DerivationBuilder no more callback soup for logging

`startBuilder` just returns the descriptor for the pipe now.

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

@@ -716,11 +716,6 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
 
                 ~DerivationBuildingGoalCallbacks() override = default;
 
-                void childStarted(Descriptor builderOut) override
-                {
-                    goal.worker.childStarted(goal.shared_from_this(), {builderOut}, true, true);
-                }
-
                 void childTerminated() override
                 {
                     goal.worker.childTerminated(&goal);
@@ -802,10 +797,11 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
 
     actLock.reset();
 
+    Descriptor builderOut;
     try {
 
         /* Okay, we have to build. */
-        builder->startBuilder();
+        builderOut = builder->startBuilder();
 
     } catch (BuildError & e) {
         builder.reset();
@@ -814,6 +810,8 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
         co_return doneFailure(std::move(e)); // InputRejected
     }
 
+    worker.childStarted(shared_from_this(), {builderOut}, true, true);
+
     started();
     co_await Suspend{};
 

src/libstore/include/nix/store/build/derivation-builder.hh

@@ -114,13 +114,6 @@ struct DerivationBuilderCallbacks
      */
     virtual void closeLogFile() = 0;
 
-    /**
-     * Hook up `builderOut` to some mechanism to ingest the log
-     *
-     * @todo this should be reworked
-     */
-    virtual void childStarted(Descriptor builderOut) = 0;
-
     /**
      * @todo this should be reworked
      */
@@ -161,8 +154,10 @@ struct DerivationBuilder : RestrictionContext
 
     /**
      * Start building a derivation.
+     *
+     * @return logging pipe
      */
-    virtual void startBuilder() = 0;
+    virtual Descriptor startBuilder() = 0;
 
     /**
      * Tear down build environment after the builder exits (either on

src/libstore/unix/build/derivation-builder.cc

@@ -216,7 +216,7 @@ public:
 
     bool prepareBuild() override;
 
-    void startBuilder() override;
+    Descriptor startBuilder() override;
 
     SingleDrvOutputs unprepareBuild() override;
 
@@ -679,7 +679,7 @@ static bool checkNotWorldWritable(std::filesystem::path path)
     return true;
 }
 
-void DerivationBuilderImpl::startBuilder()
+Descriptor DerivationBuilderImpl::startBuilder()
 {
     /* Make sure that no other processes are executing under the
        sandbox uids. This must be done before any chownToBuilder()
@@ -841,9 +841,10 @@ void DerivationBuilderImpl::startBuilder()
     startChild();
 
     pid.setSeparatePG(true);
-    miscMethods->childStarted(builderOut.get());
 
     processSandboxSetupMessages();
+
+    return builderOut.get();
 }
 
 PathsInChroot DerivationBuilderImpl::getPathsInSandbox()