registerOutputs: Hoist up optimizePath call and comment rationale

Author: Ericson2314

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

@@ -1783,16 +1783,23 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                 if (buildMode == bmRepair) {
                     /* Path already exists, need to replace it */
                     replaceValidPath(store.toRealPath(finalDestPath), actualPath);
+                    /* Optimize newly-installed store object */
+                    store.optimisePath(
+                        store.toRealPath(finalDestPath), NoRepair); // FIXME: combine with scanForReferences()
                 } else if (store.isValidPath(newInfo.path)) {
                     /* Path already exists because CA path produced by something
                        else. No moving needed. */
                     assert(newInfo.ca);
                     /* Can delete our scratch copy now. */
                     deletePath(actualPath);
+                    /* Presume already-existing store object is already optimized. */
                 } else {
                     auto destPath = store.toRealPath(finalDestPath);
                     deletePath(destPath);
                     movePath(actualPath, destPath);
+                    /* Optimize newly-installed store object */
+                    store.optimisePath(
+                        store.toRealPath(finalDestPath), NoRepair); // FIXME: combine with scanForReferences()
                 }
             }
 
@@ -1804,10 +1811,6 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                     debug("unreferenced input: '%1%'", store.printStorePath(i));
             }
 
-            if (!store.isValidPath(newInfo.path))
-                store.optimisePath(
-                    store.toRealPath(finalDestPath), NoRepair); // FIXME: combine with scanForReferences()
-
             newInfo.deriver = drvPath;
             newInfo.ultimate = true;
             store.signPathInfo(newInfo);