{libutil,libstore}: Factor out chmodIfNeeded

Using std::filesystem::path directly because we need .c_str()
anyway to interact with chmod. Path/string views don't have to be
null-terminated.

src/libutil/file-system.cc

@@ -776,4 +776,18 @@ std::filesystem::path makeParentCanonical(const std::filesystem::path & rawPath)
     }
 }
 
+bool chmodIfNeeded(const fs::path & path, mode_t mode, mode_t mask)
+{
+    auto pathString = path.string();
+    auto prevMode = lstat(pathString).st_mode;
+
+    if (((prevMode ^ mode) & mask) == 0)
+        return false;
+
+    if (chmod(pathString.c_str(), mode) != 0)
+        throw SysError("could not set permissions on '%s' to %o", pathString, mode);
+
+    return true;
+}
+
 } // namespace nix

src/libutil/file-system.hh

@@ -366,4 +366,20 @@ typedef std::function<bool(const Path & path)> PathFilter;
 
 extern PathFilter defaultPathFilter;
 
+/**
+ * Change permissions of a file only if necessary.
+ *
+ * @details
+ * Skip chmod call if the directory already has the requested permissions.
+ * This is to avoid failing when the executing user lacks permissions to change the
+ * directory's permissions even if it would be no-op.
+ *
+ * @param path Path to the file to change the permissions for.
+ * @param mode New file mode.
+ * @param mask Used for checking if the file already has requested permissions.
+ *
+ * @return true if permissions changed, false otherwise.
+ */
+bool chmodIfNeeded(const std::filesystem::path & path, mode_t mode, mode_t mask = S_IRWXU | S_IRWXG | S_IRWXO);
+
 }