pathExists: Return false on "/nix/store" in pure mode

AllowListInputAccessor has the invariant that if a path is accessible,
its parent directories are also considered accessible (though reading
them only yields the allowed subdirectories). As a result
`builtins.pathExists "/nix/store"` returns true.

However this wasn't the behaviour of previous path access control,
where `builtins.pathExists "/nix/store"` returns false even if a
subdirectory of the store is accessible.

Fixes NixOS#9672.

src/libexpr/primops.cc

@@ -1561,6 +1561,17 @@ static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args,
             mustBeDir ? SymlinkResolution::Full : SymlinkResolution::Ancestors;
         auto path = realisePath(state, pos, arg, symlinkResolution);
 
+        /* Backward compatibility hack to retain Nix 2.18 behaviour:
+           in pure mode, make `pathExists "/nix/store"` return
+           false. */
+        if ((evalSettings.restrictEval || evalSettings.pureEval)
+            && path.accessor == state.rootFS
+            && isDirOrInDir(state.store->storeDir, path.path.abs()))
+        {
+            v.mkBool(false);
+            return;
+        }
+
         auto st = path.maybeLstat();
         auto exists = st && (!mustBeDir || st->type == SourceAccessor::tDirectory);
         v.mkBool(exists);

tests/functional/flakes/flakes.sh

@@ -34,6 +34,7 @@ cat > "$flake2Dir/flake.nix" <<EOF
 
   outputs = { self, flake1 }: rec {
     packages.$system.bar = flake1.packages.$system.foo;
+    foo = builtins.pathExists (self + "/..");
   };
 }
 EOF
@@ -251,6 +252,9 @@ nix build -o "$TEST_ROOT/result" "$flake2Dir#bar" --commit-lock-file
 [[ -e "$flake2Dir/flake.lock" ]]
 [[ -z $(git -C "$flake2Dir" diff main || echo failed) ]]
 
+# Test that pathExist on the parent of a flake returns false.
+[[ $(nix eval "$flake2Dir#foo") = false ]]
+
 # Rerunning the build should not change the lockfile.
 nix build -o "$TEST_ROOT/result" "$flake2Dir#bar"
 [[ -z $(git -C "$flake2Dir" diff main || echo failed) ]]