From 1830f5f967c1726d07104fb9b65e8ae84aac287c Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <sergei@zimmerman.foo>
Date: Mon, 29 Sep 2025 23:16:28 +0300
Subject: [PATCH] libutil: Create empty directory at the root for
 makeEmptySourceAccessor

This is my SNAFU. Accidentally broken in 02c9ac445ff527a7b4c5105d20d9ab401117dcee.

There's very dubious behavior for 'builtins.readDir /.':

{
  outputs =
    { ... }:
    {
      lib.a = builtins.readDir /.;
    };
}

nix eval /tmp/test-flake#lib.a

Starting from 2.27 this now returns an empty set. This really isn't supposed
to happen, but this change in the semantics of makeEmptySourceAccessor accidentally
changed the behavior of this.
---
 src/libutil/memory-source-accessor.cc | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/src/libutil/memory-source-accessor.cc b/src/libutil/memory-source-accessor.cc
index caff5b56a..a9ffb7746 100644
--- a/src/libutil/memory-source-accessor.cc
+++ b/src/libutil/memory-source-accessor.cc
@@ -208,11 +208,16 @@ void MemorySink::createSymlink(const CanonPath & path, const std::string & targe
 
 ref<SourceAccessor> makeEmptySourceAccessor()
 {
-    static auto empty = make_ref<MemorySourceAccessor>().cast<SourceAccessor>();
-    /* Don't forget to clear the display prefix, as the default constructed
-       SourceAccessor has the «unknown» prefix. Since this accessor is supposed
-       to mimic an empty root directory the prefix needs to be empty. */
-    empty->setPathDisplay("");
+    static auto empty = []() {
+        auto empty = make_ref<MemorySourceAccessor>();
+        MemorySink sink{*empty};
+        sink.createDirectory(CanonPath::root);
+        /* Don't forget to clear the display prefix, as the default constructed
+           SourceAccessor has the «unknown» prefix. Since this accessor is supposed
+           to mimic an empty root directory the prefix needs to be empty. */
+        empty->setPathDisplay("");
+        return empty.cast<SourceAccessor>();
+    }();
     return empty;
 }