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, 13 insertions, 15 deletions

diff --git a/src/libstore/build/goal.hh b/src/libstore/build/goal.hh
index 1ccf9716b..e7a500a00 100644
--- a/src/libstore/build/goal.hh
+++ b/src/libstore/build/goal.hh
@@ -6,6 +6,7 @@
 #include "types.hh"
 #include "store-api.hh"
 #include "build-result.hh"
+#include <concepts> // IWYU pragma: keep
 #include <kj/async.h>
 
 namespace nix {
@@ -71,17 +72,6 @@ struct Goal
     const bool isDependency;
 
     /**
-     * Goals that this goal is waiting for.
-     */
-    Goals waitees;
-
-    /**
-     * Goals waiting for this one to finish.  Must use weak pointers
-     * here to prevent cycles.
-     */
-    WeakGoals waiters;
-
-    /**
      * Number of goals we are/were waiting for that have failed.
      */
     size_t nrFailed = 0;
@@ -113,6 +103,9 @@ struct Goal
      */
     BuildResult buildResult;
 
+    // for use by Worker only. will go away once work() is a promise.
+    kj::Own<kj::PromiseFulfiller<void>> notify;
+
 protected:
     AsyncSemaphore::Token slotToken;
 
@@ -122,9 +115,6 @@ public:
 
     struct [[nodiscard]] StillAlive {};
     struct [[nodiscard]] ContinueImmediately {};
-    struct [[nodiscard]] WaitForGoals {
-        Goals goals;
-    };
     struct [[nodiscard]] WaitForWorld {
         kj::Promise<Outcome<void, Finished>> promise;
     };
@@ -141,7 +131,6 @@ public:
     struct [[nodiscard]] WorkResult : std::variant<
                                           StillAlive,
                                           ContinueImmediately,
-                                          WaitForGoals,
                                           WaitForWorld,
                                           Finished>
     {
@@ -151,6 +140,15 @@ public:
 
 protected:
     kj::Promise<Result<WorkResult>> waitForAWhile();
+    kj::Promise<Result<WorkResult>>
+    waitForGoals(kj::Array<std::pair<GoalPtr, kj::Promise<void>>> dependencies) noexcept;
+
+    template<std::derived_from<Goal>... G>
+    kj::Promise<Result<Goal::WorkResult>>
+    waitForGoals(std::pair<std::shared_ptr<G>, kj::Promise<void>>... goals) noexcept
+    {
+        return waitForGoals(kj::arrOf<std::pair<GoalPtr, kj::Promise<void>>>(std::move(goals)...));
+    }
 
 public: