libstore/build: automatic clean up of unsuccessfully built scratch outputs

When a build fails, its scratch output paths are not cleaned up.

Until recently, this was deemed not a problem but as part of the effort
to harden the Nix builds and protect these paths against being part of a
staged attack (race conditions, etc.), we automatically cleanup after
failed builds.

Fixes CVE-2025-52992.

Change-Id: I58481b1cc83826298b9d80d37fecf81f117ccb09
Signed-off-by: Raito Bezarius <raito@lix.systems>

tests/nixos/default.nix

@@ -169,6 +169,9 @@ in
 
   symlinkResolvconf = runNixOSTestFor "x86_64-linux" ./symlink-resolvconf.nix;
 
+  # Use this test to test things that cannot easily be tested under chroot Nix stores in functional test suite.
+  non-chroot-misc = runNixOSTestFor "x86_64-linux" ./non-chroot-misc;
+
   noNewPrivilegesInSandbox = runNixOSTestFor "x86_64-linux" ./no-new-privileges/sandbox.nix;
 
   noNewPrivilegesOutsideSandbox = runNixOSTestFor "x86_64-linux" ./no-new-privileges/no-sandbox.nix;

tests/nixos/non-chroot-misc/default.nix

@@ -0,0 +1,34 @@
+{ ... }:
+# Misc things we want to test inside of a non redirected, non chroot Nix store.
+let
+  nonAutoCleaningFailingDerivationCode = ''
+    derivation {
+      name = "scratch-failing";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ (builtins.toFile "builder.sh" "echo bonjour > $out; echo out: $out; false") ];
+    }
+  '';
+in
+{
+  name = "non-chroot-sandbox-misc";
+
+  nodes.machine = {
+  };
+
+  testScript = { nodes }: ''
+    import re
+    start_all()
+
+    # You might ask yourself why write such a convoluted thing?
+    # The condition for fooling Nix into NOT cleaning up the output path are non trivial and unclear.
+    # This is one of those: create a derivation, mkdir or touch the $out path, communicate it back.
+    # Even with a sandboxed Lix, you will observe leftovers before 2.93.0. After this version, this test passes.
+    result = machine.fail("""nix-build --substituters "" -E '${nonAutoCleaningFailingDerivationCode}' 2>&1""")
+    match = re.search(r'out: (\S+)', result)
+    assert match is not None, "Did not find Nix store path in the result of the failing build"
+    outpath = match.group(1).strip()
+    print(f"Found Nix store path: {outpath}")
+    machine.fail(f'stat {outpath}')
+  '';
+}