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

[Philip Kaludercic]

Dmitry Gutov <dgutov@yandex.ru> writes:

> On 05.02.2022 16:38, Michael Albinus wrote:
>> Dmitry Gutov <dgutov@yandex.ru> writes:
>> 
>>> Hi Michael,
>> Hi Dmitry,
>> 
>>>>>> When invoking a command that respects xref-search-program via TRAMP,
>>>>>> e.g. on a remote system that doesn't have (in my case ripgrep)
>>>>>> installed, an error is signalled indicating that the search query
>>>>>> couldn't be executed.
>>>>>
>>>>> One way to work around this will probably involve an addition to
>>>>> find-file-hook and some code which checks (file-remote-p
>>>>> buffer-file-name) and sets xref-search-program to a particular value
>>>>> buffer-locally depending on the result.
>>>>>
>>>>> Or an around-advice for xref-matches-in-files.
>>>> There are connection-local variables exactly for this use case.
>>>
>>> Is there a documented way on how to make the variable's value on
>>> remote hosts customizable for the user too?
>> Something like
>> --8<---------------cut here---------------start------------->8---
>> (connection-local-set-profile-variables
>>   'remote-xref-variables
>>   '((xref-search-program . "/bin/grep")))
>> (with-eval-after-load 'xref
>>    (connection-local-set-profiles
>>     '(:application tramp :machine "myhost")
>>     'remote-xref-variables))
>> --8<---------------cut here---------------end--------------->8---
>
> Nice. Some integration with the Customize UI probably wouldn't hurt, though.
>
> Philip, would you like to try writing a patch along the lines of
> Michael's suggestion?
>
> It would need to use version checks, though, given that
> connection-local vars were only added in Emacs 27.

The following works, with one issue:

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 6677b4f004..b2f82f1889 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -1751,15 +1751,7 @@ 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
@@ -1770,25 +1762,56 @@ xref-matches-in-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 (version<= "27" emacs-version)
+                            (with-connection-local-variables
+                             xref-search-program)
+                          xref-search-program)))
+          ;; 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
+                    (xref--process-file-region
+                     (point-min) (point-max)
+                     shell-file-name
+                     output nil shell-command-switch
+                     (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 default-directory '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)

xref--process-file-region takes shell-file-name, that should depend on
the remote system.  I could use something like

   (with-parsed-tramp-file-name default-directory d
     (tramp-get-method-parameter d 'tramp-remote-shell))

but it seems that `with-parsed-tramp-file-name' is not used outside of
tramp, and byte-compiling triggers a warning.  Is there some better way
to do this?
  
> To get you started, here's one example of this feature's usage:
> https://github.com/company-mode/company-mode/blob/c25f1fbc3850e36e6521b77fa1641d5583365d8b/company-gtags.el#L71-L96
>
> And you can search in Emacs's own repository, of course, for other examples.

-- 
        Philip Kaludercic