Fix Bug#33735

* lisp/net/tramp-sh.el (tramp-sh-gio-monitor-process-filter):
Delete empty lines.  (Bug#33735)
(tramp-sh-inotifywait-process-filter): Use `string-match'.
(tramp-set-remote-path): Check for PIPE_BUF but PATH_MAX.

* test/lisp/filenotify-tests.el: Handle $REMOTE_FILE_NOTIFY_LIBRARY.
(file-notify--deftest-remote): Suppress vc-handled-backends.
(file-notify-test08-backup): Do not expect failure on hydra.
(Bug#33735).

lisp/net/tramp-compat.el

@@ -264,6 +264,7 @@ If NAME is a remote file name, the local part of NAME is unquoted."
 A nil value for either argument stands for the current time."
     (equal (or t1 (current-time)) (or t2 (current-time)))))
 
+;; `flatten-tree' has appeared in Emacs 27.1.
 (if (fboundp 'flatten-tree)
     (defalias 'tramp-compat-flatten-tree 'flatten-tree)
   (defun tramp-compat-flatten-tree (tree)

lisp/net/tramp-sh.el

@@ -3615,11 +3615,14 @@ Fall back to normal file name handler if no Tramp handler exists."
 	(string-match-p "Monitoring not supported\\|No locations given" string)
       (delete-process proc))
 
+    ;; Delete empty lines.
+    (setq string (replace-regexp-in-string "\n\n" "\n" string))
+
     (while (string-match
 	    (eval-when-compile
 	      (concat "^[^:]+:"
 		      "[[:space:]]\\([^:]+\\):"
-		      "[[:space:]]"  (regexp-opt tramp-gio-events t)
+		      "[[:space:]]" (regexp-opt tramp-gio-events t)
 		      "\\([[:space:]]\\([^:]+\\)\\)?$"))
 	    string)
 
@@ -3713,7 +3716,7 @@ file-notify events."
     (tramp-message proc 6 "%S\n%s" proc string)
     (dolist (line (split-string string "[\n\r]+" 'omit))
       ;; Check, whether there is a problem.
-      (unless (string-match-p
+      (unless (string-match
 	       (eval-when-compile
 		 (concat "^[^[:blank:]]+"
 			 "[[:blank:]]+\\([^[:blank:]]+\\)+"
@@ -3880,6 +3883,9 @@ This function expects to be in the right *tramp* buffer."
 	  (setq result (buffer-substring (point) (point-at-eol)))))
     result)))
 
+;; On hydra.nixos.org, the $PATH environment variable is too long to
+;; send it.  This is likely not due to PATH_MAX, but PIPE_BUF.  We
+;; check it, and use a temporary file in case of.  See Bug#33781.
 (defun tramp-set-remote-path (vec)
   "Sets the remote environment PATH to existing directories.
 I.e., for each directory in `tramp-remote-path', it is tested
@@ -3888,12 +3894,12 @@ variable PATH."
   (let ((command
 	 (format "PATH=%s; export PATH"
 		 (mapconcat 'identity (tramp-get-remote-path vec) ":")))
-	(path-max
-	 (with-tramp-connection-property vec "path-max"
-	   (tramp-send-command-and-read vec "getconf PATH_MAX /")))
+	(pipe-buf
+	 (with-tramp-connection-property vec "pipe-buf"
+	   (tramp-send-command-and-read vec "getconf PIPE_BUF /")))
 	tmpfile)
     (tramp-message vec 5 "Setting $PATH environment variable")
-    (if (< (length command) path-max)
+    (if (< (length command) pipe-buf)
 	(tramp-send-command vec command)
       ;; Use a temporary file.
       (setq tmpfile (tramp-make-tramp-temp-file vec))
@@ -5785,10 +5791,6 @@ function cell is returned to be applied on a buffer."
 ;;   gets confused about the file locking status.  Try to find out why
 ;;   the workaround doesn't work.
 ;;
-;; * Allow out-of-band methods as _last_ multi-hop.  Open a connection
-;;   until the last but one hop via `start-file-process'.  Apply it
-;;   also for ftp and smb.
-;;
 ;; * WIBNI if we had a command "trampclient"?  If I was editing in
 ;;   some shell with root privileges, it would be nice if I could
 ;;   just call

test/lisp/filenotify-tests.el

@@ -31,6 +31,21 @@
 ;; remote host, set this environment variable to "/dev/null" or
 ;; whatever is appropriate on your system.
 
+;; For the remote file-notify library, Tramp checks for the existence
+;; of a respective command.  The first command found is used.  In
+;; order to use a dedicated one, the environment variable
+;; $REMOTE_FILE_NOTIFY_LIBRARY shall be set, possible values are
+;; "inotifywait", "gio-monitor" and "gvfs-monitor-dir".
+
+;; Local file-notify libraries are auto-detected during Emacs
+;; configuration.  This can be changed with a respective configuration
+;; argument, like
+;;
+;;   --with-file-notification=inotify
+;;   --with-file-notification=kqueue
+;;   --with-file-notification=gfile
+;;   --with-file-notification=w32
+
 ;; A whole test run can be performed calling the command `file-notify-test-all'.
 
 ;;; Code:
@@ -64,6 +79,12 @@
       (format "/mock::%s" temporary-file-directory)))
   "Temporary directory for Tramp tests.")
 
+;; Filter suppressed remote file-notify libraries.
+(when (stringp (getenv "REMOTE_FILE_NOTIFY_LIBRARY"))
+  (dolist (lib '("inotifywait" "gio-monitor" "gvfs-monitor-dir"))
+    (unless (string-equal (getenv "REMOTE_FILE_NOTIFY_LIBRARY") lib)
+      (add-to-list 'tramp-connection-properties `(nil ,lib nil)))))
+
 (defvar file-notify--test-tmpdir nil)
 (defvar file-notify--test-tmpfile nil)
 (defvar file-notify--test-tmpfile1 nil)
@@ -250,7 +271,8 @@ This returns only for the local case and gfilenotify; otherwise it is nil.
      (skip-unless (not ,skip))
      (let* ((temporary-file-directory
 	     file-notify-test-remote-temporary-file-directory)
-	    (ert-test (ert-get-test ',test)))
+	    (ert-test (ert-get-test ',test))
+            vc-handled-backends)
        (skip-unless (file-notify--test-remote-enabled))
        (tramp-cleanup-connection
 	(tramp-dissect-file-name temporary-file-directory) nil 'keep-password)
@@ -1243,8 +1265,7 @@ delivered."
     (file-notify--test-cleanup)))
 
 (file-notify--deftest-remote file-notify-test08-backup
-  "Check that backup keeps file notification for remote files."
-  (if (getenv "EMACS_HYDRA_CI") :failed :passed)) ; fixme bug#33735
+  "Check that backup keeps file notification for remote files.")
 
 (ert-deftest file-notify-test09-watched-file-in-watched-dir ()
   "Watches a directory and a file in that directory separately.