Function Flags support (no-writes, no-cluster, allow-state, allow-oom) (#10066)

# Redis Functions Flags

Following the discussion on #10025 Added Functions Flags support.
The PR is divided to 2 sections:
* Add named argument support to `redis.register_function` API.
* Add support for function flags

## `redis.register_function` named argument support

The first part of the PR adds support for named argument on `redis.register_function`, example:
```
redis.register_function{
    function_name='f1',
    callback=function()
        return 'hello'
    end,
    description='some desc'
}
```

The positional arguments is also kept, which means that it still possible to write:
```
redis.register_function('f1', function() return 'hello' end)
```

But notice that it is no longer possible to pass the optional description argument on the positional
argument version. Positional argument was change to allow passing only the mandatory arguments
(function name and callback). To pass more arguments the user must use the named argument version.

As with positional arguments, the `function_name` and `callback` is mandatory and an error will be
raise if those are missing. Also, an error will be raise if an unknown argument name is given or the
arguments type is wrong.

Tests was added to verify the new syntax.

## Functions Flags

The second part of the PR is adding functions flags support.
Flags are given to Redis when the engine calls `functionLibCreateFunction`, supported flags are:

* `no-writes` - indicating the function perform no writes which means that it is OK to run it on:
   * read-only replica
   * Using FCALL_RO
   * If disk error detected
   
   It will not be possible to run a function in those situations unless the function turns on the `no-writes` flag

* `allow-oom` - indicate that its OK to run the function even if Redis is in OOM state, if the function will
  not turn on this flag it will not be possible to run it if OOM reached (even if the function declares `no-writes`
  and even if `fcall_ro` is used). If this flag is set, any command will be allow on OOM (even those that is
  marked with CMD_DENYOOM). The assumption is that this flag is for advance users that knows its
  meaning and understand what they are doing, and Redis trust them to not increase the memory usage.
  (e.g. it could be an INCR or a modification on an existing key, or a DEL command)

* `allow-state` - indicate that its OK to run the function on stale replica, in this case we will also make
  sure the function is only perform `stale` commands and raise an error if not.

* `no-cluster` - indicate to disallow running the function if cluster is enabled.

Default behaviure of functions (if no flags is given):
1. Allow functions to read and write
2. Do not run functions on OOM
3. Do not run functions on stale replica
4. Allow functions on cluster

### Lua API for functions flags

On Lua engine, it is possible to give functions flags as `flags` named argument:

```
redis.register_function{function_name='f1', callback=function() return 1 end, flags={'no-writes', 'allow-oom'}, description='description'}
```

The function flags argument must be a Lua table that contains all the requested flags, The following
will result in an error:
* Unknown flag
* Wrong flag type

Default behaviour is the same as if no flags are used.

Tests were added to verify all flags functionality

## Additional changes
* mark FCALL and FCALL_RO with CMD_STALE flag (unlike EVAL), so that they can run if the function was
  registered with the `allow-stale` flag.
* Verify `CMD_STALE` on `scriptCall` (`redis.call`), so it will not be possible to call commands from script while
  stale unless the command is marked with the `CMD_STALE` flags. so that even if the function is allowed while
  stale we do not allow it to bypass the `CMD_STALE` flag of commands.
* Flags section was added to `FUNCTION LIST` command to provide the set of flags for each function:
```
> FUNCTION list withcode
1)  1) "library_name"
    2) "test"
    3) "engine"
    4) "LUA"
    5) "description"
    6) (nil)
    7) "functions"
    8) 1) 1) "name"
          2) "f1"
          3) "description"
          4) (nil)
          5) "flags"
          6) (empty array)
    9) "library_code"
   10) "redis.register_function{function_name='f1', callback=function() return 1 end}"
```
* Added API to get Redis version from within a script, The redis version can be provided using:
   1. `redis.REDIS_VERSION` - string representation of the redis version in the format of MAJOR.MINOR.PATH
   2. `redis.REDIS_VERSION_NUM` - number representation of the redis version in the format of `0x00MMmmpp`
      (`MM` - major, `mm` - minor,  `pp` - patch). The number version can be used to check if version is greater or less 
      another version. The string version can be used to return to the user or print as logs.

   This new API is provided to eval scripts and functions, it also possible to use this API during functions loading phase.

