File Name Cache in Emacs 28.1: Add Automatic Invalidation on External Changes?

In Emacs 28.1, the file-name-cache is a performance optimization that prevents redundant system calls for functions like file-attributes and fileexists. However, because the cache lacks a native link to file-system events, it can become stale when files are modified by external processes (build tools, git, or other editors).

The Recommended Strategy: Targeted Invalidation

To ensure accuracy without sacrificing performance, the most effective approach is to use a targeted invalidation rather than a global cache purge. Clearing the entire cache via clear-file-name-cache forces Emacs to re-query every cached file on next access, which can cause noticeable lag in large projects or over network drives.

To implement this, you should utilize file-notify-add to monitor specific directories. When a modification event is detected, you should remove only the affected entry from the cache.

;; Example logic for targeted invalidation
(defun my-invalidate-cache-entry (path)
  ""Remove a specific path from the file-name-cache."
  (let ((entry (hash-file-name-cache path))
    (put file-name-cache entry nil)))

(defun my-file-changed-handler (event)
  ""Callback for file-notify-event."
  (when (eq (event-type event) 'modified)
    (my-invalidate-cache-entry (event-name event))))

Addressing the Constraints

• Performance vs. Correctness: Targeting specific entries using hash-file-name-cache is highly efficient. Since file-name-cache is a hash table, removing a single key is O(1) compared to rebuilding the entire structure.
• Multi-process/Server Mode: In server-mode, the cache lives in the server process. If an external client modifies a file, the server-side cache will not know unless the server has its own file-notify listeners. If you use multiple Emacs instances, you would need a mechanism to broadcast invalidation signals, which adds complexity.
• Reliability: Adding an advisory to file-attributes itself is not possible as it is a consumer of the cache, not the provider. The logic must reside at the file-system level (via file-notify).

Diagnostic Requirement

Are you monitoring a high-frequency directory (like a node_modules or build folder), or primarily static source files? This will determine if you need to debounce the invalidation hook to prevent CPU thrashing.