6 files changed, 239 insertions, 37 deletions

diff --git a/src/cancel.cpp b/src/cancel.cpp
index 8fa68d5..31656c4 100644
--- a/src/cancel.cpp
+++ b/src/cancel.cpp
@@ -164,7 +164,7 @@ LUAG_FUNC(lane_cancel)
             raise_luaL_argerror(L_, StackIndex{ 2 }, "duration cannot be < 0");
         }
         lua_remove(L_, 2); // argument is processed, remove it
-    } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil before the key
+    } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil
         lua_remove(L_, 2); // argument is processed, remove it
     }
 
diff --git a/src/compat.hpp b/src/compat.hpp
index f66f703..81f1665 100644
--- a/src/compat.hpp
+++ b/src/compat.hpp
@@ -272,6 +272,16 @@ LuaType luaG_getmodule(lua_State* L_, std::string_view const& name_);
 
 // #################################################################################################
 
+template<typename ENUM>
+requires std::is_enum_v<ENUM>
+[[nodiscard]]
+ENUM luaG_optenum(lua_State* const L_, StackIndex const idx_, ENUM const def_)
+{
+    return static_cast<ENUM>(luaL_optinteger(L_, idx_, static_cast<std::underlying_type_t<ENUM>>(def_)));
+}
+
+// #################################################################################################
+
 inline void luaG_registerlibfuncs(lua_State* L_, luaL_Reg const* funcs_)
 {
     // fake externs to make clang happy...
diff --git a/src/keeper.cpp b/src/keeper.cpp
index 8a99a36..e7b02e6 100644
--- a/src/keeper.cpp
+++ b/src/keeper.cpp
@@ -73,6 +73,7 @@ class KeyUD
     int first{ 1 };
     int count{ 0 };
     LindaLimit limit{ -1 };
+    LindaRestrict restrict { LindaRestrict::None };
 
     // a fifo full userdata has one uservalue, the table that holds the actual fifo contents
     [[nodiscard]]
@@ -84,6 +85,8 @@ class KeyUD
     [[nodiscard]]
     bool changeLimit(LindaLimit limit_);
     [[nodiscard]]
+    LindaRestrict changeRestrict(LindaRestrict restrict_);
+    [[nodiscard]]
     static KeyUD* Create(KeeperState K_);
     [[nodiscard]]
     static KeyUD* GetPtr(KeeperState K_, StackIndex idx_);
@@ -114,6 +117,14 @@ bool KeyUD::changeLimit(LindaLimit const limit_)
 
 // #################################################################################################
 
+[[nodiscard]]
+LindaRestrict KeyUD::changeRestrict(LindaRestrict const restrict_)
+{
+    return std::exchange(restrict, restrict_);
+}
+
+// #################################################################################################
+
 // in: nothing
 // out: { first = 1, count = 0, limit = -1}
 KeyUD* KeyUD::Create(KeeperState const K_)
@@ -280,7 +291,7 @@ bool KeyUD::reset(KeeperState const K_)
     LUA_ASSERT(K_, KeyUD::GetPtr(K_, kIdxTop) == this);
     STACK_CHECK_START_REL(K_, 0);
     bool const _wasFull{ (limit > 0) && (count >= limit) };
-    // empty the KeyUD: replace uservalue with a virgin table, reset counters, but leave limit unchanged!
+    // empty the KeyUD: replace uservalue with a virgin table, reset counters, but leave limit and restrict unchanged!
     // if we have an actual limit, use it to preconfigure the table
     lua_createtable(K_, (limit <= 0) ? 0 : limit.value(), 0);                                      // K_: KeysDB key val... KeyUD {}
     lua_setiuservalue(K_, StackIndex{ -2 }, kContentsTableIndex);                                  // K_: KeysDB key val... KeyUD
@@ -408,7 +419,7 @@ int keepercall_destruct(lua_State* const L_)
 // #################################################################################################
 