11 files changed, 377 insertions, 43 deletions

diff --git a/src/commands.c b/src/commands.c
index 8eed44e01..2183d09a1 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -6612,8 +6612,8 @@ struct redisCommand redisCommandTable[] = {
 {"evalsha","Execute a Lua script server side","Depends on the script that is executed.","2.6.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,EVALSHA_History,EVALSHA_Hints,evalShaCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_MAY_REPLICATE|CMD_NO_MANDATORY_KEYS,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_WRITE|CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},evalGetKeys,.args=EVALSHA_Args},
 {"evalsha_ro","Execute a read-only Lua script server side","Depends on the script that is executed.","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,EVALSHA_RO_History,EVALSHA_RO_Hints,evalShaRoCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_NO_MANDATORY_KEYS,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},evalGetKeys,.args=EVALSHA_RO_Args},
 {"eval_ro","Execute a read-only Lua script server side","Depends on the script that is executed.","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,EVAL_RO_History,EVAL_RO_Hints,evalRoCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_NO_MANDATORY_KEYS,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},evalGetKeys,.args=EVAL_RO_Args},
-{"fcall","PATCH__TBD__38__","PATCH__TBD__37__","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,FCALL_History,FCALL_Hints,fcallCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_MAY_REPLICATE|CMD_NO_MANDATORY_KEYS,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_WRITE|CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},functionGetKeys,.args=FCALL_Args},
-{"fcall_ro","PATCH__TBD__7__","PATCH__TBD__6__","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,FCALL_RO_History,FCALL_RO_Hints,fcallroCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_NO_MANDATORY_KEYS,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},functionGetKeys,.args=FCALL_RO_Args},
+{"fcall","PATCH__TBD__38__","PATCH__TBD__37__","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,FCALL_History,FCALL_Hints,fcallCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_MAY_REPLICATE|CMD_NO_MANDATORY_KEYS|CMD_STALE,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_WRITE|CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},functionGetKeys,.args=FCALL_Args},
+{"fcall_ro","PATCH__TBD__7__","PATCH__TBD__6__","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,FCALL_RO_History,FCALL_RO_Hints,fcallroCommand,-3,CMD_NOSCRIPT|CMD_SKIP_MONITOR|CMD_NO_MANDATORY_KEYS|CMD_STALE,ACL_CATEGORY_SCRIPTING,{{CMD_KEY_READ,KSPEC_BS_INDEX,.bs.index={2},KSPEC_FK_KEYNUM,.fk.keynum={0,1,1}}},functionGetKeys,.args=FCALL_RO_Args},
 {"function","A container for function commands","Depends on subcommand.","7.0.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,FUNCTION_History,FUNCTION_Hints,NULL,-2,0,0,.subcommands=FUNCTION_Subcommands},
 {"script","A container for Lua scripts management commands","Depends on subcommand.","2.6.0",CMD_DOC_NONE,NULL,NULL,COMMAND_GROUP_SCRIPTING,SCRIPT_History,SCRIPT_Hints,NULL,-2,0,0,.subcommands=SCRIPT_Subcommands},
 /* sentinel */
diff --git a/src/commands/fcall.json b/src/commands/fcall.json
index e5e63c638..e5dfc3898 100644
--- a/src/commands/fcall.json
+++ b/src/commands/fcall.json
@@ -11,7 +11,8 @@
             "NOSCRIPT",
             "SKIP_MONITOR",
             "MAY_REPLICATE",
-            "NO_MANDATORY_KEYS"
+            "NO_MANDATORY_KEYS",
+            "STALE"
         ],
         "acl_categories": [
             "SCRIPTING"
diff --git a/src/commands/fcall_ro.json b/src/commands/fcall_ro.json
index 4d58b2f9a..ae0bf61e3 100644
--- a/src/commands/fcall_ro.json
+++ b/src/commands/fcall_ro.json
@@ -10,7 +10,8 @@
         "command_flags": [
             "NOSCRIPT",
             "SKIP_MONITOR",
-            "NO_MANDATORY_KEYS"
+            "NO_MANDATORY_KEYS",
+            "STALE"
         ],
         "acl_categories": [
             "SCRIPTING"
diff --git a/src/function_lua.c b/src/function_lua.c
index e6b8a2727..3dbc8419e 100644
--- a/src/function_lua.c
+++ b/src/function_lua.c
@@ -68,6 +68,13 @@ typedef struct loadCtx {
     monotime start_time;
 } loadCtx;
 
