.NIL,{nilbulk=true},4}} 0
+    } {1 {} {} 4}
+
     test {EVAL - Lua table -> Redis protocol type conversion} {
         r eval {return {1,2,3,'ciao',{1,2}}} 0
     } {1 2 3 ciao {1 2}}