[PATCHv2] erc: fix erc-reuse-buffers behavior

[Mingde (Matthew) Zeng]

By definition of erc-reuse-buffers, if non-nil it should create a
new buffer when joining channels with same names on different
servers. The current behavior of erc-reuse-buffers is:
1. when non-nil, it will always reuse the same channel buffer,
resulting in server A's channel gets reconnected to the channel with
the same name of server B.
2. when nil, the buffer-name of the joined channel is
'#channel/server'. However if one tries to '/join #channel' from the
server buffer, it creates a new empty buffer with buffer-name
'#channel', instead of opening the already-joined channel buffer.

* lisp/erc/erc.el (erc-generate-new-buffer-name): since `target' is
always non-nil, buffer-name is non-nil, erc-reuse-buffers is t, it
will always use the first candidate, resulting in behavior 1. This is
now fixed, and both channel buffers with the same name will be formated
'#channel/serverA' and '#channel/serverB' for the sake of consistency.

* lisp/erc/erc.el (erc-cmd-JOIN): with some refactoring, it ensures
the joined channel buffer is selected and thereby fixes behavior 2.

* lisp/erc/erc-join.el (erc-autojoin-channels): the logic is
simplified ensuring that when autojoining channels specified in
erc-autojoin-channels-alist, if there exists an erc buffer with the
same channel name but a different server, it will create a new buffer
to join the channel. The current logic is very weak that will skip
joining same channel on different servers altogether.

Signed-off-by: Mingde (Matthew) Zeng <matthewzmd@gmail.com>
---
 lisp/erc/erc-join.el | 22 +++++++------
 lisp/erc/erc.el      | 74 ++++++++++++++++++++++++++------------------
 2 files changed, 56 insertions(+), 40 deletions(-)

