diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index a99a2fc78..bc5fd968c 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -6,98 +6,73 @@
 #include "thread-pool.hh"
 #include "topo-sort.hh"
 #include "callback.hh"
+#include "closure.hh"
 
 namespace nix {
 
-
 void Store::computeFSClosure(const StorePathSet & startPaths,
     StorePathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
 {
-    struct State
-    {
-        size_t pending;
-        StorePathSet & paths;
-        std::exception_ptr exc;
-    };
+    std::function<std::set<StorePath>(const StorePath & path, std::future<ref<const ValidPathInfo>> &)> queryDeps;
+    if (flipDirection)
+        queryDeps = [&](const StorePath& path,
+                        std::future<ref<const ValidPathInfo>> & fut) {
+            StorePathSet res;
+            StorePathSet referrers;
+            queryReferrers(path, referrers);
+            for (auto& ref : referrers)
+                if (ref != path)
+                    res.insert(ref);
 
-    Sync<State> state_(State{0, paths_, 0});
+            if (includeOutputs)
+                for (auto& i : queryValidDerivers(path))
+                    res.insert(i);
 
-    std::function<void(const StorePath &)> enqueue;
+            if (includeDerivers && path.isDerivation())
+                for (auto& i : queryDerivationOutputs(path))
+                    if (isValidPath(i) && queryPathInfo(i)->deriver == path)
+                        res.insert(i);
+            return res;
+        };
+    else
+        queryDeps = [&](const StorePath& path,
+                        std::future<ref<const ValidPathInfo>> & fut) {
+            StorePathSet res;
+            auto info = fut.get();
+            for (auto& ref : info->references)
+                if (ref != path)
+                    res.insert(ref);
 
-    std::condition_variable done;
+            if (includeOutputs && path.isDerivation())
+                for (auto& i : queryDerivationOutputs(path))
+                    if (isValidPath(i))
+                        res.insert(i);
 
-    enqueue = [&](const StorePath & path) -> void {
-        {
-            auto state(state_.lock());
-            if (state->exc) return;
-            if (!state->paths.insert(path).second) return;
-            state->pending++;
-        }
+            if (includeDerivers && info->deriver && isValidPath(*info->deriver))
+                res.insert(*info->deriver);
+            return res;
+        };
 
-        queryPathInfo(path, {[&](std::future<ref<const ValidPathInfo>> fut) {
-            // FIXME: calls to isValidPath() should be async
-
-            try {
-                auto info = fut.get();
-
-                if (flipDirection) {
-
-                    StorePathSet referrers;
-                    queryReferrers(path, referrers);
-                    for (auto & ref : referrers)
-                        if (ref != path)
-                            enqueue(ref);
-
-                    if (includeOutputs)
-                        for (auto & i : queryValidDerivers(path))
-                            enqueue(i);
-
-                    if (includeDerivers && path.isDerivation())
-                        for (auto & i : queryDerivationOutputs(path))
-                            if (isValidPath(i) && queryPathInfo(i)->deriver == path)
-                                enqueue(i);
-
-                } else {
-
-                    for (auto & ref : info->references)
-                        if (ref != path)
-                            enqueue(ref);
-
-                    if (includeOutputs && path.isDerivation())
-                        for (auto & i : queryDerivationOutputs(path))
-                            if (isValidPath(i)) enqueue(i);
-
-                    if (includeDerivers && info->deriver && isValidPath(*info->deriver))
-                        enqueue(*info->deriver);
-
-                }
-
-                {
-                    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 & startPath : startPaths)
-        enqueue(startPath);
-
-    {
-        auto state(state_.lock());
-        while (state->pending) state.wait(done);
-        if (state->exc) std::rethrow_exception(state->exc);
-    }
+    computeClosure<StorePath>(
+        startPaths, paths_,
+        [&](const StorePath& path,
+            std::function<void(std::promise<std::set<StorePath>>&)>
+                processEdges) {
+            std::promise<std::set<StorePath>> promise;
+            std::function<void(std::future<ref<const ValidPathInfo>>)>
+                getDependencies =
+                    [&](std::future<ref<const ValidPathInfo>> fut) {
+                        try {
+                            promise.set_value(queryDeps(path, fut));
+                        } catch (...) {
+                            promise.set_exception(std::current_exception());
+                        }
+                    };
+            queryPathInfo(path, getDependencies);
+            processEdges(promise);
+        });
 }
 
-
 void Store::computeFSClosure(const StorePath & startPath,
     StorePathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
 {
diff --git a/src/libutil/closure.hh b/src/libutil/closure.hh
new file mode 100644
index 000000000..779b9b2d5
--- /dev/null
+++ b/src/libutil/closure.hh
@@ -0,0 +1,69 @@
+#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
+)
+{
+    struct State
+    {
+        size_t pending;
+        set<T> & res;
+        std::exception_ptr exc;
+    };
+
+    Sync<State> state_(State{0, res, 0});
+
+    std::function<void(const T &)> enqueue;
+
+    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++;
+        }
+
+        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);
+    }
+}
+
+}
diff --git a/src/libutil/tests/closure.cc b/src/libutil/tests/closure.cc
new file mode 100644
index 000000000..7597e7807
--- /dev/null
+++ b/src/libutil/tests/closure.cc
@@ -0,0 +1,70 @@
+#include "closure.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+using namespace std;
+
+map<string, set<string>> testGraph = {
+    { "A", { "B", "C", "G" } },
+    { "B", { "A" } }, // Loops back to A
+    { "C", { "F" } }, // Indirect reference
+    { "D", { "A" } }, // Not reachable, but has backreferences
+    { "E", {} }, // Just not reachable
+    { "F", {} },
+    { "G", { "G" } }, // Self reference
+};
+
+TEST(closure, correctClosure) {
+    set<string> aClosure;
+    set<string> expectedClosure = {"A", "B", "C", "F", "G"};
+    computeClosure<string>(
+        {"A"},
+        aClosure,
+        [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+            promise<set<string>> promisedNodes;
+            promisedNodes.set_value(testGraph[currentNode]);
+            processEdges(promisedNodes);
+        }
+    );
+
+    ASSERT_EQ(aClosure, expectedClosure);
+}
+
+TEST(closure, properlyHandlesDirectExceptions) {
+    struct TestExn {};
+    set<string> aClosure;
+    EXPECT_THROW(
+        computeClosure<string>(
+            {"A"},
+            aClosure,
+            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+                throw TestExn();
+            }
+        ),
+        TestExn
+    );
+}
+
+TEST(closure, properlyHandlesExceptionsInPromise) {
+    struct TestExn {};
+    set<string> aClosure;
+    EXPECT_THROW(
+        computeClosure<string>(
+            {"A"},
+            aClosure,
+            [&](const string currentNode, function<void(promise<set<string>> &)> processEdges) {
+                promise<set<string>> promise;
+                try {
+                    throw TestExn();
+                } catch (...) {
+                    promise.set_exception(std::current_exception());
+                }
+                processEdges(promise);
+            }
+        ),
+        TestExn
+    );
+}
+
+}