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

Don't pollute /tmp

Fix error on directory existing

Cache config and use tramp-handle-* funcitons to stop TRAMP reentrant errors

flymake-ruff.el

@@ -39,6 +39,41 @@
   '(".ruff.toml" "ruff.toml" "pyproject.toml")
   "Default configuration files supported by Ruff.")
 
+(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 (file-remote-p default-directory)
+      (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
+	  (when-let* ((project-current (project-current))
+		      (config (seq-find
+                               #'tramp-handle-file-readable-p
+                               (mapcar (lambda (f)
+                                         (expand-file-name f (project-root project-current)))
+				       flymake-ruff--default-configs)))
+		      (temp-file (expand-file-name (file-name-nondirectory config) cache-dir))
+		      (temp-copy (tramp-handle-file-local-copy config)))
+	    (message "first")
+            (make-directory cache-dir)
+            (copy-file temp-copy temp-file t)
+            (delete-file temp-copy)
+            temp-file)))
+
+    ;; 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--check-buffer ()
   "Generate a list of diagnostics for the current buffer."
   (let ((code-buffer (current-buffer))
@@ -52,14 +87,8 @@
       ;; 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
@@ -94,6 +123,14 @@
   "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)