libstore: make Worker::waitForBuildSlot private

Change-Id: I02a54846cd65622edbd7a1d6c24a623b4a59e5b3

src/libstore/build/drv-output-substitution-goal.cc

@@ -41,8 +41,7 @@ Goal::WorkResult DrvOutputSubstitutionGoal::tryNext()
        if maxSubstitutionJobs == 0, we still allow a substituter to run. This
        prevents infinite waiting. */
     if (worker.runningSubstitutions >= std::max(1U, settings.maxSubstitutionJobs.get())) {
-        worker.waitForBuildSlot(shared_from_this());
+        return WaitForSlot{};
-        return StillAlive{};
     }
 
     maintainRunningSubstitutions =

src/libstore/build/goal.hh

@@ -106,12 +106,13 @@ struct Goal : public std::enable_shared_from_this<Goal>
 public:
 
     struct [[nodiscard]] StillAlive {};
+    struct [[nodiscard]] WaitForSlot {};
     struct [[nodiscard]] Finished {
         ExitCode result;
         std::unique_ptr<Error> ex;
     };
 
-    struct [[nodiscard]] WorkResult : std::variant<StillAlive, Finished>
+    struct [[nodiscard]] WorkResult : std::variant<StillAlive, WaitForSlot, Finished>
     {
         WorkResult() = delete;
         using variant::variant;

src/libstore/build/local-derivation-goal.cc

@@ -158,9 +158,8 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild()
     unsigned int curBuilds = worker.getNrLocalBuilds();
     if (curBuilds >= settings.maxBuildJobs) {
         state = &DerivationGoal::tryToBuild;
-        worker.waitForBuildSlot(shared_from_this());
         outputLocks.unlock();
-        return StillAlive{};
+        return WaitForSlot{};
     }
 
     assert(derivationType);

src/libstore/build/substitution-goal.cc

@@ -194,8 +194,7 @@ Goal::WorkResult PathSubstitutionGoal::tryToRun()
        if maxSubstitutionJobs == 0, we still allow a substituter to run. This
        prevents infinite waiting. */
     if (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
-        worker.waitForBuildSlot(shared_from_this());
+        return WaitForSlot{};
-        return StillAlive{};
     }
 
     maintainRunningSubstitutions = std::make_unique<MaintainCount<uint64_t>>(worker.runningSubstitutions);

src/libstore/build/worker.cc

@@ -187,6 +187,7 @@ void Worker::handleWorkResult(GoalPtr goal, Goal::WorkResult how)
     std::visit(
         overloaded{
             [&](Goal::StillAlive) {},
+            [&](Goal::WaitForSlot) { waitForBuildSlot(goal); },
             [&](Goal::Finished & f) { goalFinished(goal, f); },
         },
         how

src/libstore/build/worker.hh

@@ -108,6 +108,12 @@ private:
     void goalFinished(GoalPtr goal, Goal::Finished & f);
     void handleWorkResult(GoalPtr goal, Goal::WorkResult how);
 
+    /**
+     * Put `goal` to sleep until a build slot becomes available (which
+     * might be right away).
+     */
+    void waitForBuildSlot(GoalPtr goal);
+
 public:
 
     const Activity act;
@@ -233,12 +239,6 @@ public:
      */
     void childTerminated(Goal * goal);
 
-    /**
-     * Put `goal` to sleep until a build slot becomes available (which
-     * might be right away).
-     */
-    void waitForBuildSlot(GoalPtr goal);
-
     /**
      * Wait for a few seconds and then retry this goal.  Used when
      * waiting for a lock held by another process.  This kind of