2.32. Locks

There are several locking mechanisms available in Daslang. They are designed to ensure runtime safety of the code.

2.32.1. Context locks

Context can be locked and unlocked via lock and unlock functions from the C++ side. When locked Context can not be restarted. tryRestartAndLock restarts context if its not locked, and then locks it regardless. The main reason to lock context is when data on the heap is accessed externally. Heap collection is safe to do on a locked context.

2.32.2. Array and Table locks

Array or Table can be locked and unlocked explicitly. When locked, they can’t be modified. Calling resize, reserve, push, emplace, erase, etc on the locked Array` will cause panic. Accessing locked Table elements via [] operation would cause panic.

Arrays are locked when iterated over. This is done to prevent modification of the array while it is being iterated over. keys and values iterators lock Table as well. Tables are also locked during the find* operations.

2.32.3. Array and Table lock checking

Array and Table lock checking occurs on the data structures, which internally contain other Arrays or Tables.

Consider the following example:

var a : array < array<int> >
for b in a[0]
    a |> resize(100500)

The resize operation on the a array will cause panic because a[0] is locked during the iteration. This test, however, can only happen in runtime. The compiler generates custom resize code, which verifies locks:

def private builtin`resize ( var Arr:array<array<int> aka numT> explicit; newSize:int const )

The _builtin_verify_locks iterates over provided data, and for each Array or Table makes sure it does not lock. If its locked panic occurs.

Custom operations will only be generated, if the underlying type needs lock checks.

Here are the list of operations, which perform lock check on the data structures::
  • a <- b

  • return <- a

  • resize

  • reserve

  • push

  • push_clone

  • emplace

  • pop

  • erase

  • clear

  • insert

  • a[b] for the Table

Lock checking can be explicitly disabled
  • for the Array or the Table by using set_verify_array_locks and set_verify_table_locks functions.

  • for a structure type with the [skip_field_lock_check] structure annotation

  • for the entire function with the [skip_lock_check] function annotation

  • for the entire context with the options skip_lock_checks

  • for the entire context with the set_verify_context_locks function