quote from Avi's comment
> It's supposed to be illegal to call handle(...) without this->,
> because handle() is a dependent name (but many compilers don't
> insist, gcc is stricter here). So two error messages competed,
> and "unused this capture" won.
without this change, Clang complains that `this` is not used with
`-Wunused-lambda-capture`.
in this change, `this` is used. in this change, `this` is explicitly
referenced to silence Clang's warning.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
The `result_try` and `result_futurize_try` are supposed to handle both
failed results and exceptions in a way similar to a try..catch block.
In order to catch exceptions, the metaprogramming machinery invokes the
fallible code inside a stack of try..catch blocks, each one of them
handling one exception. This is done instead of creating a single
try..catch block, as to my knowledge it is not possible to create
a try..catch block with the number of "catch" clauses depending on a
variadic template parameter pack.
Unfortunately, a "try" with multiple "catches" is not functionally
equivalent to a "try block stack". Consider the following code:
try {
try {
return execute_try_block();
} catch (const derived_exception&) {
// 1
}
} catch (const base_exception&) {
// 2
}
If `execute_try_block` throws `derived_exception` and the (1) catch
handler rethrows this exception, it will also be handled in (2), which
is not the same behavior as if the try..catch stack was "flat".
This causes wrong behavior in `result_try` and `result_futurize_try`.
The following snippet has the same, wrong behavior as the previous one:
return utils::result_try([&] {
return execute_try_block();
}, utils::result_catch<derived_exception>([&] (const auto&& ex) {
// 1
}), utils::result_catch<base_exception>([&] (const auto&& ex) {
// 2
});
This commit fixes the problem by adding a boolean flag which is set just
before a catch handler is executed. If another catch handler is
accidentally matched due to exception rethrow, the catch handler is
skipped and exception is automatically rethrown.
Tests: unit(dev, debug)
Fixes: #10211Closes#10216
Adds two methods to result_try's exception handles:
- as_inner: returns a {l,r}-value reference either to the exception
container, or the exception_ptr. This allows to use them in operations
which work on both types, e.g. logging.
- clone_inner: returns a copy of the underlying exception container or
exception ptr.
Currently, the catch handlers in result_futurize_try are required to
return a future, although they are always being called with
seastar::futurize_invoke, so if their result is not future it could be
converted to one anyway. This commit relaxes the ConvertsWithTo
constraint in order to allow this conversion.
Adds result_try and result_futurize_try - functions which allow to
convert existing try..catch blocks into a version which handles C++
exceptions, failed results with exception containers and, depending on
the function variant, exceptional futures.
