receive_signal() uses the unordered map _signal_handlers (signo mapped to
signal_handler) to either register a signal or find an existing one, and
from there get a future from the promise associated with that signal.
The problem is _signal_handlers.emplace() being called unconditionally,
resulting in the constructor from signal_handler always being called to
needlessly re-register the same handler, even when the signo is already
inserted in the map.
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
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