Add notifly::exchange, fix post_and_wait's double-answer bug - #12
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post_and_wait() covers one request/reply shape: post one notification, wait
for one reply, take the first that arrives. Real protocols keep producing
shapes it cannot express, so callers hand-roll subscribe/post/wait/unsubscribe
around add_observer() and have to get the ordering right themselves.
notifly::exchange is that pattern as a scoped object. Handlers return a
notifly_verdict per delivery -- skip, keep or done -- which is what lets one
primitive cover the rest:
- skip: ignore a delivery and stay subscribed, for a sender that reports the
state it is leaving before the one it is entering. Taking the first reply
there reports the wrong state.
- several on() calls: wait on alternatives at once; wait() returns which
notification answered.
- keep + drain(quiet, deadline): a reply streamed in pieces whose length the
protocol never states.
- silent_for(window): protocols where the sender only speaks up to refuse, so
silence is the successful outcome.
- subscribe and wait() without posting: waiting on an external event rather
than on a command.
post_and_wait() is now a wrapper over it, unchanged in signature and behaviour
except that it no longer throws when a sender answers twice: the response
observer called promise->set_value() unguarded, and the observer was only
removed after the wait returned, so a second delivery in that window raised
std::future_error out of the dispatch loop -- which runs with the centre
locked and no try/catch, so it surfaced on the poster's thread. An exchange
ignores deliveries after the first done, so the guard now applies everywhere.
The private is_tuple trait moves to notifly_detail so exchange::capture() can
use it; add_response_observer() goes away with its only caller.
Tests cover each shape plus lifetime, subscription errors and the
double-answer regression (16 new, 54 total green). example/exchange_example.cpp
walks a simulated device through all five shapes.
capture() is what post_and_wait() is built on, and it's public on its own -- useful shorthand for a branch of a multi-alternative exchange that just needs the payload, no handler. It had no test that exercised it directly (only indirectly, through post_and_wait()'s own tests) and no mention outside the doxygen comment on the declaration. Add both. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Summary
Adds
notifly::exchange, the subscribe/wait primitivepost_and_wait()is now built on top of,and fixes a real bug in
post_and_wait()along the way: a second answer from a sender that repliestwice used to call
promise->set_value()twice, throwingstd::future_error.exchangeholds itsmutex across the "first done wins" decision, so that can't happen.
post_and_wait()covers one shape: post one notification, wait for one reply, take the first thatarrives.
exchangecovers what that shape can't express — several possible answers where which onefired matters, ignoring a delivery that isn't the one being waited for, a reply streamed in pieces of
unstated length, treating silence as success, or waiting on something no command asked for. See the
new "Waiting For A Reply" section in the README for the full shape table and
capture(), theconvenience
on()wraps for a plain value or tuple readback.Tests
56 tests (30 existing
notifly+ 8NotiflyMemoryLeak, both untouched, + 16 newexchangetests +2 for
capture()added while reviewing this before tagging).example/exchange_example.cppdrives asimulated device through each shape in the table.
Version
NOTIFLY_VERSION_{MAJOR,MINOR,PATCH}ininclude/notifly.his already at 3.6.0. Taggingv3.6onthis merge commit.
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