bug#12972: [PATCH] Avoid regression in mailcap-view-file similar to Bug#44824

[Maxim Nikulin <manikulin@gmail.com>]

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On 01/06/2021 13:56, Lars Ingebrigtsen wrote:
> So I've now added this to Emacs 28 under the name `mailcap-view-file'.

I am attaching a patch similar to proposed to Org mode that should help 
to avoid obscure failures of viewers due to unnecessary terminal sessions.

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From de55b623810736df04641a4d8f6027ccb04caa7f Mon Sep 17 00:00:00 2001
From: Max Nikulin <manikulin@gmail.com>
Date: Thu, 1 Jul 2021 23:41:16 +0700
Subject: [PATCH] mailcap.el: Avoid xdg-open silent failure

* lisp/net/mailcap.el (mailcap-view-file): Use 'pipe :connection-type
instead of 'pty to prevent killing of background process on handler
exit.  Avoid regression similar to Bug#44824.

Problem happens only in some desktop environments where mailcap handler
launches actual viewer (as defined in .desktop files and obtained from
mimeapps.list) in background.  E.g. xdg-open invokes "gio open" or
kde-open5 for Gnome or KDE accordingly and these handlers launch e.g.
eog or okular in background.  As soon as main process exits, temporary
terminal session created by `start-process-shell-command' is terminated.
As a result background processes receive SIGHUP.

Previously command were executed with no buffer as well, so the change
does not affect "needsterminal" and "copiousoutput" mailcap features,
they are not supported as earlier.

If main process of the handler fails then show a message with exit
reason.  Output (including error messages) is ignored as before.
Gtk applications tend to report significant amount of failed asserts
hardly informative for majority of users.
---
 lisp/net/mailcap.el | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/lisp/net/mailcap.el b/lisp/net/mailcap.el
index 54f7f416ab..a53e385662 100644
--- a/lisp/net/mailcap.el
+++ b/lisp/net/mailcap.el
@@ -1177,7 +1177,23 @@ See \"~/.mailcap\", `mailcap-mime-data' and related files and variables."
 		   (shell-quote-argument (convert-standard-filename file))
 		   command
 		   nil t))
-    (start-process-shell-command command nil command)))
+    ;; Handlers such as "gio open" and kde-open5 start viewer in background
+    ;; and exit immediately.  Avoid `start-process' since it assumes
+    ;; :connection-type 'pty and kills children processes with SIGHUP
+    ;; when temporary terminal session is finished (Bug#44824).
+    ;; An alternative is `process-connection-type' let-bound to nil for
+    ;; `start-process-shell-command' call (with no chance to report failure).
+    (make-process
+     :name "mailcap-view-file" :connection-type 'pipe :noquery t
+     :buffer nil ; "*Messages*" may be suitable for debugging
+     :sentinel (lambda (proc event)
+                 (when (and (memq (process-status proc) '(exit signal))
+                            (/= (process-exit-status proc) 0))
+                   (message
+                    "Command %s: %s."
+                    (mapconcat #'identity (process-command proc) " ")
+                    (substring event 0 -1))))
+     :command (list shell-file-name shell-command-switch command))))
 
 (provide 'mailcap)
 
-- 
2.25.1