Henry Robinson created KUDU-2028:
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Summary: Async Proxy callbacks cannot capture non-copyable types
Key: KUDU-2028
URL: https://issues.apache.org/jira/browse/KUDU-2028
Project: Kudu
Issue Type: Improvement
Reporter: Henry Robinson
Assignee: Henry Robinson
Priority: Minor
Callbacks that are passed to {{Proxy::AsyncRequest()}} are passed by reference.
They are then passed by value into {{OutboundCall()}}, where that copy is saved.
This means that you can't have a callback that captures a non-copyable type:
{code}
unique_ptr<Foo> my_foo = make_unique<Foo>();
auto cb = [f(move(my_foo))]() {
};
proxy->AsyncRequest("method", req, response, controller, cb); // <-- error,
missing copy c'tor for unique_ptr<Foo>
{code}
This is a sometimes useful idiom for passing ownership of something into the
callback, where it can get destroyed some time in the future, after the caller
has ceased to exist.
A solution would be for {{AsyncRequest()}} (and its generated wrappers) to take
a {{ResponseCallback}} by value, and to {{move()}} it until it was copied. This
kind of transformation has its drawbacks (if for some reason the parameter
doesn't get copied, it is more expensive than pass-by-reference), but seems a
reasonable tradeoff here.
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