 // in: linda_ud key [count]
-// out: bool + at most <count> values
+// out: N <N values>|kRestrictedChannel
 int keepercall_get(lua_State* const L_)
 {
     KeeperState const _K{ L_ };
@@ -423,7 +434,13 @@ int keepercall_get(lua_State* const L_)
     lua_remove(_K, 1);                                                                             // _K: KeyUD
     KeyUD const* const _key{ KeyUD::GetPtr(_K, kIdxTop) };
     if (_key != nullptr) {
-        _key->peek(_K, _count);                                                                    // _K: N val...
+        if (_key->restrict == LindaRestrict::SendReceive) { // can we use set/get?
+            lua_settop(_K, 0);                                                                     // _K:
+            kRestrictedChannel.pushKey(_K);                                                        // _K: kRestrictedChannel
+            return 1;
+        } else {
+            _key->peek(_K, _count);                                                                // _K: N val...
+        }
     } else {
         // no fifo was ever registered for this key, or it is empty
         lua_pop(_K, 1);                                                                            // _K:
@@ -492,6 +509,17 @@ int keepercall_receive(lua_State* const L_)
         lua_rawget(_K, 1);                                                                         // _K: KeysDB keys... KeyUD
         KeyUD* const _key{ KeyUD::GetPtr(_K, kIdxTop) };
         if (_key != nullptr) { // it's fine to attempt a read on a key that wasn't yet written to
+            if (_key->restrict == LindaRestrict::SetGet) { // can we use send/receive?
+                kRestrictedChannel.pushKey(_K);                                                    // _K: KeysDB keys... key[i] kRestrictedChannel
+                lua_replace(_K, 1);                                                                // _K: kRestrictedChannel keys... key[i]
+                lua_settop(_K, _keyIdx);                                                           // _K: kRestrictedChannel keys... key[i]
+                if (_keyIdx != 2) {
+                    lua_replace(_K, 2);                                                            // _K: kRestrictedChannel key[i] keys...
+                    lua_settop(_K, 2);                                                             // _K: kRestrictedChannel key[i]
+                }
+                lua_insert(_K, 1);                                                                 // _K: key kRestrictedChannel
+                return 2;
+            }
             int const _popped{ _key->pop(_K, 1, 1) };                                              // _K: KeysDB keys... val
             if (_popped > 0) {
                 lua_replace(_K, 1);                                                                // _K: val keys...
@@ -527,19 +555,89 @@ int keepercall_receive_batched(lua_State* const L_)
     lua_rawget(_K, 2);                                                                             // _K: key KeysDB KeyUD
     lua_remove(_K, 2);                                                                             // _K: key KeyUD
     KeyUD* const _key{ KeyUD::GetPtr(_K, kIdxTop) };
-    if (_key == nullptr || _key->pop(_K, _min_count, _max_count) == 0) {                           // _K: [key val...]|crap
-        // Lua will adjust the stack for us when we return
-        return 0;
+    if (!_key) {
+        return 0; // Lua will adjust the stack for us when we return
+    }
+    if (_key->restrict == LindaRestrict::SetGet) { // can we use send/receive?
+        lua_settop(_K, 1);                                                                         // _K: key
+        kRestrictedChannel.pushKey(_K);                                                            // _K: key kRestrictedChannel
+        return 2;
+    }
