The exception_container is supposed to be a cheaper, but possibly harder
to use alternative to std::exception_ptr. Before this commit, the
exception was kept behind foreign_ptr<std::unique_ptr<>> so that moving
the container is very cheap. However, the original std::exception_ptr
supports copying in a thread-safe manner, and it turns out that some of
the read coordinator logic intentionally copies the pointer in order to
be able to fail two different promises with the same exception.
The pointer type is changed to std::shared_ptr. Although it uses atomics
for reference counting, this is also probably what std::exception_ptr
does, so the performance should not be worse. The exception stored
inside the container is immutable, so this allows for a non-throwing
implementation of copying.
To encourage moves instead of copying, the copy constructor is deleted
and instead the `clone()` method should be used if it is really
necessary.
This commit changes the behavior of `exception_container::accept`. Now,
instead of throwing an `utils::bad_exception_container_access` exception
when the container is empty, the provided visitor is invoked with that
exception instead. There are two reasons for this change:
- The exception_container is supposed to allow handling exceptions
without using the costly C++'s exception runtime. Although an empty
container is an edge case, I think it the new behavior is more aligned
with the class' purpose. The old behavior can be simulated by
providing a visitor which throws when called with bad access
exception.
- The new behavior fixes a bug in `result_try`/`result_futurize_try`.
Before the change, if the `try` block returned a failed result with an
empty exception container, a bad access exception would either be
thrown or returned as an exceptional future without being handled by
the `catch` clauses. Although nobody is supposed to return such
result<>s on purpose, a moved out result can be returned by accident
and it's important for the exception handling logic to be correct in
such a situation.
Tests: unit(dev)
Closes#10086
Adds `exception_container` - a helper type used to hold exceptions as a
value, without involving the std::exception_ptr.
The motivation behind this type is that it allows inspecting exception's
type and value without having to rethrow that exception and catch it,
unlike std::exception_ptr. In our current codebase, some exception
handling paths need to rethrow the exception multiple times in order to
account it into metrics or encode it as an error response to the CQL
client. Some types of exceptions can be thrown very frequently in case
of overload (e.g. timeouts) and inspecting those exceptions with
rethrows can make the overload even worse. For those kinds of exceptions
it is important to handle them as cheaply as possible, and
exception_container used with conjunction with boost::outcome::result
can help achieve that.
