libstore: remove Goal::notify

Goal::work() is a fully usable promise that does not rely on the worker
to report completion conditions. as such we no longer need the `notify`
field that enabled this interplay. we do have to clear goal caches when
destroying the worker though, otherwise goal promises may (incorrectly)
keep goals alive due to strong shared pointers created by childStarted.

Change-Id: Ie607209aafec064dbdf3464fe207d70ba9ee158a

src/libstore/build/goal.cc

@@ -26,7 +26,6 @@ try {
 
     trace("done");
 
-    notify->fulfill(result);
     cleanup();
 
     co_return std::move(result);

src/libstore/build/worker.cc

@@ -48,6 +48,10 @@ Worker::~Worker()
        their destructors). */
     children.clear();
 
+    derivationGoals.clear();
+    drvOutputSubstitutionGoals.clear();
+    substitutionGoals.clear();
+
     assert(expectedSubstitutions == 0);
     assert(expectedDownloadSize == 0);
     assert(expectedNarSize == 0);
@@ -71,21 +75,21 @@ std::pair<std::shared_ptr<G>, kj::Promise<Result<Goal::WorkResult>>> Worker::mak
         auto goal = goal_weak.goal.lock();
         if (!goal) {
             goal = create();
-            goal->notify = std::move(goal_weak.fulfiller);
             goal_weak.goal = goal;
             // do not start working immediately. if we are not yet running we
             // may create dependencies as though they were toplevel goals, in
             // which case the dependencies will not report build errors. when
             // we are running we may be called for this same goal more times,
             // and then we want to modify rather than recreate when possible.
-            childStarted(goal, kj::evalLater([goal] { return goal->work(); }));
+            goal_weak.promise = kj::evalLater([goal] { return goal->work(); }).fork();
+            childStarted(goal, goal_weak.promise.addBranch());
         } else {
             if (!modify(*goal)) {
                 goal_weak = {};
                 continue;
             }
         }
-        return {goal, goal_weak.promise->addBranch()};
+        return {goal, goal_weak.promise.addBranch()};
     }
     assert(false && "could not make a goal. possible concurrent worker access");
 }
@@ -224,8 +228,6 @@ void Worker::childStarted(GoalPtr goal, kj::Promise<Result<Goal::WorkResult>> pr
         .then([this, goal](auto result) {
             if (result.has_value()) {
                 goalFinished(goal, result.assume_value());
-            } else {
-                goal->notify->fulfill(result.assume_error());
             }
         }));
 }

src/libstore/build/worker.hh

@@ -96,15 +96,7 @@ private:
     struct CachedGoal
     {
         std::weak_ptr<G> goal;
-        kj::Own<kj::ForkedPromise<Result<Goal::WorkResult>>> promise;
-        kj::Own<kj::PromiseFulfiller<Result<Goal::WorkResult>>> fulfiller;
-
-        CachedGoal()
-        {
-            auto pf = kj::newPromiseAndFulfiller<Result<Goal::WorkResult>>();
-            promise = kj::heap(pf.promise.fork());
-            fulfiller = std::move(pf.fulfiller);
-        }
+        kj::ForkedPromise<Result<Goal::WorkResult>> promise{nullptr};
     };
     /**
      * Maps used to prevent multiple instantiations of a goal for the