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+;;; filelock-tests.el --- test file locking -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021  Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file tests code in src/filelock.c and, to some extent, the
+;; related code in src/fileio.c.
+;;
+;; See also (info "(emacs)Interlocking") and (info "(elisp)File Locks")
+
+;;; Code:
+
+(require 'cl-macs)
+(require 'ert)
+(require 'seq)
+
+(defun filelock-tests--fixture (test-function)
+  "Call TEST-FUNCTION under a test fixture.
+Create a test directory and a buffer whose `buffer-file-name' and
+`buffer-file-truename' are a file within it, then call
+TEST-FUNCTION.  Finally, delete the buffer and the test
+directory."
+  (let* ((temp-dir (make-temp-file "filelock-tests" t))
+         (name (concat (file-name-as-directory temp-dir)
+                       "userfile"))
+         (create-lockfiles t))
+    (unwind-protect
+        (with-temp-buffer
+          (setq buffer-file-name name
+                buffer-file-truename name)
+          (unwind-protect
+              (save-current-buffer
+                (funcall test-function))
+            ;; Set `buffer-file-truename' nil to prevent unlocking,
+            ;; which might prompt the user and/or signal errors.
+            (setq buffer-file-name nil
+                  buffer-file-truename nil)))
+      (delete-directory temp-dir t nil))))
+
+(defun filelock-tests--make-lock-name (file-name)
+  "Return the lock file name for FILE-NAME.
+Equivalent logic in Emacs proper is implemented in C and
+unavailable to Lisp."
+  (concat (file-name-directory (expand-file-name file-name))
+          ".#"
+          (file-name-nondirectory file-name)))
+
+(defun filelock-tests--spoil-lock-file (file-name)
+  "Spoil the lock file for FILE-NAME.
+Cause Emacs to report errors for various file locking operations
+on FILE-NAME going forward.  Create a file that is incompatible
+with Emacs' file locking protocol, but uses the same name as
+FILE-NAME's lock file.  A directory file is used, which is
+portable in practice."
+  (make-directory (filelock-tests--make-lock-name file-name)))
+
+(defun filelock-tests--unspoil-lock-file (file-name)
+  "Remove the lock file spoiler for FILE-NAME.
+See `filelock-tests--spoil-lock-file'."
+  (delete-directory (filelock-tests--make-lock-name file-name) t))
+
+(defun filelock-tests--should-be-locked ()
+  "Abort the current test if the current buffer is not locked.
+Exception: on systems without lock file support, aborts the
+current test if the current file is locked (which should never
+the case)."
+  (if (eq system-type 'ms-dos)
+      (should-not (file-locked-p buffer-file-truename))
+    (should (file-locked-p buffer-file-truename))))
+
+(ert-deftest filelock-tests-lock-unlock-no-errors ()
+  "Check that locking and unlocking works without error."
+  (filelock-tests--fixture
+   (lambda ()
+     (should-not (file-locked-p (buffer-file-name)))
+
+     ;; inserting text should lock the buffer's file.
+     (insert "this locks the buffer's file")
+     (filelock-tests--should-be-locked)
+     (unlock-buffer)
+     (set-buffer-modified-p nil)
+     (should-not (file-locked-p (buffer-file-name)))
+
+     ;; `set-buffer-modified-p' should lock the buffer's file.
+     (set-buffer-modified-p t)
+     (filelock-tests--should-be-locked)
+     (unlock-buffer)
+     (should-not (file-locked-p (buffer-file-name)))
+
+     (should-not (file-locked-p (buffer-file-name))))))
+
+(ert-deftest filelock-tests-lock-spoiled ()
+  "Check `lock-buffer' ."
+  (skip-unless (not (eq system-type 'ms-dos))) ; no filelock support
+  (filelock-tests--fixture
+   (lambda ()
+     (filelock-tests--spoil-lock-file buffer-file-truename)
+     ;; FIXME: errors when locking a file are ignored; should they be?
+     (set-buffer-modified-p t)
+     (filelock-tests--unspoil-lock-file buffer-file-truename)
+     (should-not (file-locked-p buffer-file-truename)))))
+
+(ert-deftest filelock-tests-file-locked-p-spoiled ()
+  "Check that `file-locked-p' fails if the lockfile is \"spoiled\"."
+  (skip-unless (not (eq system-type 'ms-dos))) ; no filelock support
+  (filelock-tests--fixture
+   (lambda ()
+     (filelock-tests--spoil-lock-file buffer-file-truename)
+     (let ((err (should-error (file-locked-p (buffer-file-name)))))
+       (should (equal (seq-subseq err 0 2)
+                      '(file-error "Testing file lock")))))))
+
+(ert-deftest filelock-tests-unlock-spoiled ()
+  "Check that `unlock-buffer' fails if the lockfile is \"spoiled\"."
+  (skip-unless (not (eq system-type 'ms-dos))) ; no filelock support
+  (filelock-tests--fixture
+   (lambda ()
+     ;; Set the buffer modified with file locking temporarily
+     ;; disabled.
+     (let ((create-lockfiles nil))
+       (set-buffer-modified-p t))
+     (should-not (file-locked-p buffer-file-truename))
+     (filelock-tests--spoil-lock-file buffer-file-truename)
+
+     ;; Errors from `unlock-buffer' should call
+     ;; `userlock--handle-unlock-error' (bug#46397).
+     (let (errors)
+       (cl-letf (((symbol-function 'userlock--handle-unlock-error)
+                  (lambda (err) (push err errors))))
+         (unlock-buffer))
+       (should (consp errors))
+       (should (equal '(file-error "Unlocking file")
+                      (seq-subseq (car errors) 0 2)))
+       (should (equal (length errors) 1))))))
+
+(ert-deftest filelock-tests-kill-buffer-spoiled ()
+  "Check that `kill-buffer' fails if a lockfile is \"spoiled\"."
+  (skip-unless (not (eq system-type 'ms-dos))) ; no filelock support
+  (filelock-tests--fixture
+   (lambda ()
+     ;; Set the buffer modified with file locking temporarily
+     ;; disabled.
+     (let ((create-lockfiles nil))
+       (set-buffer-modified-p t))
+     (should-not (file-locked-p buffer-file-truename))
+     (filelock-tests--spoil-lock-file buffer-file-truename)
+
+     ;; Kill the current buffer.  Because the buffer is modified Emacs
+     ;; will attempt to unlock it.  Temporarily bind `yes-or-no-p' to
+     ;; a function that fakes a "yes" answer for the "Buffer modified;
+     ;; kill anyway?" prompt.
+     ;;
+     ;; File errors from unlocking files should call
+     ;; `userlock--handle-unlock-error' (bug#46397).
+     (let (errors)
+       (cl-letf (((symbol-function 'yes-or-no-p)
+                  (lambda (&rest _) t))
+                 ((symbol-function 'userlock--handle-unlock-error)
+                  (lambda (err) (push err errors))))
+         (kill-buffer))
+       (should (consp errors))
+       (should (equal '(file-error "Unlocking file")
+                      (seq-subseq (car errors) 0 2)))
+       (should (equal (length errors) 1))))))
+
+(provide 'filelock-tests)
+;;; filelock-tests.el ends here