+typedef struct registerFunctionArgs {
+    sds name;
+    sds desc;
+    luaFunctionCtx *lua_f_ctx;
+    uint64_t f_flags;
+} registerFunctionArgs;
+
 /* Hook for FUNCTION LOAD execution.
  * Used to cancel the execution in case of a timeout (500ms).
  * This execution should be fast and should only register
@@ -224,56 +231,214 @@ static void luaEngineFreeFunction(void *engine_ctx, void *compiled_function) {
     zfree(f_ctx);
 }
 
-static int luaRegisterFunction(lua_State *lua) {
-    int argc = lua_gettop(lua);
-    if (argc < 2 || argc > 3) {
-        luaPushError(lua, "wrong number of arguments to redis.register_function");
-        return luaRaiseError(lua);
+static void luaRegisterFunctionArgsInitialize(registerFunctionArgs *register_f_args,
+    sds name,
+    sds desc,
+    luaFunctionCtx *lua_f_ctx,
+    uint64_t flags)
+{
+    *register_f_args = (registerFunctionArgs){
+        .name = name,
+        .desc = desc,
+        .lua_f_ctx = lua_f_ctx,
+        .f_flags = flags,
+    };
+}
+
+static void luaRegisterFunctionArgsDispose(lua_State *lua, registerFunctionArgs *register_f_args) {
+    sdsfree(register_f_args->name);
+    if (register_f_args->desc) sdsfree(register_f_args->desc);
+    lua_unref(lua, register_f_args->lua_f_ctx->lua_function_ref);
+    zfree(register_f_args->lua_f_ctx);
+}
+
+/* Read function flags located on the top of the Lua stack.
+ * On success, return C_OK and set the flags to 'flags' out parameter
+ * Return C_ERR if encounter an unknown flag. */
+static int luaRegisterFunctionReadFlags(lua_State *lua, uint64_t *flags) {
+    int j = 1;
+    int ret = C_ERR;
+    int f_flags = 0;
+    while(1) {
+        lua_pushnumber(lua,j++);
+        lua_gettable(lua,-2);
+        int t = lua_type(lua,-1);
+        if (t == LUA_TNIL) {
+            lua_pop(lua,1);
+            break;
+        }
+        if (!lua_isstring(lua, -1)) {
+            lua_pop(lua,1);
+            goto done;
+        }
+
+        const char *flag_str = lua_tostring(lua, -1);
+        int found = 0;
+        for (scriptFlag *flag = scripts_flags_def; flag->str ; ++flag) {
+            if (!strcasecmp(flag->str, flag_str)) {
+                f_flags |= flag->flag;
+                found = 1;
+                break;
+            }
+        }
+        /* pops the value to continue the iteration */
+        lua_pop(lua,1);
+        if (!found) {
+            /* flag not found */
+            goto done;
+        }
     }
-    loadCtx *load_ctx = luaGetFromRegistry(lua, REGISTRY_LOAD_CTX_NAME);
-    if (!load_ctx) {
-        luaPushError(lua, "redis.register_function can only be called on FUNCTION LOAD command");
-        return luaRaiseError(lua);
+
+    *flags = f_flags;
+    ret = C_OK;
+
+done:
+    return ret;
+}
+
+static int luaRegisterFunctionReadNamedArgs(lua_State *lua, registerFunctionArgs *register_f_args) {
+    char *err = NULL;
+    sds name = NULL;
+    sds desc = NULL;
+    luaFunctionCtx *lua_f_ctx = NULL;
+    uint64_t flags = 0;
+    if (!lua_istable(lua, 1)) {
+        err = "calling redis.register_function with a single argument is only applicable to Lua table (representing named arguments).";
+        goto error;
     }
 