+    if (_key->pop(_K, _min_count, _max_count) == 0) {                                              // _K: [key val...]|crap
+        return 0; // Lua will adjust the stack for us when we return
+    }
+    // return whatever remains on the stack at that point: the key and the values we pulled from the fifo
+    return lua_gettop(_K);
+}
+
+// #################################################################################################
+
+// in: linda key [mode]
+// out: mode
+int keepercall_restrict(lua_State* const L_)
+{
+    KeeperState const _K{ L_ };
+    STACK_CHECK_START_ABS(_K, lua_gettop(_K));
+    // no restriction to set, means we read and return the current restriction instead
+    bool const _reading{ lua_gettop(_K) == 2 };
+    auto _decodeRestrict = [_K, _reading]() {
+        if (_reading) {
+            return LindaRestrict::None;
+        }
+        std::string_view const _val{ luaG_tostring(_K, StackIndex{ 3 }) };
+        if (_val == "set/get") {
+            return LindaRestrict::SetGet;
+        }
+        if (_val == "send/receive") {
+            return LindaRestrict::SendReceive;
+        }
+        return LindaRestrict::None;
+    };
+    auto _encodeRestrict = [](LindaRestrict const val_) {
+        switch (val_) {
+        default:
+        case LindaRestrict::None:
+            return std::string_view{ "none" };
+        case LindaRestrict::SetGet:
+            return std::string_view{ "set/get" };
+        case LindaRestrict::SendReceive:
+            return std::string_view{ "send/receive" };
+        }
+    };
+    LindaRestrict const _rstrct{ _decodeRestrict() }; // if we read nil because the argument is absent
+    lua_settop(_K, 2);                                                                             // _K: linda key
+    PushKeysDB(_K, StackIndex{ 1 });                                                               // _K: linda key KeysDB
+    lua_replace(_K, 1);                                                                            // _K: KeysDB key
+    lua_pushvalue(_K, -1);                                                                         // _K: KeysDB key key
+    lua_rawget(_K, -3);                                                                            // _K: KeysDB key KeyUD|nil
+    KeyUD* _key{ KeyUD::GetPtr(_K, kIdxTop) };
+    if (_reading) {
+        // remove any clutter on the stack
+        lua_settop(_K, 0);                                                                         // _K:
+        auto const _prevRstrct{ _key ? _key->restrict : LindaRestrict::None };
+        // return a single value: the restrict mode of the key
+        luaG_pushstring(_K, _encodeRestrict(_prevRstrct));                                          // _K: _previous
     } else {
-        // return whatever remains on the stack at that point: the key and the values we pulled from the fifo
-        return lua_gettop(_K);
+        if (_key == nullptr) {                                                                     // _K: KeysDB key nil
+            lua_pop(_K, 1);                                                                        // _K: KeysDB key
+            _key = KeyUD::Create(_K);                                                              // _K: KeysDB key KeyUD
+            lua_rawset(_K, -3);                                                                    // _K: KeysDB
+        }
+        // remove any clutter on the stack
+        lua_settop(_K, 0);                                                                         // _K:
+        // return true if we decide that blocked threads waiting to write on that key should be awakened
+        // this is the case if we detect the key was full but it is no longer the case
+        LindaRestrict const _previous{ _key->changeRestrict(_rstrct) };
+        luaG_pushstring(_K, _encodeRestrict(_previous));                                           // _K: _previous
     }
+    STACK_CHECK(_K, 1);
+    return 1;
 }
 
