return thread-id of thread hosting the fiber (suspened)

example/fiber/Jamfile.v2

@@ -79,6 +79,10 @@ exe migrateable
     : migrateable.cpp
     ;
 
+exe hosting_thread
+    : hosting_thread.cpp
+    ;
+
 #exe backtrace
 #    : backtrace.cpp
 #    ;

example/fiber/hosting_thread.cpp

@@ -0,0 +1,29 @@
+
+//          Copyright Oliver Kowalke 2016.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#define UNW_LOCAL_ONLY
+
+#include <cstdlib>
+#include <iostream>
+#include <thread>
+
+#include <boost/context/fiber.hpp>
+
+namespace ctx = boost::context;
+
+int main() {
+    std::cout << "main-thread: " << std::this_thread::get_id() << std::endl;
+    ctx::fiber f{
+            [](ctx::fiber && m){
+                m = std::move( m).resume();
+                return std::move( m);
+            }};
+    std::cout << "hosting thread ID before resumption: " << f.previous_thread() << std::endl;
+    f = std::move( f).resume();
+    std::cout << "hosting thread ID after resumption: " << f.previous_thread() << std::endl;
+    std::cout << "main: done" << std::endl;
+    return EXIT_SUCCESS;
+}

include/boost/context/fiber_fcontext.hpp

@@ -19,6 +19,7 @@
 #include <functional>
 #include <memory>
 #include <ostream>
+#include <thread>
 #include <tuple>
 #include <utility>
 #include <variant>
@@ -60,11 +61,12 @@ namespace detail {
 
 struct data {
     enum flag_t {
-        flag_side_stack = 0
+        flag_side_stack = 0,
+        flag_hosting_thread
     };
 
-    flag_t                  f;
-    std::variant< bool >    v;
+    flag_t                                  f;
+    std::variant< bool, std::thread::id >   v;
 };
 
 inline
@@ -112,6 +114,8 @@ rep:
             data * d = static_cast< data * >( t.data);
             if ( data::flag_side_stack == d->f) {
                 d->v = fiber_uses_side_stack();
+            } else if ( data::flag_hosting_thread == d->f) {
+                d->v = std::thread::id{};
             }
             goto rep;
         }
@@ -316,6 +320,7 @@ public:
 
     fiber resume() && {
         BOOST_ASSERT( nullptr != fctx_);
+        auto tid = std::this_thread::get_id();
 rep:
         auto t = detail::jump_fcontext(
 #if defined(BOOST_NO_CXX14_STD_EXCHANGE)
@@ -328,6 +333,8 @@ rep:
             detail::data * d = static_cast< detail::data * >( t.data);
             if ( detail::data::flag_side_stack == d->f) {
                 d->v = detail::fiber_uses_side_stack();
+            } else if ( detail::data::flag_hosting_thread == d->f) {
+                d->v = tid;
             }
             fctx_ = t.fctx;
             goto rep;
@@ -338,6 +345,7 @@ rep:
     template< typename Fn >
     fiber resume_with( Fn && fn) && {
         BOOST_ASSERT( nullptr != fctx_);
+        auto tid = std::this_thread::get_id();
         auto p = std::make_tuple( std::forward< Fn >( fn) );
         auto t = detail::ontop_fcontext(
 #if defined(BOOST_NO_CXX14_STD_EXCHANGE)
@@ -351,6 +359,8 @@ rep:
             detail::data * d = static_cast< detail::data * >( t.data);
             if ( detail::data::flag_side_stack == d->f) {
                 d->v = detail::fiber_uses_side_stack();
+            } else if ( detail::data::flag_hosting_thread == d->f) {
+                d->v = tid;
             }
             fctx_ = t.fctx;
             t = detail::jump_fcontext(
@@ -378,6 +388,20 @@ rep:
         return ! std::get< bool >( d.v);
     }
 
+    std::thread::id previous_thread() {
+        BOOST_ASSERT( * this);
+        detail::data d = { detail::data::flag_hosting_thread };
+        auto t = detail::jump_fcontext(
+#if defined(BOOST_NO_CXX14_STD_EXCHANGE)
+                    detail::exchange( fctx_, nullptr),
+#else
+                    std::exchange( fctx_, nullptr),
+#endif
+                    & d);
+        fctx_ = t.fctx;
+        return std::get< std::thread::id >( d.v);
+    }
+
     explicit operator bool() const noexcept {
         return nullptr != fctx_;
     }

test/test_fiber.cpp

@@ -452,6 +452,19 @@ void test_use_system_stack() {
     }
 }
 
+void test_hosting_thread() {
+    ctx::fiber f{
+            [](ctx::fiber && m){
+                m = std::move( m).resume();
+                return std::move( m);
+            }};
+    BOOST_CHECK( f);
+    BOOST_CHECK_EQUAL( std::thread::id{}, f.previous_thread() );
+    f = std::move( f).resume();
+    BOOST_CHECK_EQUAL( std::this_thread::get_id(), f.previous_thread() );
+
+}
+
 #ifdef BOOST_WINDOWS
 void test_bug12215() {
         ctx::fiber{
@@ -480,6 +493,7 @@ boost::unit_test::test_suite * init_unit_test_suite( int, char* [])
     test->add( BOOST_TEST_CASE( & test_sscanf) );
     test->add( BOOST_TEST_CASE( & test_snprintf) );
     test->add( BOOST_TEST_CASE( & test_use_system_stack) );
+    test->add( BOOST_TEST_CASE( & test_hosting_thread) );
 #ifdef BOOST_WINDOWS
     test->add( BOOST_TEST_CASE( & test_bug12215) );
 #endif