Restore erc-last-saved-position from previous session

* lisp/erc/erc-log.el (erc-log-setup-logging): Restore
`erc-last-saved-position' from previous session.  By default, a
non-/QUIT disconnect does not write out any remaining buffer text to
logs, instead leaving it until the buffer or Emacs is killed.  But if a
successful reconnect occurs beforehand, the uncommitted portion must be
seen to somehow.  Before this change, it would be lost because the
function `erc-initialize-log-marker' remakes the marker at the prompt
instead of recovering the previous value as now done here.  Moreover,
the traditional workaround of customizing `erc-log-write-after-insert'
and `erc-log-write-after-send' to t should not be required to prevent
gaps in any decent IRC client.
* test/lisp/erc/erc-scenarios-log.el (erc-scenarios-log--reconnect): New
function.
(erc-scenarios-log--reconnect/auto, erc-scenarios-log--reconnect/manual):
New tests.
;; * test/lisp/erc/resources/join/reconnect/foonet-again.eld: Add QUIT.

lisp/erc/erc-log.el

@@ -256,6 +256,12 @@ This function is destined to be run from `erc-connect-pre-hook'.
 The current buffer is given by BUFFER."
   (when (erc-logging-enabled buffer)
     (with-current-buffer buffer
+      (when-let* ((erc-last-saved-position)
+                  (priors (or erc--server-reconnecting erc--target-priors))
+                  (val (alist-get 'erc-last-saved-position priors))
+                  (_ (eq buffer (marker-buffer val))))
+        ;; Will have been initialized by `erc-initialize-log-marker'.
+        (move-marker erc-last-saved-position val))
       (auto-save-mode -1)
       (setq buffer-file-name nil)
       (add-hook 'write-file-functions #'erc-save-buffer-in-logs nil t)

test/lisp/erc/erc-scenarios-log.el

@@ -460,4 +460,116 @@
     (erc-truncate-mode -1)
     (when noninteractive (delete-directory tempdir :recursive))))
 