 // #################################################################################################
 
 // in: linda, key, ...
-// out: true|false
+// out: true|false|kRestrictedChannel
 int keepercall_send(lua_State* const L_)
 {
     KeeperState const _K{ L_ };
@@ -561,7 +659,11 @@ int keepercall_send(lua_State* const L_)
     lua_pop(_K, 1);                                                                                // _K: linda KeyUD val...
     STACK_CHECK(_K, 0);
     KeyUD* const _key{ KeyUD::GetPtr(_K, StackIndex{ 2 }) };
-    if (_key && _key->push(_K, _n, true)) { // not enough room?
+    if (_key->restrict == LindaRestrict::SetGet) { // can we use send/receive?
+        lua_settop(_K, 0);                                                                         // _K:
+        kRestrictedChannel.pushKey(_K);                                                            // _K: kRestrictedChannel
+    }
+    else if (_key->push(_K, _n, true)) { // not enough room?
         lua_settop(_K, 0);                                                                         // _K:
         lua_pushboolean(_K, 1);                                                                    // _K: true
     } else {
@@ -575,7 +677,7 @@ int keepercall_send(lua_State* const L_)
 // #################################################################################################
 
 // in: linda key [val...]
-// out: true if the linda was full but it's no longer the case, else false
+// out: true if the linda was full but it's no longer the case, else false, or kRestrictedChannel if the key is restricted
 int keepercall_set(lua_State* const L_)
 {
     KeeperState const _K{ L_ };
@@ -588,11 +690,16 @@ int keepercall_set(lua_State* const L_)
     lua_pushvalue(_K, 2);                                                                          // _K: KeysDB key val... key
     lua_rawget(_K, 1);                                                                             // _K: KeysDB key val KeyUD|nil
     KeyUD* _key{ KeyUD::GetPtr(_K, kIdxTop) };
+    if (_key && _key->restrict == LindaRestrict::SendReceive) { // can we use send/receive?
+        lua_settop(_K, 0);                                                                         // _K:
+        kRestrictedChannel.pushKey(_K);                                                            // _K: kRestrictedChannel
+        return 1;
+    }
 
     if (lua_gettop(_K) == 3) { // no value to set                                                  // _K: KeysDB key KeyUD|nil
         // empty the KeyUD for the specified key: replace uservalue with a virgin table, reset counters, but leave limit unchanged!
         if (_key != nullptr) { // might be nullptr if we set a nonexistent key to nil              // _K: KeysDB key KeyUD
-            if (_key->limit < 0) { // KeyUD limit value is the default (unlimited): we can totally remove it
+            if (_key->limit < 0 && _key->restrict == LindaRestrict::None) { // KeyUD limit value and restrict mode are the default (unlimited/none): we can totally remove it
                 lua_pop(_K, 1);                                                                    // _K: KeysDB key
                 lua_pushnil(_K);                                                                   // _K: KeysDB key nil
                 lua_rawset(_K, -3);                                                                // _K: KeysDB
@@ -777,6 +884,20 @@ int Keeper::PushLindaStorage(Linda& linda_, DestState const L_)
         }
         STACK_CHECK(L_, 5);
         lua_setfield(L_, -3, "limit");                                                             // _K: KeysDB key                                     L_: out key keyout fifo
+        // keyout.restrict
+        switch (_key->restrict) {
+        case LindaRestrict::None:
+            luaG_pushstring(L_, "none");                                                           // _K: KeysDB key                                     L_: out key keyout fifo restrict
+            break;
+        case LindaRestrict::SetGet:
+            luaG_pushstring(L_, "set/get");                                                        // _K: KeysDB key                                     L_: out key keyout fifo restrict
+            break;
+        case LindaRestrict::SendReceive:
+            luaG_pushstring(L_, "send/receive");                                                   // _K: KeysDB key                                     L_: out key keyout fifo restrict
+            break;
+        }
+        STACK_CHECK(L_, 5);
+        lua_setfield(L_, -3, "restrict");                                                          // _K: KeysDB key                                     L_: out key keyout fifo
         // keyout.fifo
         lua_setfield(L_, -2, "fifo");                                                              // _K: KeysDB key                                     L_: out key keyout
         // out[key] = keyout
diff --git a/src/keeper.hpp b/src/keeper.hpp
index b77f1a9..e2ad445 100644
--- a/src/keeper.hpp
+++ b/src/keeper.hpp
@@ -13,6 +13,15 @@ DECLARE_UNIQUE_TYPE(KeeperIndex, int);
 
 // #################################################################################################
 
+enum class LindaRestrict
+{
+    None,
+    SetGet,
+    SendReceive
+};
+
+// #################################################################################################
+
 struct Keeper
 {
     std::mutex mutex;
@@ -79,6 +88,9 @@ DECLARE_UNIQUE_TYPE(KeeperCallResult, std::optional<int>);
 // xxh64 of string "kNilSentinel" generated at https://www.pelock.com/products/hash-calculator
 static constexpr UniqueKey kNilSentinel{ 0xC457D4EDDB05B5E4ull, "lanes.null" };
 
+// xxh64 of string "kRestrictedChannel" generated at https://www.pelock.com/products/hash-calculator
+static constexpr UniqueKey kRestrictedChannel{ 0x4C8B879ECDE110F7ull };
+
 using keeper_api_t = lua_CFunction;
 #define KEEPER_API(_op) keepercall_##_op
 
@@ -96,6 +108,8 @@ int keepercall_receive(lua_State* L_);
 [[nodiscard]]
 int keepercall_receive_batched(lua_State* L_);
 [[nodiscard]]
+int keepercall_restrict(lua_State* L_);
+[[nodiscard]]
 int keepercall_send(lua_State* L_);
 [[nodiscard]]
 int keepercall_set(lua_State* L_);
diff --git a/src/lanes.lua b/src/lanes.lua
index dfa959d..8d8f25d 100644
--- a/src/lanes.lua
+++ b/src/lanes.lua
@@ -448,6 +448,7 @@ local configure_timers = function()
 