-    if (!lua_isstring(lua, 1)) {
-        luaPushError(lua, "first argument to redis.register_function must be a string");
-        return luaRaiseError(lua);
+    /* Iterating on all the named arguments */
+    lua_pushnil(lua);
+    while (lua_next(lua, -2)) {
+        /* Stack now: table, key, value */
+        if (!lua_isstring(lua, -2)) {
+            err = "named argument key given to redis.register_function is not a string";
+            goto error;
+        }
+        const char *key = lua_tostring(lua, -2);
+        if (!strcasecmp(key, "function_name")) {
+            if (!(name = luaGetStringSds(lua, -1))) {
+                err = "function_name argument given to redis.register_function must be a string";
+                goto error;
+            }
+        } else if (!strcasecmp(key, "description")) {
+            if (!(desc = luaGetStringSds(lua, -1))) {
+                err = "description argument given to redis.register_function must be a string";
+                goto error;
+            }
+        } else if (!strcasecmp(key, "callback")) {
+            if (!lua_isfunction(lua, -1)) {
+                err = "callback argument given to redis.register_function must be a function";
+                goto error;
+            }
+            int lua_function_ref = luaL_ref(lua, LUA_REGISTRYINDEX);
+
+            lua_f_ctx = zmalloc(sizeof(*lua_f_ctx));
+            lua_f_ctx->lua_function_ref = lua_function_ref;
+            continue; /* value was already popped, so no need to pop it out. */
+        } else if (!strcasecmp(key, "flags")) {
+            if (!lua_istable(lua, -1)) {
+                err = "flags argument to redis.register_function must be a table representing function flags";
+                goto error;
+            }
+            if (luaRegisterFunctionReadFlags(lua, &flags) != C_OK) {
+                err = "unknown flag given";
+                goto error;
+            }
+        } else {
+            /* unknown argument was given, raise an error */
+            err = "unknown argument given to redis.register_function";
+            goto error;
+        }
+        lua_pop(lua, 1); /* pop the value to continue the iteration */
     }
 
-    if (!lua_isfunction(lua, 2)) {
-        luaPushError(lua, "second argument to redis.register_function must be a function");
-        return luaRaiseError(lua);
+    if (!name) {
+        err = "redis.register_function must get a function name argument";
+        goto error;
     }
 
-    if (argc == 3 && !lua_isstring(lua, 3)) {
-        luaPushError(lua, "third argument to redis.register_function must be a string");
-        return luaRaiseError(lua);
+    if (!lua_f_ctx) {
+        err = "redis.register_function must get a callback argument";
+        goto error;
+    }
+
+    luaRegisterFunctionArgsInitialize(register_f_args, name, desc, lua_f_ctx, flags);
+
+    return C_OK;
+
+error:
+    if (name) sdsfree(name);
+    if (desc) sdsfree(desc);
+    if (lua_f_ctx) {
+        lua_unref(lua, lua_f_ctx->lua_function_ref);
+        zfree(lua_f_ctx);
     }
+    luaPushError(lua, err);
+    return C_ERR;
+}
 
-    size_t function_name_len;
-    const char *function_name = lua_tolstring(lua, 1, &function_name_len);
-    sds function_name_sds = sdsnewlen(function_name, function_name_len);
+static int luaRegisterFunctionReadPositionalArgs(lua_State *lua, registerFunctionArgs *register_f_args) {
+    char *err = NULL;
+    sds name = NULL;
+    sds desc = NULL;
+    luaFunctionCtx *lua_f_ctx = NULL;
+    if (!(name = luaGetStringSds(lua, 1))) {
+        err = "first argument to redis.register_function must be a string";
+        goto error;
+    }
 
-    sds desc_sds = NULL;
-    if (argc == 3){
-        size_t desc_len;
-        const char *desc = lua_tolstring(lua, 3, &desc_len);
-        desc_sds = sdsnewlen(desc, desc_len);
-        lua_pop(lua, 1); /* pop out the description */
+    if (!lua_isfunction(lua, 2)) {
+        err = "second argument to redis.register_function must be a function";
+        goto error;
     }
 
     int lua_function_ref = luaL_ref(lua, LUA_REGISTRYINDEX);
 
