Factor out a `GcStore` interface

Starts progress on #5729.

The idea is that we should not have these default methods throwing
"unimplemented". This is a small step in that direction.

I kept `addTempRoot` because it is a no-op, rather than failure. Also,
as a practical matter, it is called all over the place, while doing
other tasks, so the downcasting would be annoying.

Maybe in the future I could move the "real" `addTempRoot` to `GcStore`,
and the existing usecases use a `tryAddTempRoot` wrapper to downcast or
do nothing, but I wasn't sure whether that was a good idea so with a
bias to less churn I didn't do it yet.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/nix/store-gc.cc b/src/nix/store-gc.cc
index a2d74066e..21718dc0c 100644
--- a/src/nix/store-gc.cc
+++ b/src/nix/store-gc.cc
@@ -2,6 +2,7 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "gc-store.hh"
 
 using namespace nix;
 
@@ -33,10 +34,12 @@ struct CmdStoreGC : StoreCommand, MixDryRun
 
     void run(ref<Store> store) override
     {
+        auto & gcStore = requireGcStore(*store);
+
         options.action = dryRun ? GCOptions::gcReturnDead : GCOptions::gcDeleteDead;
         GCResults results;
         PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
-        store->collectGarbage(options, results);
+        gcStore.collectGarbage(options, results);
     }
 };