Add fs::parent_canonical()

src/libutil/file-system.cc

@@ -765,4 +765,19 @@ bool isExecutableFileAmbient(const fs::path & exe) {
         ) == 0;
 }
 
+namespace fs {
+
+std::filesystem::path parent_canonical(const std::filesystem::path & path)
+{
+    auto parent = path.parent_path();
+    if (parent == path) {
+        // `path` is a root directory => trivially canonical
+        return parent;
+    }
+    return std::filesystem::canonical(parent) / path.filename();
 }
+
+} // namespace nix::fs
+
+
+} // namespace nix

src/libutil/file-system.hh

@@ -141,6 +141,19 @@ inline bool symlink_exists(const std::filesystem::path & path) {
     return std::filesystem::exists(std::filesystem::symlink_status(path));
 }
 
+/**
+ * Canonicalize a path except for the last component.
+ *
+ * This is useful for getting the canonical location of a symlink.
+ *
+ * Consider the case where `foo/l` is a symlink. `canonical("foo/l")` will
+ * resolve the symlink `l` to its target.
+ * `parent_canonical("foo/l")` will not resolve the symlink `l` to its target,
+ * but does ensure that the returned parent path's parent equals its `canonical`,
+ * and can therefore be retrieved without traversing any symlinks.
+ */
+std::filesystem::path parent_canonical(const std::filesystem::path & path);
+
 } // namespace fs
 
 /**