Extract a generic computeClosure function

Move the `closure` logic of `computeFSClosure` to its own (templated) function.

This doesn’t bring much by itself (except for the ability to properly
test the “closure” functionality independently from the rest), but it
allows reusing it (in particular for the realisations which will require
a very similar closure computation)

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
+    );
+}
+
+}