libutil: remove callback.hh

it's no longer used. it really shouldn't have existed this long since it
was just a mashup of both std::promise and std::packaged_task in a shape
that makes composition unnecessarily difficult. all but a single case of
Callback pattern calls were fully synchronous anyway, and even this sole
outlier was by far not important enough to justify the extra complexity.

Change-Id: I208aec4572bf2501cdbd0f331f27d505fca3a62f

3 files changed, 0 insertions, 54 deletions

diff --git a/src/libutil/callback.hh b/src/libutil/callback.hh
deleted file mode 100644
index 3710d1239..000000000
--- a/src/libutil/callback.hh
+++ /dev/null
@@ -1,49 +0,0 @@
-#pragma once
-///@file
-
-#include <future>
-#include <functional>
-
-namespace nix {
-
-/**
- * A callback is a wrapper around a lambda that accepts a valid of
- * type T or an exception. (We abuse std::future<T> to pass the value or
- * exception.)
- */
-template<typename T>
-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) { }
-
-    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) noexcept
-    {
-        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()) noexcept
-    {
-        auto prev = done.test_and_set();
-        assert(!prev);
-        std::promise<T> promise;
-        promise.set_exception(exc);
-        fun(promise.get_future());
-    }
-};
-
-}
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 8caa0532a..13266f6bd 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -43,7 +43,6 @@ libutil_headers = files(
   'args/root.hh',
   'args.hh',
   'box_ptr.hh',
-  'callback.hh',
   'canon-path.hh',
   'cgroup.hh',
   'chunked-vector.hh',
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index ac4aa1d3a..5adabae28 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -836,10 +836,6 @@ std::optional<typename T::value_type> pop(T & c)
 }
 
 
-template<typename T>
-class Callback;
-
-
 /**
  * A RAII helper that increments a counter on construction and
  * decrements it on destruction.