bug#53644: 29.0.50; xref-search-program breaks if programm not installed on a remote host

[Philip Kaludercic <philipk@posteo.net>]

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Michael Albinus <michael.albinus@gmx.de> writes:

> Philip Kaludercic <philipk@posteo.net> writes:
>
> Hi Philip,
>
>> But the question here remains precisely what to use instead of the
>> literal "/bin/sh"?
>
> Well, if there isn't a better value, "/bin/sh" should serve as
> fallback. In my experience with Tramp, it works on most of the remote
> systems. It doesn't work on remote Android devices, for example. But
> the
> number of Emacs < 27 users, running xref-* on a remote Android device,
> is rather limited I believe.
>
> POSIX declines this assumptions. It recommends to call "command -v sh"
> in order to determine the shell path, see
> <https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html>.
>
> Best regards, Michael.

In that case, I'd propose this patch:


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From da83d403688b4b5771d08c95975c73a1cfd593c1 Mon Sep 17 00:00:00 2001
From: Philip Kaludercic <philipk@posteo.net>
Date: Wed, 9 Feb 2022 10:14:38 +0100
Subject: [PATCH] Check if xref-search-program exists on a remote system

* lisp/progmodes/xref.el (xref-matches-in-files): Loop through all
programmes in xref-search-program-alist as a fallback before raising
an error that the search query couldn't be executed.
---
 lisp/progmodes/xref.el | 84 ++++++++++++++++++++++++++++--------------
 1 file changed, 57 insertions(+), 27 deletions(-)

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 6677b4f004..759dbb9778 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -1751,44 +1751,74 @@ xref-matches-in-files
        (remote-id (file-remote-p dir))
        ;; The 'auto' default would be fine too, but ripgrep can't handle
        ;; the options we pass in that case.
-       (grep-highlight-matches nil)
-       (command (grep-expand-template (cdr
-                                       (or
-                                        (assoc
-                                         xref-search-program
-                                         xref-search-program-alist)
-                                        (user-error "Unknown search program `%s'"
-                                                    xref-search-program)))
-                                      (xref--regexp-to-extended regexp))))
+       (grep-highlight-matches nil))
     (when remote-id
       (require 'tramp)
       (setq files (mapcar
                    (if (tramp-tramp-file-p dir)
                        #'tramp-file-local-name
-                       #'file-local-name)
+                     #'file-local-name)
                    files)))
     (when (file-name-quoted-p (car files))
       (setq files (mapcar #'file-name-unquote files)))
     (with-current-buffer output
-      (erase-buffer)
       (with-temp-buffer
         (insert (mapconcat #'identity files "\0"))
-        (setq default-directory dir)
-        (setq status
-              (xref--process-file-region (point-min)
-                                         (point-max)
-                                         shell-file-name
-                                         output
-                                         nil
-                                         shell-command-switch
-                                         command)))
-      (goto-char (point-min))
-      (when (and (/= (point-min) (point-max))
-                 (not (looking-at grep-re))
-                 ;; TODO: Show these matches as well somehow?
-                 (not (looking-at "Binary file .* matches")))
-        (user-error "Search failed with status %d: %s" status
-                    (buffer-substring (point-min) (line-end-position))))
+        (let* ((default-directory dir)
+               (xref-search-program-alist xref-search-program-alist)
+               (program (if (bound-and-true-p enable-connection-local-variables)
+                            (with-connection-local-variables
+                             xref-search-program)
+                          xref-search-program))
+               (process-file-side-effects nil))
+          ;; In case `xref-search-program' is not installed on a
+          ;; remote system, we will speculatively try to start a
+          ;; different searcher to see if it is installed and works.
+          (catch 'ok
+            (while xref-search-program-alist
+              (with-current-buffer output
+                (erase-buffer))
+              (setq status
+                    (let ((sfn shell-file-name)
+                          (scs shell-command-switch))
+                      (when remote-id
+                        (if (bound-and-true-p enable-connection-local-variables)
+                            (with-connection-local-variables
+                             (setq sfn shell-file-name
+                                   scs shell-command-switch))
+                          (setq sfn "/bin/sh")))
+                      (xref--process-file-region
+                       (point-min) (point-max)
+                       sfn output nil scs
+                       (grep-expand-template
+                        (or (alist-get program xref-search-program-alist)
+                            (user-error "Unknown search program `%s'" program))
+                        (xref--regexp-to-extended regexp)))))
+              (with-current-buffer output
+                (goto-char (point-min))
+                (unless (and (/= (point-min) (point-max))
+                             (not (looking-at grep-re))
+                             ;; TODO: Show these matches as well somehow?
+                             (not (looking-at "Binary file .* matches")))
+                  (when (and (not (eq program xref-search-program))
+                             (version<= "27" emacs-version))
+                    ;; If possible and necessary, save whatever program
+                    ;; was found as a connection local variable.
+                    (let* ((host (file-remote-p dir 'host))
+                           (profile (intern (concat "xref-remote-" host))))
+                      (connection-local-set-profile-variables
+                       profile `((xref-search-program . ,program)))
+                      (connection-local-set-profiles
+                       (list :machine host) profile)))
+                  (throw 'ok t)))
+              (setq xref-search-program-alist
+                    (remq (assq program xref-search-program-alist)
+                          xref-search-program-alist)
+                    program (caar xref-search-program-alist)))
+            (with-current-buffer output
+              (user-error "Search failed with status %d: %s" status
+                          (buffer-string)
+                          (buffer-substring (point-min) (line-end-position)))))))
       (while (re-search-forward grep-re nil t)
         (push (list (string-to-number (match-string line-group))
                     (match-string file-group)
-- 
2.34.0


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-- 
	Philip Kaludercic