Fix remote path setting in Eshell

This ensures that we supply Tramp with the local PATH so that it can
do its job of starting the local "ssh", or whatever the method uses
(bug#65551).

* lisp/eshell/esh-proc.el (eshell-gather-process-output): Add special
handling for remote processes.

* test/lisp/eshell/esh-proc-tests.el
(esh-var-test/remote/remote-path): New test.

lisp/eshell/esh-proc.el

@@ -265,6 +265,8 @@ nil, write to `eshell-output-handle'."
   "A marker that tracks the beginning of output of the last subprocess.
 Used only on systems which do not support async subprocesses.")
 
+(defvar tramp-remote-path)
+
 (defun eshell-gather-process-output (command args)
   "Gather the output from COMMAND + ARGS."
   (require 'esh-var)
@@ -272,7 +274,9 @@ Used only on systems which do not support async subprocesses.")
   (unless (and (file-executable-p command)
 	       (file-regular-p (file-truename command)))
     (error "%s: not an executable file" command))
-  (let* ((delete-exited-processes
+  (let* ((real-path (getenv "PATH"))
+         (tramp-remote-path (bound-and-true-p tramp-remote-path))
+         (delete-exited-processes
 	  (if eshell-current-subjob-p
 	      eshell-delete-exited-processes
 	    delete-exited-processes))
@@ -280,6 +284,16 @@ Used only on systems which do not support async subprocesses.")
          (coding-system-for-read coding-system-for-read)
          (coding-system-for-write coding-system-for-write)
 	 proc stderr-proc decoding encoding changed)
+    ;; HACK: We want to supply our subprocess with the all the
+    ;; environment variables we've set in Eshell.  However, supplying
+    ;; a remote PATH this way can break Tramp, which needs the *local*
+    ;; PATH for calling "ssh", etc.  Instead, set the local path in
+    ;; our `process-environment' and pass the remote PATH via
+    ;; `tramp-remote-path'.  (If we handle this some better way in the
+    ;; future, remember to remove `tramp-remote-path' above, too.)
+    (when (file-remote-p default-directory)
+      (push (concat "PATH=" real-path) process-environment)
+      (setq tramp-remote-path (eshell-get-path)))
     ;; MS-Windows needs special setting of encoding/decoding, because
     ;; (a) non-ASCII text in command-line arguments needs to be
     ;; encoded in the system's codepage; and (b) because many Windows

test/lisp/eshell/esh-proc-tests.el

@@ -259,4 +259,19 @@ write the exit status to the pipe.  See bug#54136."
                      output-start (eshell-end-of-output))
                     "")))))
 
+
+;; Remote processes
+
+(ert-deftest esh-var-test/remote/remote-path ()
+  "Ensure that setting the remote PATH in Eshell doesn't interfere with Tramp.
+See bug#65551."
+  (skip-unless (and (eshell-tests-remote-accessible-p)
+                    (executable-find "echo")))
+  (let ((default-directory ert-remote-temporary-file-directory))
+    (with-temp-eshell
+     (eshell-insert-command "set PATH ''")
+     (eshell-match-command-output
+      (format "%s hello" (executable-find "echo" t))
+      "\\`hello\n"))))
+
 ;;; esh-proc-tests.el ends here