libutil: Improve handling of non-directory root in MemorySourceAccessor

src/libutil/include/nix/util/memory-source-accessor.hh

@@ -58,7 +58,7 @@ struct MemorySourceAccessor : virtual SourceAccessor
         Stat lstat() const;
     };
 
-    File root{File::Directory{}};
+    std::optional<File> root;
 
     bool operator==(const MemorySourceAccessor &) const noexcept = default;
 

src/libutil/memory-source-accessor.cc

@@ -4,7 +4,22 @@ namespace nix {
 
 MemorySourceAccessor::File * MemorySourceAccessor::open(const CanonPath & path, std::optional<File> create)
 {
-    File * cur = &root;
+    bool hasRoot = root.has_value();
+
+    // Special handling of root directory.
+    if (path.isRoot() && !hasRoot) {
+        if (create) {
+            root = std::move(*create);
+            return &root.value();
+        }
+        return nullptr;
+    }
+
+    // Root does not exist.
+    if (!hasRoot)
+        return nullptr;
+
+    File * cur = &root.value();
 
     bool newF = false;
 
@@ -112,6 +127,10 @@ std::string MemorySourceAccessor::readLink(const CanonPath & path)
 
 SourcePath MemorySourceAccessor::addFile(CanonPath path, std::string && contents)
 {
+    // Create root directory automatically if necessary as a convenience.
+    if (!root && !path.isRoot())
+        open(CanonPath::root, File::Directory{});
+
     auto * f = open(path, File{File::Regular{}});
     if (!f)
         throw Error("file '%s' cannot be made because some parent file is not a directory", path);