     -- Timer lane; initialize only on the first 'require "lanes"' instance (which naturally has 'table' always declared)
     local first_time_key = "first time"
+    timerLinda:restrict(first_time_key, "set/get")
     local _, _first_time_val = timerLinda:get(first_time_key)
     local first_time = (_first_time_val == nil)
     timerLinda:set(first_time_key, true)
diff --git a/src/linda.cpp b/src/linda.cpp
index 43d2a91..6ebbc1f 100644
--- a/src/linda.cpp
+++ b/src/linda.cpp
@@ -57,7 +57,7 @@ namespace {
 
             case LuaType::USERDATA:
                 if (!DeepFactory::IsDeepUserdata(L_, _i)) {
-                    raise_luaL_error(L_, "argument #%d: can't use non-deep userdata as a key", _i);
+                    raise_luaL_error(L_, "argument #%d: can't use non-deep userdata as a slot", _i);
                 }
                 break;
 
@@ -66,7 +66,7 @@ namespace {
                     static constexpr std::array<std::reference_wrapper<UniqueKey const>, 3> kKeysToCheck{ kLindaBatched, kCancelError, kNilSentinel };
                     for (UniqueKey const& _key : kKeysToCheck) {
                         if (_key.equals(L_, _i)) {
-                            raise_luaL_error(L_, "argument #%d: can't use %s as a key", _i, _key.debugName.data());
+                            raise_luaL_error(L_, "argument #%d: can't use %s as a slot", _i, _key.debugName.data());
                             break;
                         }
                     }
@@ -74,7 +74,7 @@ namespace {
                 break;
 
             default:
-                raise_luaL_error(L_, "argument #%d: invalid key type (not a boolean, string, number or light userdata)", _i);
+                raise_luaL_error(L_, "argument #%d: invalid slot type (not a boolean, string, number or light userdata)", _i);
             }
         }
         STACK_CHECK(L_, 0);
@@ -416,7 +416,7 @@ static LUAG_FUNC(linda_index)
 // #################################################################################################
 
 /*
- * [val] = linda_count( linda_ud, [key [, ...]])
+ * [val] = linda_count( linda_ud, [slot [, ...]])
  *
  * Get a count of the pending elements in the specified keys
  */
@@ -430,7 +430,7 @@ LUAG_FUNC(linda_count)
 
             Keeper* const _keeper{ _linda->whichKeeper() };
             KeeperCallResult const _pushed{ keeper_call(_keeper->K, KEEPER_API(count), L_, _linda, StackIndex{ 2 }) };
-            return OptionalValue(_pushed, L_, "tried to count an invalid key");
+            return OptionalValue(_pushed, L_, "Tried to count an invalid slot");
         }
     };
     return Linda::ProtectedCall(L_, _count);
@@ -487,13 +487,16 @@ LUAG_FUNC(linda_get)
             lua_Integer const _count{ luaL_optinteger(L_, 3, 1) };
             luaL_argcheck(L_, _count >= 1, 3, "count should be >= 1");
             luaL_argcheck(L_, lua_gettop(L_) <= 3, 4, "too many arguments");
-            // make sure the key is of a valid type (throws an error if not the case)
+            // make sure the slot is of a valid type (throws an error if not the case)
             CheckKeyTypes(L_, StackIndex{ 2 }, StackIndex{ 2 });
 
             KeeperCallResult _pushed;
             if (_linda->cancelStatus == Linda::Active) {
                 Keeper* const _keeper{ _linda->whichKeeper() };
                 _pushed = keeper_call(_keeper->K, KEEPER_API(get), L_, _linda, StackIndex{ 2 });
+                if (_pushed.has_value() && kRestrictedChannel.equals(L_, kIdxTop)) {
+                    raise_luaL_error(L_, "Key is restricted");
+                }
             } else { // linda is cancelled
                 // do nothing and return nil,lanes.cancel_error
                 lua_pushnil(L_);
@@ -511,7 +514,7 @@ LUAG_FUNC(linda_get)
 
