From 7348653ff4fc4e9b2dc24943aabdb57179b1c75a Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 3 Sep 2019 12:51:35 +0200
Subject: Ensure that Callback is called only once

Also, make Callback movable but uncopyable.
---
 src/libutil/util.hh | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

(limited to 'src/libutil')

diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index b538a0b41..686e81d3f 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -445,21 +445,34 @@ string get(const T & map, const string & key, const string & def = "")
    type T or an exception. (We abuse std::future<T> to pass the value or
    exception.) */
 template<typename T>
-struct Callback
+class Callback
 {
     std::function<void(std::future<T>)> fun;
+    std::atomic_flag done = ATOMIC_FLAG_INIT;
+
+public:
 
     Callback(std::function<void(std::future<T>)> fun) : fun(fun) { }
 
-    void operator()(T && t) const
+    Callback(Callback && callback) : fun(std::move(callback.fun))
+    {
+        auto prev = callback.done.test_and_set();
+        if (prev) done.test_and_set();
+    }
+
+    void operator()(T && t)
     {
+        auto prev = done.test_and_set();
+        assert(!prev);
         std::promise<T> promise;
         promise.set_value(std::move(t));
         fun(promise.get_future());
     }
 
-    void rethrow(const std::exception_ptr & exc = std::current_exception()) const
+    void rethrow(const std::exception_ptr & exc = std::current_exception())
     {
+        auto prev = done.test_and_set();
+        assert(!prev);
         std::promise<T> promise;
         promise.set_exception(exc);
         fun(promise.get_future());
-- 
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