libstore: make LocalDerivationGoal::needsHashRewrite virtual

This rather simple function existed just to check some flags,
but the response varies by platform. This is a perfect case for
our subclasses.

Change-Id: Ieb1732a8d024019236e0d0028ad843a24ec3dc59

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

@@ -117,14 +117,9 @@ LocalDerivationGoal::~LocalDerivationGoal() noexcept(false)
 }
 
 
-inline bool LocalDerivationGoal::needsHashRewrite()
+bool LocalDerivationGoal::needsHashRewrite()
 {
-#if __linux__
     return !useChroot;
-#else
-    /* Darwin requires hash rewriting even when sandboxing is enabled. */
-    return true;
-#endif
 }
 
 

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

@@ -206,7 +206,7 @@ struct LocalDerivationGoal : public DerivationGoal
     /**
      * Whether we need to perform hash rewriting if there are valid output paths.
      */
-    bool needsHashRewrite();
+   virtual bool needsHashRewrite();
 
     /**
      * The additional states.

src/libstore/platform/darwin.hh

@@ -52,6 +52,13 @@ private:
      * Set process flags to enter or leave rosetta, then execute the builder
      */
     void execBuilder(std::string builder, Strings args, Strings envStrs) override;
+
+    /**
+     * Whether we need to rewrite output hashes.
+     * Always true on Darwin since Darwin requires hash rewriting
+     * even when sandboxing is enabled.
+     */
+    bool needsHashRewrite() override { return true; };
 };
 
 }