-    luaFunctionCtx *lua_f_ctx = zmalloc(sizeof(*lua_f_ctx));
-    *lua_f_ctx = (luaFunctionCtx ) { .lua_function_ref = lua_function_ref, };
+    lua_f_ctx = zmalloc(sizeof(*lua_f_ctx));
+    lua_f_ctx->lua_function_ref = lua_function_ref;
+
+    luaRegisterFunctionArgsInitialize(register_f_args, name, NULL, lua_f_ctx, 0);
+
+    return C_OK;
+
+error:
+    if (name) sdsfree(name);
+    if (desc) sdsfree(desc);
+    luaPushError(lua, err);
+    return C_ERR;
+}
+
+static int luaRegisterFunctionReadArgs(lua_State *lua, registerFunctionArgs *register_f_args) {
+    int argc = lua_gettop(lua);
+    if (argc < 1 || argc > 2) {
+        luaPushError(lua, "wrong number of arguments to redis.register_function");
+        return C_ERR;
+    }
+
+    if (argc == 1) {
+        return luaRegisterFunctionReadNamedArgs(lua, register_f_args);
+    } else {
+        return luaRegisterFunctionReadPositionalArgs(lua, register_f_args);
+    }
+}
+
+static int luaRegisterFunction(lua_State *lua) {
+    registerFunctionArgs register_f_args = {0};
+
+    loadCtx *load_ctx = luaGetFromRegistry(lua, REGISTRY_LOAD_CTX_NAME);
+    if (!load_ctx) {
+        luaPushError(lua, "redis.register_function can only be called on FUNCTION LOAD command");
+        return luaRaiseError(lua);
+    }
+
+    if (luaRegisterFunctionReadArgs(lua, &register_f_args) != C_OK) {
+        return luaRaiseError(lua);
+    }
 
     sds err = NULL;
-    if (functionLibCreateFunction(function_name_sds, lua_f_ctx, load_ctx->li, desc_sds, &err) != C_OK) {
-        sdsfree(function_name_sds);
-        if (desc_sds) sdsfree(desc_sds);
-        lua_unref(lua, lua_f_ctx->lua_function_ref);
-        zfree(lua_f_ctx);
+    if (functionLibCreateFunction(register_f_args.name, register_f_args.lua_f_ctx, load_ctx->li, register_f_args.desc, register_f_args.f_flags, &err) != C_OK) {
+        luaRegisterFunctionArgsDispose(lua, &register_f_args);
         luaPushError(lua, err);
         sdsfree(err);
         return luaRaiseError(lua);
@@ -298,6 +463,7 @@ int luaEngineInitEngine() {
     lua_settable(lua_engine_ctx->lua, -3);
 
     luaRegisterLogFunction(lua_engine_ctx->lua);
+    luaRegisterVersion(lua_engine_ctx->lua);
 
     lua_settable(lua_engine_ctx->lua, LUA_REGISTRYINDEX);
 
diff --git a/src/functions.c b/src/functions.c
index 5a8cd27c1..76e8c21d7 100644
--- a/src/functions.c
+++ b/src/functions.c
@@ -198,7 +198,7 @@ functionsLibCtx* functionsLibCtxCreate() {
  *       the function will verify that the given name is following the naming format
  *       and return an error if its not.
  */
-int functionLibCreateFunction(sds name, void *function, functionLibInfo *li, sds desc, sds *err) {
+int functionLibCreateFunction(sds name, void *function, functionLibInfo *li, sds desc, uint64_t f_flags, sds *err) {
     if (functionsVerifyName(name) != C_OK) {
         *err = sdsnew("Function names can only contain letters and numbers and must be at least one character long");
         return C_ERR;
@@ -215,6 +215,7 @@ int functionLibCreateFunction(sds name, void *function, functionLibInfo *li, sds
         .function = function,
         .li = li,
         .desc = desc,
+        .f_flags = f_flags,
     };
 
     int res = dictAdd(li->functions, fi->name, fi);
@@ -410,6 +411,24 @@ void functionStatsCommand(client *c) {
     dictReleaseIterator(iter);
 }
 
+static void functionListReplyFlags(client *c, functionInfo *fi) {
+    /* First count the number of flags we have */
+    int flagcount = 0;
+    for (scriptFlag *flag = scripts_flags_def; flag->str ; ++flag) {
+        if (fi->f_flags & flag->flag) {
+            ++flagcount;
+        }
+    }
+
+    addReplySetLen(c, flagcount);
+
+    for (scriptFlag *flag = scripts_flags_def; flag->str ; ++flag) {
+        if (fi->f_flags & flag->flag) {
+            addReplyStatus(c, flag->str);
+        }
+    }
+}
+
 /*
  * FUNCTION LIST [LIBRARYNAME PATTERN] [WITHCODE]
  *
@@ -480,7 +499,7 @@ void functionListCommand(client *c) {
         dictEntry *function_entry = NULL;
         while ((function_entry = dictNext(functions_iter))) {
             functionInfo *fi = dictGetVal(function_entry);
-            addReplyMapLen(c, 2);
+            addReplyMapLen(c, 3);
             addReplyBulkCString(c, "name");
             addReplyBulkCBuffer(c, fi->name, sdslen(fi->name));
             addReplyBulkCString(c, "description");
@@ -489,6 +508,8 @@ void functionListCommand(client *c) {
             } else {
                 addReplyNull(c);
             }
+            addReplyBulkCString(c, "flags");
+            functionListReplyFlags(c, fi);
         }
         dictReleaseIterator(functions_iter);
 
