Remove obsolete server buffers on MOTD in erc-track

* lisp/erc/erc-networks.el
(erc-networks--copy-server-buffer-functions): New internal hook
through which modules can perform housekeeping when server buffers
belonging to the same network context are merged.
(erc-networks--copy-over-server-buffer-contents): Run new internal
hook `erc-networks--copy-server-buffer-functions'.
* lisp/erc/erc-track.el (erc-track-enable, erc-track-disable): Manage
membership in `erc-networks--copy-server-buffer-functions' hook.
(erc-track--replace-killed-buffer): New function to replace server
buffer being killed in `erc-modified-channels-alist'.
* test/lisp/erc/erc-scenarios-base-association.el
(erc-scenarios-networks-merge-server-track): New test.
* test/lisp/erc/resources/networks/merge-server/track.eld: New test
data.  (Bug#60560.)

2 files changed, 16 insertions, 2 deletions

diff --git a/lisp/erc/erc-networks.el b/lisp/erc/erc-networks.el
index 4044be08f92..95fd8990c99 100644
--- a/lisp/erc/erc-networks.el
+++ b/lisp/erc/erc-networks.el
@@ -1366,6 +1366,11 @@ ANNOUNCED is the server's reported host name."
                erc-server-connected t
                erc-networks--id nid))))))
 
+(defvar erc-networks--copy-server-buffer-functions nil
+  "Abnormal hook run in new server buffers when deduping.
+Passed the existing buffer to be killed, whose contents have
+already been copied over to the current, replacement buffer.")
+
 (defun erc-networks--copy-over-server-buffer-contents (existing name)
   "Kill off existing server buffer after copying its contents.
 Must be called from the replacement buffer."
@@ -1386,6 +1391,7 @@ Must be called from the replacement buffer."
         erc-kill-server-hook
         erc-kill-buffer-hook)
     (erc-networks--insert-transplanted-content text)
+    (run-hook-with-args 'erc-networks--copy-server-buffer-functions existing)
     (kill-buffer name)))
 
 ;; This stands alone for testing purposes
diff --git a/lisp/erc/erc-track.el b/lisp/erc/erc-track.el
index 61c0c66abfb..7fd7b53602e 100644
--- a/lisp/erc/erc-track.el
+++ b/lisp/erc/erc-track.el
@@ -521,7 +521,9 @@ keybindings will not do anything useful."
        (add-hook 'erc-disconnected-hook #'erc-modified-channels-update))
      ;; enable the tracking keybindings
      (add-hook 'erc-connect-pre-hook #'erc-track-minor-mode-maybe)
-     (erc-track-minor-mode-maybe)))
+     (erc-track-minor-mode-maybe))
+   (add-hook 'erc-networks--copy-server-buffer-functions
+             #'erc-track--replace-killed-buffer))
   ;; Disable:
   ((when (boundp 'erc-track-when-inactive)
      (erc-track-remove-from-mode-line)
@@ -539,7 +541,9 @@ keybindings will not do anything useful."
      ;; disable the tracking keybindings
      (remove-hook 'erc-connect-pre-hook #'erc-track-minor-mode-maybe)
      (when erc-track-minor-mode
-       (erc-track-minor-mode -1)))))
+       (erc-track-minor-mode -1)))
+   (remove-hook 'erc-networks--copy-server-buffer-functions
+                #'erc-track--replace-killed-buffer)))
 
 (defcustom erc-track-when-inactive nil
   "Enable channel tracking even for visible buffers, if you are inactive."
@@ -942,6 +946,10 @@ reverse it."
   (interactive "p")
   (erc-track--switch-buffer 'switch-to-buffer-other-window arg))
 
+(defun erc-track--replace-killed-buffer (existing)
+  (when-let ((found (assq existing erc-modified-channels-alist)))
+    (setcar found (current-buffer))))
+
 (provide 'erc-track)
 
 ;;; erc-track.el ends here