Allow flymake-ruff to use Ruff config files from remote TRAMP hosts

flymake-ruff.el

@@ -38,6 +38,10 @@
 
 (defvar flymake-ruff--output-regex "\\(.*\\):\\([0-9]+\\):\\([0-9]+\\): \\([A-Za-z0-9-]+\\):? \\(.*\\)")
 
+(defvar flymake-ruff--no-config-tramp-dirs
+  nil
+  "List of TRAMP directories that ruff checked for config in but found none. This prevents repeatedly looking for config files that do not exist.")
+
 (defconst flymake-ruff--default-configs
   '(".ruff.toml" "ruff.toml" "pyproject.toml")
   "Default configuration files supported by Ruff.")
@@ -57,6 +61,46 @@
     ("PERF"           . :warning))  ; Performance issues
   "Mapping of Ruff diagnostic code prefixes to Flymake severities.")
 
+(defun flymake-ruff--get-config ()
+  "Look for configuration files supported by Ruff in project root.
+When project is on remote host, cache config using `default-directory' as key."
+  (if (tramp-handle-file-remote-p (or buffer-file-name
+				      default-directory))
+      (when (not (seq-contains-p flymake-ruff--no-config-tramp-dirs default-directory #'string-equal))
+	(let ((cache-dir (expand-file-name 
+                          (concat "ruff-config-" (sha1 default-directory))
+                          "/tmp")))
+          (if (file-directory-p cache-dir)
+              ;; Return cached config if exists
+              (seq-find #'file-readable-p
+			(mapcar (lambda (f)
+				  (expand-file-name f cache-dir))
+				flymake-ruff--default-configs))
+            ;; Create cache and return config path
+	    (if-let* ((project-current (project-current))
+		      (config (seq-find
+                               #'tramp-handle-file-readable-p
+                               (mapcar (lambda (f)
+                                         (tramp-handle-expand-file-name f (project-root project-current)))
+				       flymake-ruff--default-configs)))
+		      (temp-file (tramp-handle-expand-file-name (tramp-handle-file-name-nondirectory config) cache-dir))
+		      (temp-copy (tramp-handle-file-local-copy config)))
+		(progn
+		  (make-directory cache-dir)
+		  (copy-file temp-copy temp-file t)
+		  (delete-file temp-copy)
+		  temp-file)
+	      (progn
+		(push default-directory flymake-ruff--no-config-tramp-dirs)
+		nil)))))
+
+    ;; Local project path
+    (when-let ((project-current (project-current)))
+      (seq-find #'file-readable-p
+		(mapcar (lambda (f)
+			  (expand-file-name f (project-root project-current)))
+			flymake-ruff--default-configs)))))
+
 (defun flymake-ruff--severity-for-code (code)
   "Return Flymake severity for Ruff CODE."
   (let ((matched-prefix (seq-find (lambda (pfx)
@@ -82,14 +126,8 @@
       ;; check if the current buffer belongs to a project before
       ;; trying to build a path using `project-current' otherwise it
       ;; will fail silently
-      (let* ((config (and (project-current)
-                          (seq-find
-                           #'file-readable-p
-                           (mapcar
-                            (lambda (f)
-                              (expand-file-name f (project-root (project-current))))
-                            flymake-ruff--default-configs))))
-             (args (if config
+      (let* ((config (flymake-ruff--get-config))
+	     (args (if config
                        ;; for version > 0.5 the work "check" is
                        ;; included so we need to extract it and put it
                        ;; before --config argument
@@ -130,6 +168,14 @@
   "Load hook for the current buffer to tell flymake to run checker."
   (interactive)
   (when (derived-mode-p 'python-mode 'python-ts-mode)
+    ;; If cached TRAMP config file exists for current buffer, remove it so that cache can be refreshed on reverting buffer.
+    (when-let* ((_remote (file-remote-p default-directory))
+		(cache-dir (expand-file-name 
+			    (concat "ruff-config-" (sha1 default-directory))
+			    "/tmp"))
+		(_cache_exist (file-directory-p cache-dir)))
+      (delete-directory cache-dir t))
+
     (add-hook 'flymake-diagnostic-functions #'flymake-ruff--run-checker nil t)))
 
 (defun flymake-ruff--run-checker (report-fn &rest _args)