@@ -549,12 +570,69 @@ static void fcallCommandGeneric(client *c, int ro) {
         return;
     }
 
+    if ((fi->f_flags & SCRIPT_FLAG_NO_CLUSTER) && server.cluster_enabled) {
+        addReplyError(c, "Can not run function on cluster, 'no-cluster' flag is set.");
+        return;
+    }
+
+    if (!(fi->f_flags & SCRIPT_FLAG_ALLOW_OOM) && server.script_oom && server.maxmemory) {
+        addReplyError(c, "-OOM allow-oom flag is not set on the function, "
+                         "can not run it when used memory > 'maxmemory'");
+        return;
+    }
+
+    if (server.masterhost && server.repl_state != REPL_STATE_CONNECTED &&
+        server.repl_serve_stale_data == 0 && !(fi->f_flags & SCRIPT_FLAG_ALLOW_STALE))
+    {
+        addReplyError(c, "-MASTERDOWN Link with MASTER is down, "
+                         "replica-serve-stale-data is set to 'no' "
+                         "and 'allow-stale' flag is not set on the function.");
+        return;
+    }
+
+    if (!(fi->f_flags & SCRIPT_FLAG_NO_WRITES)) {
+        /* Function may perform writes we need to verify:
+         * 1. we are not a readonly replica
+         * 2. no disk error detected
+         * 3. command is not 'fcall_ro' */
+        if (server.masterhost && server.repl_slave_ro && c->id != CLIENT_ID_AOF
+            && !(c->flags & CLIENT_MASTER))
+        {
+            addReplyError(c, "Can not run a function with write flag on readonly replica");
+            return;
+        }
+
+        int deny_write_type = writeCommandsDeniedByDiskError();
+        if (deny_write_type != DISK_ERROR_TYPE_NONE && server.masterhost == NULL) {
+            if (deny_write_type == DISK_ERROR_TYPE_RDB)
+                addReplyError(c, "-MISCONF Redis is configured to save RDB snapshots, "
+                                 "but it is currently not able to persist on disk. "
+                                 "So its impossible to run functions that has 'write' flag on.");
+            else
+                addReplyErrorFormat(c, "-MISCONF Redis is configured to persist data to AOF, "
+                                       "but it is currently not able to persist on disk. "
+                                       "So its impossible to run functions that has 'write' flag on. "
+                                       "AOF error: %s", strerror(server.aof_last_write_errno));
+            return;
+        }
+
+        if (ro) {
+            addReplyError(c, "Can not execute a function with write flag using fcall_ro.");
+            return;
+        }
+    }
+
     scriptRunCtx run_ctx;
 
     scriptPrepareForRun(&run_ctx, fi->li->ei->c, c, fi->name);
-    if (ro) {
+    if (ro || (fi->f_flags & SCRIPT_FLAG_NO_WRITES)) {
+        /* On fcall_ro or on functions that do not have the 'write'
+         * flag, we will not allow write commands. */
         run_ctx.flags |= SCRIPT_READ_ONLY;
     }
+    if (fi->f_flags & SCRIPT_FLAG_ALLOW_OOM) {
+        run_ctx.flags |= SCRIPT_ALLOW_OOM;
+    }
     engine->call(&run_ctx, engine->engine_ctx, fi->function, c->argv + 3, numkeys,
                  c->argv + 3 + numkeys, c->argc - 3 - numkeys);
     scriptResetRun(&run_ctx);
diff --git a/src/functions.h b/src/functions.h
index f49040193..fb2b74de9 100644
--- a/src/functions.h
+++ b/src/functions.h
@@ -96,6 +96,7 @@ typedef struct functionInfo {
                             to run the function, usually it's the function compiled code. */
     functionLibInfo* li; /* Pointer to the library created the function */
     sds desc;            /* Function description */
+    uint64_t f_flags;    /* Function flags */
 } functionInfo;
 