+;; These tests check whether logs contain gaps when reconnecting.
+(defun erc-scenarios-log--reconnect (autop)
+
+  (erc-scenarios-common-with-cleanup
+      ((erc-scenarios-common-dialog "join/reconnect")
+       (dumb-server (erc-d-run "localhost" t 'foonet 'foonet-again))
+       (tempdir (make-temp-file "erc-tests-log." t nil nil))
+       (erc-log-channels-directory tempdir)
+       (erc-modules `(log ,@erc-modules))
+       (erc-timestamp-format-left "\n[@@DATE__STAMP@@]\n")
+       (port (process-contact dumb-server :service))
+       (erc-server-auto-reconnect autop)
+       (erc-server-flood-penalty 0.1)
+       (expect (erc-d-t-make-expecter))
+       ;; Bind these so they'll be killed on teardown.
+       (server-log-buffer (get-buffer-create "*erc-log FooNet*"))
+       (chan-log-buffer (get-buffer-create "*erc-log #chan*"))
+       (spam-log-buffer (get-buffer-create "*erc-log #spam*")))
+
+    (ert-info ("Connect")
+      (with-current-buffer (erc :server "127.0.0.1"
+                                :port port
+                                :nick "tester"
+                                :password "changeme"
+                                :full-name "tester")
+        (funcall expect 10 "debug mode")))
+
+    (ert-info ("#chan populated")
+      (with-current-buffer (erc-d-t-wait-for 10 (get-buffer "#chan"))
+        (funcall expect 10 "@@DATE__STAMP@@")
+        (funcall expect 10 "<alice> tester, welcome")))
+
+    (ert-info ("#spam populated")
+      (with-current-buffer (erc-d-t-wait-for 10 (get-buffer "#spam"))
+        (funcall expect 10 "@@DATE__STAMP@@")
+        (funcall expect 10 "<alice> tester, welcome")))
+
+    (ert-info ("Reconnect")
+      (with-current-buffer "FooNet"
+        (funcall expect 10 "Connection failed!")
+
+        (if autop
+            (funcall expect 10 "Reconnecting")
+          (erc-scenarios-common-say "/reconnect"))
+
+        (funcall expect 10 "Welcome")
+        (funcall expect 10 "debug mode")))
+
+    (with-current-buffer "#chan"
+      (funcall expect -0.01 "@@DATE__STAMP@@")
+      (funcall expect 10 "<alice> bob: Well, this"))
+
+    (with-current-buffer "#spam"
+      (funcall expect -0.01 "@@DATE__STAMP@@")
+      (funcall expect 10 "<alice> bob: Our queen and all"))
+
+    (with-current-buffer "FooNet"
+      (erc-scenarios-common-say "/quit")
+      (funcall expect 10 "Quit"))
+
+    (with-current-buffer "FooNet"
+      (let ((file (erc-current-logfile (current-buffer))))
+        (with-current-buffer server-log-buffer
+          (insert-file-contents file)
+          (funcall expect 1 "@@DATE__STAMP@@")
+          (funcall expect 1 "*** Welcome to the foonet")
+          (funcall expect 1 "debug mode")
+          (funcall expect 1 "*** Connection failed!")
+          ;; Full output again on reconnect.
+          (funcall expect -0.01 "@@DATE__STAMP@@") ; but no stamp
+          (funcall expect 1 "*** Welcome to the foonet")
+          (funcall expect 1 "debug mode"))))
+
+    (with-current-buffer "#chan"
+      (let ((file (erc-current-logfile (current-buffer))))
+        (with-current-buffer chan-log-buffer
+          (insert-file-contents file)
+          (funcall expect 1 "@@DATE__STAMP@@")
+          (funcall expect 1 "*** You have joined channel #chan")
+          (funcall expect 1 "<alice> tester, welcome!")
+          ;; No stamp on reconnect.
+          (funcall expect -0.01 "@@DATE__STAMP@@")
+          (funcall expect 1 "*** You have joined channel #chan")
+          (funcall expect 1 "<alice> bob: Well, this is the forest"))))
+
+    (with-current-buffer "#spam"
+      (let ((file (erc-current-logfile (current-buffer))))
+        (with-current-buffer spam-log-buffer
+          (insert-file-contents file)
+          (funcall expect 1 "@@DATE__STAMP@@")
+          (funcall expect 1 "*** You have joined channel #spam")
+          (funcall expect 1 "<alice> tester, welcome!")
+          ;; No stamp on reconnect.
+          (funcall expect -0.01 "@@DATE__STAMP@@")
+          (funcall expect 1 "*** You have joined channel #spam")
+          (funcall expect 1 "<alice> bob: Our queen and all her elves come"))))
+
+    (erc-log-mode -1)
+
+    (if noninteractive
+        (delete-directory tempdir :recursive)
+      (add-hook 'kill-emacs-hook
+                (lambda () (delete-directory tempdir :recursive))))))
+
+(ert-deftest erc-scenarios-log--reconnect/auto ()
+  :tags '(:expensive-test)
+  (erc-scenarios-log--reconnect 'autop))
+
+(ert-deftest erc-scenarios-log--reconnect/manual ()
+  :tags '(:expensive-test)
+  (erc-scenarios-log--reconnect nil))
+
 ;;; erc-scenarios-log.el ends here

test/lisp/erc/resources/join/reconnect/foonet-again.eld

@@ -43,3 +43,7 @@
  (0 ":irc.foonet.org 329 tester #spam 1620104779")
  (0.1 ":bob!~u@rz2v467q4rwhy.irc PRIVMSG #spam :alice: Signior Iachimo will not from it. Pray, let us follow 'em.")
  (0.1 ":alice!~u@rz2v467q4rwhy.irc PRIVMSG #spam :bob: Our queen and all her elves come here anon."))
+
+((quit 10 "QUIT :\2ERC\2")
+ (0.07 ":tester!~u@h3f95zveyc38a.irc QUIT :Quit: \2ERC\2 5.5 (IRC client for GNU Emacs 30.0.50)")
+ (0.01 "ERROR :Quit: \2ERC\2 5.5 (IRC client for GNU Emacs 30.0.50)"))