Skip to main content

Memoization Options

The __Memoize and __MemoizeLSB attributes in Hack support experimental options that control how the function behaves with respect to an Implicit Context value.

Introduction​

The Implicit Context (IC) is an experimental feature. It is a value that can be associated with the execution of code and propagated implicitly from caller to callee in a way that is safe for concurrently running code.

In order to prevent the problem of "memoization cache poisoning" where results computed using one IC value are available to callers using a different IC value, we require that memoized functions executed with an IC value are explicit about how they should behave relative to the IC value. Broadly, functions can specify one of two choices:

  • This function's memoization cache key should incorporate the propagated IC value.
  • This function's memoization cache key should not incorporate the propagated IC value AND this function and its dependencies cannot depend on the propagated IC value.

These requirements are enforced at runtime via thrown exceptions and not statically by the typechecker. If static requirements are desired, one can make use of special contexts and capabilities specifically zoned.

Available Options​

A memoized function may specify its behavior relative to the IC by passing an option to the __Memoize (or __MemoizeLSB) attribute.

<<__Memoize(#MakeICInaccessible)>>
function get_same_random_int(): int {
  return HH\Lib\PseudoRandom\int();
}
  • #KeyedByIC indicates that the function’s memoization cache should include the IC value in the cache key.
    • Memoized functions that depend on the IC should use this option.
    • Memoized functions with the [zoned] context must use this option.
    • A function must have one of the following contexts to use this option: defaults (implicitly or explicitly), zoned, zoned_local, zoned_shallow
    • Note that the cache may also be keyed on metadata necessary for sufficiently complete migration logging and so you should not assume that just because your function isn’t currently being called with a propagated IC value that it is only called once.
  • #MakeICInaccessible indicates that the function’s memoization cache should not include the IC value in the cache key and that the function does not access the IC.
    • Any direct or indirect calls to the builtin function that fetches the IC will result in an exception.
    • Any direct or indirect calls to an uncategorized memoized function will throw.
    • A function must have one of the following contexts to use this option: defaults (implicitly or explicitly), leak_safe_shallow, leak_safe_local

Note that this syntax uses “enum class labels” as arguments. Currently, only the memoization attributes are allowed to use labels as arguments.

  • Executing code with an IC value and accessing the propagated value is done by extending and using methods of the class HH\ImplicitContext.
  • Functions HH\ImplicitContext\soft_run_with and HH\ImplicitContext\soft_run_with_async are passthrough functions that do not affect the Implicit Context state.