libstore: turn Goal::WaitForGoals into a promise

also gets rid of explicit strong references to dependencies of any goal,
and weak references to dependers as well. those are now only held within
promises representing goal completion and thus independent of the goal's
relation to each other. the weak references to dependers was only needed
for notifications, and that's much better handled entirely by kj itself.

Change-Id: I00d06df9090f8d6336ee4bb0c1313a7052fb016b

1 files changed, 6 insertions, 4 deletions

diff --git a/src/libstore/build/substitution-goal.cc b/src/libstore/build/substitution-goal.cc
index 6d90196fa..74a63ca21 100644
--- a/src/libstore/build/substitution-goal.cc
+++ b/src/libstore/build/substitution-goal.cc
@@ -3,6 +3,8 @@
 #include "nar-info.hh"
 #include "signals.hh"
 #include "finally.hh"
+#include <kj/array.h>
+#include <kj/vector.h>
 
 namespace nix {
 
@@ -160,16 +162,16 @@ try {
 
     /* To maintain the closure invariant, we first have to realise the
        paths referenced by this one. */
-    WaitForGoals result;
+    kj::Vector<std::pair<GoalPtr, kj::Promise<void>>> dependencies;
     for (auto & i : info->references)
         if (i != storePath) /* ignore self-references */
-            result.goals.insert(worker.goalFactory().makePathSubstitutionGoal(i));
+            dependencies.add(worker.goalFactory().makePathSubstitutionGoal(i));
 
-    if (result.goals.empty()) {/* to prevent hang (no wake-up event) */
+    if (dependencies.empty()) {/* to prevent hang (no wake-up event) */
         return referencesValid(inBuildSlot);
     } else {
         state = &PathSubstitutionGoal::referencesValid;
-        return {std::move(result)};
+        return waitForGoals(dependencies.releaseAsArray());
     }
 } catch (...) {
     return {std::current_exception()};