feat(signal_set): add constructors from executor

Mirror the executor-construction pattern used by tcp_socket and
tcp_acceptor: add a bare signal_set(Ex const&) constructor plus an
executor variant of the variadic signals constructor, each delegating
to the executor's context.

Add tests covering both new constructors.

Author: sgerbino

include/boost/corosio/signal_set.hpp

@@ -18,6 +18,7 @@
 
 #include <concepts>
 #include <system_error>
+#include <type_traits>
 
 /*
     Signal Set Public API
@@ -224,6 +225,36 @@ class BOOST_COROSIO_DECL signal_set : public io_signal_set
         (check(add(signals)), ...);
     }
 
+    /** Construct an empty signal set from an executor.
+
+        The signal set is associated with the executor's context.
+
+        @param ex The executor whose context will own this signal set.
+    */
+    template<class Ex>
+        requires(!std::same_as<std::remove_cvref_t<Ex>, signal_set>) &&
+        capy::Executor<Ex>
+    explicit signal_set(Ex const& ex) : signal_set(ex.context())
+    {
+    }
+
+    /** Construct a signal set with initial signals from an executor.
+
+        The signal set is associated with the executor's context.
+
+        @param ex The executor whose context will own this signal set.
+        @param signal First signal number to add.
+        @param signals Additional signal numbers to add.
+
+        @throws std::system_error Thrown on failure.
+    */
+    template<class Ex, std::convertible_to<int>... Signals>
+        requires capy::Executor<Ex>
+    signal_set(Ex const& ex, int signal, Signals... signals)
+        : signal_set(ex.context(), signal, signals...)
+    {
+    }
+
     /** Move constructor.
 
         Transfers ownership of the signal set resources.

test/unit/signal_set.cpp

@@ -67,6 +67,22 @@ struct signal_set_test
         BOOST_TEST_PASS();
     }
 
+    void testConstructFromExecutor()
+    {
+        io_context ioc(Backend);
+        signal_set s(ioc.get_executor());
+
+        BOOST_TEST_PASS();
+    }
+
+    void testConstructFromExecutorWithSignals()
+    {
+        io_context ioc(Backend);
+        signal_set s(ioc.get_executor(), SIGINT, SIGTERM);
+
+        BOOST_TEST_PASS();
+    }
+
     void testMoveConstruct()
     {
         io_context ioc(Backend);
@@ -795,6 +811,8 @@ struct signal_set_test
         testConstructWithOneSignal();
         testConstructWithTwoSignals();
         testConstructWithThreeSignals();
+        testConstructFromExecutor();
+        testConstructFromExecutorWithSignals();
         testMoveConstruct();
         testMoveAssign();
         testMoveAssignCrossContext();