 /*
  * [bool]|nil,cancel_error = linda:limit(key_num|str|bool|lightuserdata, [int])
- * "unlimited"|number = linda:limit(key)
+ * "unlimited"|number = linda:limit(slot)
  *
  * Read or set limit to 1 Linda keys.
  * Optionally wake threads waiting to write on the linda, in case the limit enables them to do so
@@ -522,7 +525,7 @@ LUAG_FUNC(linda_limit)
     static constexpr lua_CFunction _limit{
         +[](lua_State* const L_) {
             Linda* const _linda{ ToLinda<false>(L_, StackIndex{ 1 }) };
-            // make sure we got 2 or 3 arguments: the linda, a key and optionally a limit
+            // make sure we got 2 or 3 arguments: the linda, a slot and optionally a limit
             int const _nargs{ lua_gettop(L_) };
             luaL_argcheck(L_, _nargs == 2 || _nargs == 3, 2, "wrong number of arguments");
             // make sure we got a numeric limit, or "unlimited", (or nothing)
@@ -531,7 +534,7 @@ LUAG_FUNC(linda_limit)
             if (_val < 0) {
                 raise_luaL_argerror(L_, StackIndex{ 3 }, "limit must be >= 0");
             }
-            // make sure the key is of a valid type
+            // make sure the slot is of a valid type
             CheckKeyTypes(L_, StackIndex{ 2 }, StackIndex{ 2 });
 
             KeeperCallResult _pushed;
@@ -539,8 +542,8 @@ LUAG_FUNC(linda_limit)
                 if (_unlimited) {
                     LUA_ASSERT(L_, lua_gettop(L_) == 3 && luaG_tostring(L_, StackIndex{ 3 }) == "unlimited");
                     // inside the Keeper, unlimited is signified with a -1 limit (can't use nil because of nil kNilSentinel conversions!)
-                    lua_pop(L_, 1);                                                                // L_: linda key
-                    lua_pushinteger(L_, -1);                                                       // L_: linda key nil
+                    lua_pop(L_, 1);                                                                // L_: linda slot
+                    lua_pushinteger(L_, -1);                                                       // L_: linda slot nil
                 }
                 Keeper* const _keeper{ _linda->whichKeeper() };
                 _pushed = keeper_call(_keeper->K, KEEPER_API(limit), L_, _linda, StackIndex{ 2 });
@@ -572,12 +575,12 @@ LUAG_FUNC(linda_limit)
 
 /*
  * 2 modes of operation
- * [val, key]= linda:receive([timeout_secs_num=nil], key_num|str|bool|lightuserdata [, ...] )
- * Consumes a single value from the Linda, in any key.
- * Returns: received value (which is consumed from the slot), and the key which had it
+ * [val, slot]= linda:receive([timeout_secs_num=nil], key_num|str|bool|lightuserdata [, ...] )
+ * Consumes a single value from the Linda, in any slot.
+ * Returns: received value (which is consumed from the slot), and the slot which had it
 
  * [val1, ... valCOUNT]= linda_receive( linda_ud, [timeout_secs_num=-1], linda.batched, key_num|str|bool|lightuserdata, min_COUNT[, max_COUNT])
- * Consumes between min_COUNT and max_COUNT values from the linda, from a single key.
+ * Consumes between min_COUNT and max_COUNT values from the linda, from a single slot.
  * returns the actual consumed values, or nil if there weren't enough values to consume
  */
 LUAG_FUNC(linda_receive)
@@ -585,7 +588,7 @@ LUAG_FUNC(linda_receive)
     static constexpr lua_CFunction _receive{
         +[](lua_State* const L_) {
             Linda* const _linda{ ToLinda<false>(L_, StackIndex{ 1 }) };
-            StackIndex _key_i{ 2 }; // index of first key, if timeout not there
+            StackIndex _key_i{ 2 }; // index of first slot, if timeout not there
 
             std::chrono::time_point<std::chrono::steady_clock> _until{ std::chrono::time_point<std::chrono::steady_clock>::max() };
             if (luaG_type(L_, StackIndex{ 2 }) == LuaType::NUMBER) { // we don't want to use lua_isnumber() because of autocoercion
@@ -596,7 +599,7 @@ LUAG_FUNC(linda_receive)
                     raise_luaL_argerror(L_, StackIndex{ 2 }, "duration cannot be < 0");
                 }
                 ++_key_i;
-            } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil before the key
+            } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil before the slot
                 ++_key_i;
             }
 
