[COMMIT seastar master] program_options: allow configure switch-stytle option programmatically

[Commit Bot]

From: Kefu Chai <tcha...@gmail.com>
Committer: Avi Kivity <a...@scylladb.com>
Branch: master

program_options: allow configure switch-stytle option programmatically

before this change, a `program_options::value<>` cannot be configured
programmatically. if we configure a switch option and then mutate an
`program_options::option_group` with this option, the switch option is
always reset if the mutator does not contain that option even we
set that option with a default value beforehand.

after this change, `program_options::value<>` tracks its value with an
`optional<bool>` like the other specialization `program_options::value<T>`.
so its value is overriden only if it is not preset. this allows us to
configure switch-style options programmatically. so its behavior is more
consistent with other `program_options::value<T>`.

Signed-off-by: Kefu Chai <tcha...@gmail.com>

---
diff --git a/include/seastar/util/program-options.hh b/include/seastar/util/program-options.hh
--- a/include/seastar/util/program-options.hh
+++ b/include/seastar/util/program-options.hh
@@ -459,14 +459,22 @@ public:
/// Contains no value, can be set or not.
template <>
class value<std::monostate> : public basic_value {
- bool _set = false;
+ std::optional<bool> _set;

private:
virtual void do_describe(options_descriptor& descriptor) const override {
descriptor.visit_value();
}
virtual void do_mutate(options_mutator& mutator) override {
- _set = mutator.visit_value();
+ bool is_set = mutator.visit_value();
+ if (_set.has_value()) {
+ // override the value only if it is not preset
+ if (is_set) {
+ _set = true;
+ }
+ } else {
+ _set = is_set;
+ }
}

public:
@@ -483,7 +491,7 @@ public:
: basic_value(group, false, std::move(name), {})
{ }
/// Is the option set?
- operator bool () const { return _set; }
+ operator bool () const { return _set ? _set.value() : false; }
void set_value() { _set = true; }
void unset_value() { _set = false; }
};