 /* Hold information about the specific library.
@@ -124,7 +125,7 @@ void functionsLibCtxFree(functionsLibCtx *lib_ctx);
 void functionsLibCtxClear(functionsLibCtx *lib_ctx);
 void functionsLibCtxSwapWithCurrent(functionsLibCtx *lib_ctx);
 
-int functionLibCreateFunction(sds name, void *function, functionLibInfo *li, sds desc, sds *err);
+int functionLibCreateFunction(sds name, void *function, functionLibInfo *li, sds desc, uint64_t f_flags, sds *err);
 
 int luaEngineInitEngine();
 int functionsInit();
diff --git a/src/script.c b/src/script.c
index bef4f383c..8b1545b1a 100644
--- a/src/script.c
+++ b/src/script.c
@@ -31,6 +31,14 @@
 #include "script.h"
 #include "cluster.h"
 
+scriptFlag scripts_flags_def[] = {
+    {.flag = SCRIPT_FLAG_NO_WRITES, .str = "no-writes"},
+    {.flag = SCRIPT_FLAG_ALLOW_OOM, .str = "allow-oom"},
+    {.flag = SCRIPT_FLAG_ALLOW_STALE, .str = "allow-stale"},
+    {.flag = SCRIPT_FLAG_NO_CLUSTER, .str = "no-cluster"},
+    {.flag = 0, .str = NULL}, /* flags array end */
+};
+
 /* On script invocation, holding the current run context */
 static scriptRunCtx *curr_run_ctx = NULL;
 
@@ -281,6 +289,11 @@ static int scriptVerifyWriteCommandAllow(scriptRunCtx *run_ctx, char **err) {
 }
 
 static int scriptVerifyOOM(scriptRunCtx *run_ctx, char **err) {
+    if (run_ctx->flags & SCRIPT_ALLOW_OOM) {
+        /* Allow running any command even if OOM reached */
+        return C_OK;
+    }
+
     /* If we reached the memory limit configured via maxmemory, commands that
      * could enlarge the memory usage are not allowed, but only if this is the
      * first write in the context of this script, otherwise we can't stop
@@ -348,6 +361,32 @@ int scriptSetRepl(scriptRunCtx *run_ctx, int repl) {
     return C_OK;
 }
 
+static int scriptVerifyAllowStale(client *c, sds *err) {
+    if (!server.masterhost) {
+        /* Not a replica, stale is irrelevant */
+        return C_OK;
+    }
+
+    if (server.repl_state == REPL_STATE_CONNECTED) {
+        /* Connected to replica, stale is irrelevant */
+        return C_OK;
+    }
+
+    if (server.repl_serve_stale_data == 1) {
+        /* Disconnected from replica but allow to serve data */
+        return C_OK;
+    }
+
+    if (c->cmd->flags & CMD_STALE) {
+        /* Command is allow while stale */
+        return C_OK;
+    }
+
+    /* On stale replica, can not run the command */
+    *err = sdsnew("Can not execute the command on a stale replica");
+    return C_ERR;
+}
+
 /* Call a Redis command.
  * The reply is written to the run_ctx client and it is
  * up to the engine to take and parse.
@@ -379,6 +418,10 @@ void scriptCall(scriptRunCtx *run_ctx, robj* *argv, int argc, sds *err) {
         return;
     }
 
+    if (scriptVerifyAllowStale(c, err) != C_OK) {
+        return;
+    }
+
     if (scriptVerifyACL(c, err) != C_OK) {
         return;
     }
diff --git a/src/script.h b/src/script.h
index b2e253ee0..122db1dcf 100644
--- a/src/script.h
+++ b/src/script.h
@@ -62,6 +62,7 @@
 #define SCRIPT_TIMEDOUT               (1ULL<<3) /* indicate that the current script timedout */
 #define SCRIPT_KILLED                 (1ULL<<4) /* indicate that the current script was marked to be killed */
 #define SCRIPT_READ_ONLY              (1ULL<<5) /* indicate that the current script should only perform read commands */
+#define SCRIPT_ALLOW_OOM              (1ULL<<6) /* indicate to allow any command even if OOM reached */
 #define SCRIPT_EVAL_MODE              (1ULL<<7) /* Indicate that the current script called from legacy Lua */
 typedef struct scriptRunCtx scriptRunCtx;
 
@@ -75,6 +76,20 @@ struct scriptRunCtx {
     mstime_t snapshot_time;
 };
 
