libutil: Implement second overload of createDirectory for RestoreSink

Now the intermediate symlink following issue should be completely plugged.

src/libutil/fs-sink.cc

@@ -71,6 +71,29 @@ static std::filesystem::path append(const std::filesystem::path & src, const Can
     return dst;
 }
 
+#ifndef _WIN32
+void RestoreSink::createDirectory(const CanonPath & path, DirectoryCreatedCallback callback)
+{
+    if (path.isRoot()) {
+        createDirectory(path);
+        callback(*this, path);
+        return;
+    }
+
+    createDirectory(path);
+    assert(dirFd); // If that's not true the above call must have thrown an exception.
+
+    RestoreSink dirSink{startFsync};
+    dirSink.dstPath = append(dstPath, path);
+    dirSink.dirFd = ::openat(dirFd.get(), path.rel_c_str(), O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+
+    if (!dirSink.dirFd)
+        throw SysError("opening directory '%s'", dirSink.dstPath.string());
+
+    callback(dirSink, CanonPath::root);
+}
+#endif
+
 void RestoreSink::createDirectory(const CanonPath & path)
 {
     auto p = append(dstPath, path);

src/libutil/include/nix/util/fs-sink.hh

@@ -120,6 +120,10 @@ struct RestoreSink : FileSystemObjectSink
 
     void createDirectory(const CanonPath & path) override;
 
+#ifndef _WIN32
+    void createDirectory(const CanonPath & path, DirectoryCreatedCallback callback) override;
+#endif
+
     void createRegularFile(const CanonPath & path, std::function<void(CreateRegularFileSink &)>) override;
 
     void createSymlink(const CanonPath & path, const std::string & target) override;

tests/functional/nars.sh

@@ -114,7 +114,7 @@ if (( unicodeTestCode == 1 )); then
     # If the command failed (MacOS or ZFS + normalization), checks that it failed
     # with the expected "already exists" error, and that this is the same
     # behavior as `touch`
-    echo "$unicodeTestOut" | grepQuiet "path '.*/out/â' already exists"
+    echo "$unicodeTestOut" | grepQuiet "creating directory '.*/out/â': File exists"
 
     (( touchFilesCount == 1 ))
 elif (( unicodeTestCode == 0 )); then