Create DerivationBuilder::killChild

Then the derivation building goal doesn't need to snoop around as much.

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

@@ -84,22 +84,8 @@ void DerivationBuildingGoal::killChild()
     hook.reset();
 #endif
 #ifndef _WIN32 // TODO enable `DerivationBuilder` on Windows
-    if (builder && builder->pid != -1) {
+    if (builder && builder->killChild())
         worker.childTerminated(this);
-
-        // FIXME: move this into DerivationBuilder.
-
-        /* If we're using a build user, then there is a tricky race
-           condition: if we kill the build user before the child has
-           done its setuid() to the build user uid, then it won't be
-           killed, and we'll potentially lock up in pid.wait().  So
-           also send a conventional kill to the child. */
-        ::kill(-builder->pid, SIGKILL); /* ignore the result */
-
-        builder->killSandbox(true);
-
-        builder->pid.wait();
-    }
 #endif
 }
 

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

@@ -193,10 +193,12 @@ struct DerivationBuilder : RestrictionContext
     virtual SingleDrvOutputs unprepareBuild() = 0;
 
     /**
-     * Kill any processes running under the build user UID or in the
+     * Forcibly kill the child process, if any.
-     * cgroup of the build.
+     *
+     * @returns whether the child was still alive and needed to be
+     * killed.
      */
-    virtual void killSandbox(bool getStats) = 0;
+    virtual bool killChild() = 0;
 };
 
 #ifndef _WIN32 // TODO enable `DerivationBuilder` on Windows

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

@@ -370,9 +370,15 @@ protected:
      */
     virtual void cleanupBuild(bool force);
 
+    /**
+     * Kill any processes running under the build user UID or in the
+     * cgroup of the build.
+     */
+    virtual void killSandbox(bool getStats);
+
 public:
 
-    void killSandbox(bool getStats) override;
+    bool killChild() override;
 
 private:
 
@@ -435,6 +441,24 @@ void DerivationBuilderImpl::killSandbox(bool getStats)
     }
 }
 
+bool DerivationBuilderImpl::killChild()
+{
+    bool ret = pid != -1;
+    if (ret) {
+        /* If we're using a build user, then there is a tricky race
+           condition: if we kill the build user before the child has
+           done its setuid() to the build user uid, then it won't be
+           killed, and we'll potentially lock up in pid.wait().  So
+           also send a conventional kill to the child. */
+        ::kill(-pid, SIGKILL); /* ignore the result */
+
+        killSandbox(true);
+
+        pid.wait();
+    }
+    return ret;
+}
+
 bool DerivationBuilderImpl::prepareBuild()
 {
     if (useBuildUsers()) {