+/* Scripts flags */
+#define SCRIPT_FLAG_NO_WRITES (1ULL<<0)
+#define SCRIPT_FLAG_ALLOW_OOM (1ULL<<1)
+#define SCRIPT_FLAG_ALLOW_STALE (1ULL<<3)
+#define SCRIPT_FLAG_NO_CLUSTER (1ULL<<4)
+
+/* Defines a script flags */
+typedef struct scriptFlag {
+    uint64_t flag;
+    const char *str;
+} scriptFlag;
+
+extern scriptFlag scripts_flags_def[];
+
 void scriptPrepareForRun(scriptRunCtx *r_ctx, client *engine_client, client *caller, const char *funcname);
 void scriptResetRun(scriptRunCtx *r_ctx);
 int scriptSetResp(scriptRunCtx *r_ctx, int resp);
diff --git a/src/script_lua.c b/src/script_lua.c
index fc9fc812a..76e9eb7ad 100644
--- a/src/script_lua.c
+++ b/src/script_lua.c
@@ -35,6 +35,7 @@
 #include "cluster.h"
 #include "monotonic.h"
 #include "resp_parser.h"
+#include "version.h"
 #include <lauxlib.h>
 #include <lualib.h>
 #include <ctype.h>
@@ -1039,6 +1040,19 @@ static void luaRemoveUnsupportedFunctions(lua_State *lua) {
     lua_setglobal(lua,"dofile");
 }
 
+/* Return sds of the string value located on stack at the given index.
+ * Return NULL if the value is not a string. */
+sds luaGetStringSds(lua_State *lua, int index) {
+    if (!lua_isstring(lua, index)) {
+        return NULL;
+    }
+
+    size_t len;
+    const char *str = lua_tolstring(lua, index, &len);
+    sds str_sds = sdsnewlen(str, len);
+    return str_sds;
+}
+
 /* This function installs metamethods in the global table _G that prevent
  * the creation of globals accidentally.
  *
@@ -1142,6 +1156,16 @@ void luaSetGlobalProtection(lua_State *lua) {
     serverAssert(res == 0);
 }
 
+void luaRegisterVersion(lua_State* lua) {
+    lua_pushstring(lua,"REDIS_VERSION_NUM");
+    lua_pushnumber(lua,REDIS_VERSION_NUM);
+    lua_settable(lua,-3);
+
+    lua_pushstring(lua,"REDIS_VERSION");
+    lua_pushstring(lua,REDIS_VERSION);
+    lua_settable(lua,-3);
+}
+
 void luaRegisterLogFunction(lua_State* lua) {
     /* redis.log and log levels. */
     lua_pushstring(lua,"log");
@@ -1184,6 +1208,8 @@ void luaRegisterRedisAPI(lua_State* lua) {
 
     luaRegisterLogFunction(lua);
 
+    luaRegisterVersion(lua);
+
     /* redis.setresp */
     lua_pushstring(lua,"setresp");
     lua_pushcfunction(lua,luaSetResp);
diff --git a/src/script_lua.h b/src/script_lua.h
index b7862bee1..ac13178ca 100644
--- a/src/script_lua.h
+++ b/src/script_lua.h
@@ -59,10 +59,12 @@
 #define REDIS_API_NAME "redis"
 
 void luaRegisterRedisAPI(lua_State* lua);
+sds luaGetStringSds(lua_State *lua, int index);
 void luaEnableGlobalsProtection(lua_State *lua, int is_eval);
 void luaRegisterGlobalProtectionFunction(lua_State *lua);
 void luaSetGlobalProtection(lua_State *lua);
 void luaRegisterLogFunction(lua_State* lua);
+void luaRegisterVersion(lua_State* lua);
 void luaPushError(lua_State *lua, char *error);
 int luaRaiseError(lua_State *lua);
 void luaSaveOnRegistry(lua_State* lua, const char* name, void* ptr);
diff --git a/src/server.c b/src/server.c
index 36143caa5..300d50198 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3543,7 +3543,8 @@ int processCommand(client *c) {
          * arguments might interfere. */
         if (c->cmd->proc == evalCommand ||
             c->cmd->proc == evalShaCommand ||
-            c->cmd->proc == fcallCommand)
+            c->cmd->proc == fcallCommand ||
+            c->cmd->proc == fcallroCommand)
         {
             server.script_oom = out_of_memory;
         }