@@ -616,7 +619,7 @@ LUAG_FUNC(linda_receive)
                     raise_luaL_argerror(L_, StackIndex{ _key_i + 1 }, "bad min count");
                 }
                 _expected_pushed_max = (int) luaL_optinteger(L_, _key_i + 2, _expected_pushed_min);
-                // don't forget to count the key in addition to the values
+                // don't forget to count the slot in addition to the values
                 ++_expected_pushed_min;
                 ++_expected_pushed_max;
                 if (_expected_pushed_min > _expected_pushed_max) {
@@ -627,7 +630,7 @@ LUAG_FUNC(linda_receive)
                 CheckKeyTypes(L_, _key_i, StackIndex{ lua_gettop(L_) });
                 // receive a single value, checking multiple slots
                 _selected_keeper_receive = KEEPER_API(receive);
-                // we expect a single (value, key) pair of returned values
+                // we expect a single (value, slot) pair of returned values
                 _expected_pushed_min = _expected_pushed_max = 2;
             }
 
@@ -660,6 +663,9 @@ LUAG_FUNC(linda_receive)
                 }
                 if (_pushed.value() > 0) {
                     LUA_ASSERT(L_, _pushed.value() >= _expected_pushed_min && _pushed.value() <= _expected_pushed_max);
+                    if (kRestrictedChannel.equals(L_, StackIndex{ kIdxTop })) {
+                        raise_luaL_error(L_, "Key is restricted");
+                    }
                     _linda->readHappened.notify_all();
                     break;
                 }
@@ -730,6 +736,49 @@ LUAG_FUNC(linda_receive)
 // #################################################################################################
 
 /*
+ * "string" = linda:restrict(key_num|str|bool|lightuserdata, [string])
+ * "string" = linda:restrict(slot)
+ *
+ * Read or set restrict mode to 1 Linda slot.
+ */
+LUAG_FUNC(linda_restrict)
+{
+    static constexpr lua_CFunction _rstrct{
+        +[](lua_State* const L_) {
+            Linda* const _linda{ ToLinda<false>(L_, StackIndex{ 1 }) };
+            // make sure we got 2 or 3 arguments: the linda, a slot and optionally a restrict mode
+            int const _nargs{ lua_gettop(L_) };
+            luaL_argcheck(L_, _nargs == 2 || _nargs == 3, 2, "wrong number of arguments");
+            // make sure we got a known restrict mode, (or nothing)
+            std::string_view const _mode{ luaG_tostring(L_, StackIndex{ 3 }) };
+            if (!_mode.empty() && (_mode != "none" && _mode != "set/get" && _mode != "send/receive")) {
+                raise_luaL_argerror(L_, StackIndex{ 3 }, "unknown restrict mode");
+            }
+            // make sure the slot is of a valid type
+            CheckKeyTypes(L_, StackIndex{ 2 }, StackIndex{ 2 });
+
+            KeeperCallResult _pushed;
+            if (_linda->cancelStatus == Linda::Active) {
+                Keeper* const _keeper{ _linda->whichKeeper() };
+                _pushed = keeper_call(_keeper->K, KEEPER_API(restrict), L_, _linda, StackIndex{ 2 });
+                // we should get a single return value: the string describing the previous restrict mode
+                LUA_ASSERT(L_, _pushed.has_value() && (_pushed.value() == 1) && luaG_type(L_, kIdxTop) == LuaType::STRING);
+            } else { // linda is cancelled
+                // do nothing and return nil,lanes.cancel_error
+                lua_pushnil(L_);
+                kCancelError.pushKey(L_);
+                _pushed.emplace(2);
+            }
+            // propagate returned values
+            return _pushed.value();
+        }
+    };
+    return Linda::ProtectedCall(L_, _rstrct);
+}
+
+// #################################################################################################
+
+/*
  * bool= linda:linda_send([timeout_secs=nil,] key_num|str|bool|lightuserdata, ...)
  *
  * Send one or more values to a Linda. If there is a limit, all values must fit.
@@ -743,7 +792,7 @@ LUAG_FUNC(linda_send)
     static constexpr lua_CFunction _send{
         +[](lua_State* const L_) {
             Linda* const _linda{ ToLinda<false>(L_, StackIndex{ 1 }) };
-            StackIndex _key_i{ 2 }; // index of first key, if timeout not there
+            StackIndex _key_i{ 2 }; // index of first slot, if timeout not there
 