diff --git a/lisp/erc/erc-join.el b/lisp/erc/erc-join.el
index 280d6bfe0f..c59f7118d1 100644
--- a/lisp/erc/erc-join.el
+++ b/lisp/erc/erc-join.el
@@ -153,18 +153,20 @@ This function is run from `erc-nickserv-identified-hook'."
                              'erc-autojoin-channels-delayed
                              server nick (current-buffer))))
     ;; `erc-autojoin-timing' is `connect':
-    (dolist (l erc-autojoin-channels-alist)
-      (when (string-match (car l) server)
-       (let ((server (or erc-session-server erc-server-announced-name)))
+    (let ((server (or erc-session-server erc-server-announced-name)))
+      (dolist (l erc-autojoin-channels-alist)
+        (when (string-match (car l) server)
          (dolist (chan (cdr l))
-           (let ((buffer (erc-get-buffer chan)))
-             ;; Only auto-join the channels that we aren't already in
-             ;; using a different nick.
+           (let ((buffer
+                   (car (erc-buffer-filter
+                         (lambda ()
+                           (let ((current (erc-default-target)))
+                             (and (stringp current)
+                                  (string-match (car l)
+                                                (or erc-session-server 
erc-server-announced-name))
+                                  (string-equal (erc-downcase chan)
+                                                (erc-downcase current)))))))))
              (when (or (not buffer)
-                       ;; If the same channel is joined on another
-                       ;; server the best-effort is to just join
-                       (not (string-match (car l)
-                                          (process-name erc-server-process)))
                        (not (with-current-buffer buffer
                               (erc-server-process-alive))))
                (erc-server-join-channel server chan))))))))
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 3880778794..2b2cc578d9 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -1606,33 +1606,45 @@ symbol, it may have these values:
 (defun erc-generate-new-buffer-name (server port target)
   "Create a new buffer name based on the arguments."
   (when (numberp port) (setq port (number-to-string port)))
-  (let ((buf-name (or target
-                      (or (let ((name (concat server ":" port)))
-                            (when (> (length name) 1)
-                              name))
-                          ;; This fallback should in fact never happen
-                          "*erc-server-buffer*")))
-        buffer-name)
+  (let* ((buf-name (or target
+                       (or (let ((name (concat server ":" port)))
+                             (when (> (length name) 1)
+                               name))
+                           ;; This fallback should in fact never happen
+                           "*erc-server-buffer*")))
+         (full-buf-name (concat buf-name "/" server))
+         (dup-buf-name (car (mapcar #'(lambda (chanbuf)
+                                        (buffer-name chanbuf))
+                                    (erc-channel-list nil))))
+         buffer-name)
     ;; Reuse existing buffers, but not if the buffer is a connected server
     ;; buffer and not if its associated with a different server than the
     ;; current ERC buffer.
     ;; if buf-name is taken by a different connection (or by something !erc)
     ;; then see if "buf-name/server" meets the same criteria
-    (dolist (candidate (list buf-name (concat buf-name "/" server)))
-      (if (and (not buffer-name)
-               erc-reuse-buffers
-               (or (not (get-buffer candidate))
-                   (or target
-                       (with-current-buffer (get-buffer candidate)
-                         (and (erc-server-buffer-p)
-                              (not (erc-server-process-alive)))))
-                   (with-current-buffer (get-buffer candidate)
-                     (and (string= erc-session-server server)
-                          (erc-port-equal erc-session-port port)))))
-          (setq buffer-name candidate)))
+    (if (and dup-buf-name (string-match (concat buf-name "/") dup-buf-name))
+        (setq buffer-name full-buf-name) ; already exists erc buffer with the 
full name
+      (dolist (candidate (list buf-name full-buf-name))
+        (if (and (not buffer-name)
+                 erc-reuse-buffers
+                 (or (not (get-buffer candidate))
+                     (with-current-buffer (get-buffer candidate)
+                       (and (erc-server-buffer-p)
+                            (not (erc-server-process-alive))))
+                     (with-current-buffer (get-buffer candidate)
+                       (and (string= erc-session-server server)
+                            (erc-port-equal erc-session-port port)))))
+            (setq buffer-name candidate)
+          (when (and (not buffer-name) (get-buffer buf-name) erc-reuse-buffers)
+            ;; a new buffer will be created with the name buf-name/server, 
rename
+            ;; the existing name-duplicated buffer with the same format as well
+            (with-current-buffer (get-buffer buf-name)
+              (when (derived-mode-p 'erc-mode) ; ensure it's an erc buffer
+                (rename-buffer
+                 (concat buf-name "/" (or erc-session-server 
erc-server-announced-name)))))))))
     ;; if buffer-name is unset, neither candidate worked out for us,
     ;; fallback to the old <N> uniquification method:
-    (or buffer-name (generate-new-buffer-name (concat buf-name "/" server)))))
+    (or buffer-name (generate-new-buffer-name full-buf-name))))

 (defun erc-get-buffer-create (server port target)
   "Create a new buffer based on the arguments."
@@ -3097,16 +3109,18 @@ were most recently invited.  See also `invitation'."
       (setq chnl (erc-ensure-channel-name channel)))
     (when chnl
       ;; Prevent double joining of same channel on same server.
-      (let ((joined-channels
-             (mapcar #'(lambda (chanbuf)
-                         (with-current-buffer chanbuf (erc-default-target)))
-                     (erc-channel-list erc-server-process))))
-        (if (erc-member-ignore-case chnl joined-channels)
-            (switch-to-buffer (car (erc-member-ignore-case chnl
-                                                           joined-channels)))
-         (let ((server (with-current-buffer (process-buffer erc-server-process)
-                         (or erc-session-server erc-server-announced-name))))
-           (erc-server-join-channel server chnl key))))))
+      (let* ((joined-channels
+              (mapcar #'(lambda (chanbuf)
+                          (with-current-buffer chanbuf (erc-default-target)))
+                      (erc-channel-list erc-server-process)))
+             (server (with-current-buffer (process-buffer erc-server-process)
+                      (or erc-session-server erc-server-announced-name)))
+             (chnl-name (car (erc-member-ignore-case chnl joined-channels))))
+        (if chnl-name
+            (switch-to-buffer (if (get-buffer chnl-name)
+                                  chnl-name
+                                (concat chnl-name "/" server)))
+         (erc-server-join-channel server chnl key)))))
   t)

 (defalias 'erc-cmd-CHANNEL 'erc-cmd-JOIN)
--
2.27.0