POC: Make nix store delete --recursive best effort

Only delete the free paths in the closure rather than trying to delete
everything.

Author: Théophane Hufschmitt

src/libstore/gc.cc

@@ -778,11 +778,11 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 
     /* Either delete all garbage paths, or just the specified
        paths (for gcDeleteSpecific). */
-    if (options.action == GCOptions::gcDeleteSpecific) {
+    if (options.action == GCOptions::gcDeleteSpecific || options.action == GCOptions::gcDeleteDead) {
 
         for (auto & i : options.pathsToDelete) {
             deleteReferrersClosure(i);
-            if (!dead.count(i))
+            if (options.action == GCOptions::gcDeleteSpecific && !dead.count(i))
                 throw Error(
                     "Cannot delete path '%1%' since it is still alive. "
                     "To find out why, use: "

src/nix/store-delete.cc

@@ -9,7 +9,7 @@ using namespace nix;
 
 struct CmdStoreDelete : StorePathsCommand
 {
-    GCOptions options { .action = GCOptions::gcDeleteSpecific };
+    GCOptions options { .action = GCOptions::gcDeleteDead };
 
     CmdStoreDelete()
     {