             std::chrono::time_point<std::chrono::steady_clock> _until{ std::chrono::time_point<std::chrono::steady_clock>::max() };
             if (luaG_type(L_, StackIndex{ 2 }) == LuaType::NUMBER) { // we don't want to use lua_isnumber() because of autocoercion
@@ -754,11 +803,11 @@ LUAG_FUNC(linda_send)
                     raise_luaL_argerror(L_, StackIndex{ 2 }, "duration cannot be < 0");
                 }
                 ++_key_i;
-            } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil before the key
+            } else if (lua_isnil(L_, 2)) { // alternate explicit "infinite timeout" by passing nil before the slot
                 ++_key_i;
             }
 
-            // make sure the key is of a valid type
+            // make sure the slot is of a valid type
             CheckKeyTypes(L_, _key_i, _key_i);
 
             STACK_GROW(L_, 1);
@@ -799,6 +848,9 @@ LUAG_FUNC(linda_send)
                     }
                     LUA_ASSERT(L_, _pushed.value() == 1);
 
+                    if (kRestrictedChannel.equals(L_, StackIndex{ kIdxTop })) {
+                        raise_luaL_error(L_, "Key is restricted");
+                    }
                     _ret = lua_toboolean(L_, -1) ? true : false;
                     lua_pop(L_, 1);
 
@@ -884,7 +936,7 @@ LUAG_FUNC(linda_set)
         +[](lua_State* const L_) {
             Linda* const _linda{ ToLinda<false>(L_, StackIndex{ 1 }) };
             bool const _has_data{ lua_gettop(L_) > 2 };
-            // make sure the key is of a valid type (throws an error if not the case)
+            // make sure the slot is of a valid type (throws an error if not the case)
             CheckKeyTypes(L_, StackIndex{ 2 }, StackIndex{ 2 });
 
             KeeperCallResult _pushed;
@@ -892,6 +944,9 @@ LUAG_FUNC(linda_set)
                 Keeper* const _keeper{ _linda->whichKeeper() };
                 _pushed = keeper_call(_keeper->K, KEEPER_API(set), L_, _linda, StackIndex{ 2 });
                 if (_pushed.has_value()) { // no error?
+                    if (kRestrictedChannel.equals(L_, kIdxTop)) {
+                        raise_luaL_error(L_, "Key is restricted");
+                    }
                     LUA_ASSERT(L_, _pushed.value() == 2 && luaG_type(L_, kIdxTop) == LuaType::STRING && luaG_type(L_, StackIndex{ -2 }) == LuaType::BOOLEAN);
 
                     if (_has_data) {
@@ -899,7 +954,7 @@ LUAG_FUNC(linda_set)
                         _linda->writeHappened.notify_all(); // To be done from within the 'K' locking area
                     }
                     if (lua_toboolean(L_, -2)) {
-                        // the key was full, but it is no longer the case, tell writers they should wake
+                        // the slot was full, but it is no longer the case, tell writers they should wake
                         _linda->readHappened.notify_all(); // To be done from within the 'K' locking area
                     }
                 }
@@ -992,6 +1047,7 @@ namespace {
             { "get", LG_linda_get },
             { "limit", LG_linda_limit },
             { "receive", LG_linda_receive },
+            { "restrict", LG_linda_restrict },
             { "send", LG_linda_send },
             { "set", LG_linda_set },
             { "wake", LG_linda_wake },