Move user eval function to be located on Lua registry.

Today, Redis wrap the user Lua code with a Lua function.
For example, assuming the user code is:

```
return redis.call('ping')
```

The actual code that would have sent to the Lua interpreter was:

```
f_b3a02c833904802db9c34a3cf1292eee3246df3c() return redis.call('ping') end
```

The wraped code would have been saved on the global dictionary with the
following name: `f_<script sha>` (in our example `f_b3a02c833904802db9c34a3cf1292eee3246df3c`).

This approach allows one user to easily override the implementation a another user code, example:

```
f_b3a02c833904802db9c34a3cf1292eee3246df3c = function() return 'hacked' end
```

Running the above code will cause `evalsha b3a02c833904802db9c34a3cf1292eee3246df3c 0` to return
hacked although it should have returned `pong`.

Another disadventage is that Redis basically runs code on the loading (compiling) phase without been
aware of it. User can do code injection like this:

```
return 1 end <run code on compling phase> function() return 1
```

The wraped code will look like this and the entire `<run code on compling phase>` block will run outside
of eval or evalsha context:

```
f_<sha>() return 1 end <run code on compling phase> function() return 1 end
```

src/eval.c

@@ -269,7 +269,7 @@ void scriptingInit(int setup) {
     /* Lua beginners often don't use "local", this is likely to introduce
      * subtle bugs in their code. To prevent problems we protect accesses
      * to global variables. */
-    luaEnableGlobalsProtection(lua, 1);
+    luaEnableGlobalsProtection(lua);
 
     lctx.lua = lua;
 }
@@ -378,35 +378,20 @@ sds luaCreateFunction(client *c, robj *body) {
         sdsfreesplitres(parts, numparts);
     }
 
-    /* Build the lua function to be loaded */
-    sds funcdef = sdsempty();
-    funcdef = sdscat(funcdef,"function ");
-    funcdef = sdscatlen(funcdef,funcname,42);
-    funcdef = sdscatlen(funcdef,"() ",3);
     /* Note that in case of a shebang line we skip it but keep the line feed to conserve the user's line numbers */
-    funcdef = sdscatlen(funcdef,(char*)body->ptr + shebang_len,sdslen(body->ptr) - shebang_len);
-    funcdef = sdscatlen(funcdef,"\nend",4);
-
-    if (luaL_loadbuffer(lctx.lua,funcdef,sdslen(funcdef),"@user_script")) {
+    if (luaL_loadbuffer(lctx.lua,(char*)body->ptr + shebang_len,sdslen(body->ptr) - shebang_len,"@user_script")) {
         if (c != NULL) {
             addReplyErrorFormat(c,
                 "Error compiling script (new function): %s",
                 lua_tostring(lctx.lua,-1));
         }
         lua_pop(lctx.lua,1);
-        sdsfree(funcdef);
         return NULL;
     }
-    sdsfree(funcdef);
 
-    if (lua_pcall(lctx.lua,0,0,0)) {
-        if (c != NULL) {
-            addReplyErrorFormat(c,"Error running script (new function): %s",
-                lua_tostring(lctx.lua,-1));
-        }
-        lua_pop(lctx.lua,1);
-        return NULL;
-    }
+    serverAssert(lua_isfunction(lctx.lua, -1));
+
+    lua_setfield(lctx.lua, LUA_REGISTRYINDEX, funcname);
 
     /* We also save a SHA1 -> Original script map in a dictionary
      * so that we can replicate / write in the AOF all the
@@ -479,7 +464,7 @@ void evalGenericCommand(client *c, int evalsha) {
     lua_getglobal(lua, "__redis__err__handler");
 
     /* Try to lookup the Lua function */
-    lua_getglobal(lua, funcname);
+    lua_getfield(lua, LUA_REGISTRYINDEX, funcname);
     if (lua_isnil(lua,-1)) {
         lua_pop(lua,1); /* remove the nil from the stack */
         /* Function not defined... let's define it if we have the
@@ -497,7 +482,7 @@ void evalGenericCommand(client *c, int evalsha) {
             return;
         }
         /* Now the following is guaranteed to return non nil */
-        lua_getglobal(lua, funcname);
+        lua_getfield(lua, LUA_REGISTRYINDEX, funcname);
         serverAssert(!lua_isnil(lua,-1));
     }
 

src/script_lua.c

@@ -1144,7 +1144,7 @@ sds luaGetStringSds(lua_State *lua, int index) {
  * On Legacy Lua (eval) we need to check 'w ~= \"main\"' otherwise we will not be able
  * to create the global 'function <sha> ()' variable. On Functions Lua engine we do not use
  * this trick so it's not needed. */
-void luaEnableGlobalsProtection(lua_State *lua, int is_eval) {
+void luaEnableGlobalsProtection(lua_State *lua) {
     char *s[32];
     sds code = sdsempty();
     int j = 0;
@@ -1157,7 +1157,7 @@ void luaEnableGlobalsProtection(lua_State *lua, int is_eval) {
     s[j++]="mt.__newindex = function (t, n, v)\n";
     s[j++]="  if dbg.getinfo(2) then\n";
     s[j++]="    local w = dbg.getinfo(2, \"S\").what\n";
-    s[j++]=     is_eval ? "    if w ~= \"main\" and w ~= \"C\" then\n" : "    if w ~= \"C\" then\n";
+    s[j++]="    if w ~= \"C\" then\n";
     s[j++]="      error(\"Script attempted to create global variable '\"..tostring(n)..\"'\", 2)\n";
     s[j++]="    end\n";
     s[j++]="  end\n";

src/script_lua.h

@@ -67,7 +67,7 @@ typedef struct errorInfo {
 
 void luaRegisterRedisAPI(lua_State* lua);
 sds luaGetStringSds(lua_State *lua, int index);
-void luaEnableGlobalsProtection(lua_State *lua, int is_eval);
+void luaEnableGlobalsProtection(lua_State *lua);
 void luaRegisterGlobalProtectionFunction(lua_State *lua);
 void luaSetGlobalProtection(lua_State *lua);
 void luaRegisterLogFunction(lua_State* lua);