libutil: de-callback-ify computeClosure

only two users of this function exist. only one used it in a way that
even bears resemblance to asynchronicity, and even that one didn't do
it right. fully async and parallel computation would have only worked
if any getEdgesAsync never calls the continuation it receives itself,
only from more derived callbacks running on other threads. calling it
directly would cause the decoupling promise to be awaited immediately
*on the original thread*, completely negating all nice async effects.

Change-Id: I0aa640950cf327533a32dee410105efdabb448df

1 files changed, 14 insertions, 54 deletions

diff --git a/src/libutil/closure.hh b/src/libutil/closure.hh
index 16e3b93e4..146931fc8 100644
--- a/src/libutil/closure.hh
+++ b/src/libutil/closure.hh
@@ -1,72 +1,32 @@
 #pragma once
 ///@file
 
+#include <functional>
 #include <set>
-#include <future>
-#include "sync.hh"
-
-using std::set;
 
 namespace nix {
 
 template<typename T>
-using GetEdgesAsync = std::function<void(const T &, std::function<void(std::promise<set<T>> &)>)>;
-
-template<typename T>
-void computeClosure(
-    const set<T> startElts,
-    set<T> & res,
-    GetEdgesAsync<T> getEdgesAsync
+std::set<T> computeClosure(
+    std::set<T> startElts,
+    std::function<std::set<T>(const T &)> getEdges
 )
 {
-    struct State
-    {
-        size_t pending;
-        set<T> & res;
-        std::exception_ptr exc;
-    };
-
-    Sync<State> state_(State{0, res, 0});
+    std::set<T> res, queue = std::move(startElts);
 
-    std::function<void(const T &)> enqueue;
+    while (!queue.empty()) {
+        std::set<T> next;
 
-    std::condition_variable done;
-
-    enqueue = [&](const T & current) -> void {
-        {
-            auto state(state_.lock());
-            if (state->exc) return;
-            if (!state->res.insert(current).second) return;
-            state->pending++;
+        for (auto & e : queue) {
+            if (res.insert(e).second) {
+                next.merge(getEdges(e));
+            }
         }
 
-        getEdgesAsync(current, [&](std::promise<set<T>> & prom) {
-            try {
-                auto children = prom.get_future().get();
-                for (auto & child : children)
-                    enqueue(child);
-                {
-                    auto state(state_.lock());
-                    assert(state->pending);
-                    if (!--state->pending) done.notify_one();
-                }
-            } catch (...) {
-                auto state(state_.lock());
-                if (!state->exc) state->exc = std::current_exception();
-                assert(state->pending);
-                if (!--state->pending) done.notify_one();
-            };
-        });
-    };
-
-    for (auto & startElt : startElts)
-        enqueue(startElt);
-
-    {
-        auto state(state_.lock());
-        while (state->pending) state.wait(done);
-        if (state->exc) std::rethrow_exception(state->exc);
+        queue = std::move(next);
     }
+
+    return res;
 }
 
 }