Prashant Varanasi 1505d282ac Fix Swap and CompareAndSwap for Value wrappers (#130) ...

* Regenerate code to update copyright end year to 2023

* Test behaviour of default values initialized in different ways

This adds repro tests for #126 and #129

* Fix Swap and CompareAndSwap for Value wrappers

Fixes #126, #129

All atomic types can be used without initialization, e.g., `var v
<AtomicType>`. This works fine for integer types as the initialized
value of 0 matches the default value for the user-facing type.  However,
for Value wrappers, they are initialized to `nil`, which is a value that
can't be set (triggers a panic) so the default value for the user-facing
type is forced to be stored as a different value. This leads to multiple
possible values representing the default user-facing type.

E.g., an `atomic.String` with value `""` may be represented by the
underlying atomic as either `nil`, or `""`. This causes issues when we
don't handle the `nil` value correctly, causing to panics in `Swap` and
incorrectly not swapping values in `CompareAndSwap`.

This change fixes the above issues by:
 * Requiring `pack` and `unpack` function in gen-atomicwrapper as the
   only place we weren't supplying them was for `String`, and the
   branching adds unnecessary complexity, especially with added `nil`
   handling.
 * Extending `CompareAndSwap` for `Value` wrappers to try an additional
   `CompareAndSwap(nil, <new>)` only if the original `CompareAndSwap`
   fails and the old value is the zero value.

2023-02-06 01:17:57 -08:00

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