bug#46397: 27.1; Cannot delete buffer pointing to a file in a path that includes a file

[Matt Armstrong]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Matt Armstrong <gmatta@gmail.com>
>> Cc: 46397@debbugs.gnu.org, eggert@cs.ucla.edu, craven@gmx.net
>> Date: Sun, 14 Feb 2021 14:16:12 -0800
>> 
>> When releasing the lock, I have a less clear opinion but I'm thinking
>> that warnings are better. A warning is still quite intrusive and
>> obvious. Maybe we don't need to decide this part now.
>
> The problem with warnings is that they can go unnoticed, unless
> followed by sit-for.

Eli, Paul,

Attached is test code for some of the scenarios we are discussing here.

My FSF papers are out of date. Mind sending me new ones?  I am in the
United States.

These tests operate by constructing buffers within directories that the
test then restricts permissions on, which induces errors from the varous
locking calls.  Each test is a FIXME that allows us to verify the
behavior change as work on bug#46397 progresses.

In doing this I discovered that Emacs silently ignores some IO errors
when constructing lock files.  Paul noticed this a while back and added
FIXME.  This patch adds a test with a corresponding FIXME.

The test isn't complete.  It relies on POSIX semantics from the
filesystem.  It might fail on Windows.  As the review progresses I can
work on handling that scenario more gracefully.

If this kind of a test is a bad idea, feel free to let me know.  I like
to have test coverage before I do code surgery, but I don't have strong
opinions here.

>From 99577c5a0b582e6d0c3d9fd495dec9f27d0cb7a2 Mon Sep 17 00:00:00 2001
From: Matt Armstrong <matt@rfc20.org>
Date: Mon, 15 Feb 2021 12:59:08 -0800
Subject: [PATCH] Add some test coverage for src/filelock.c (Bug#46397).

* test/src/filelock-tests.el: new file
---
 test/src/filelock-tests.el | 115 +++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 test/src/filelock-tests.el

diff --git a/test/src/filelock-tests.el b/test/src/filelock-tests.el
new file mode 100644
index 0000000000..8cdcec09e7
--- /dev/null
+++ b/test/src/filelock-tests.el
@@ -0,0 +1,115 @@
+;;; 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.
+
+;;; Code:
+
+(require 'ert)
+
+(defun filelock-tests--call-with-fixture (body)
+  (let ((test-dir (make-temp-file "filelock-tests-" t)))
+    (unwind-protect
+        (let ((file-name (concat (file-name-as-directory test-dir))))
+          (with-temp-buffer
+            (setq-local buffer-file-name file-name)
+            (setq-local buffer-file-truename file-name)
+            (unwind-protect
+                (save-current-buffer
+                  (let ((create-lockfiles t))
+                    (funcall body)))
+              (set-buffer-modified-p nil))))
+      (delete-directory test-dir t nil))))
+
+(defmacro filelock-tests--with-fixture (&rest body)
+  "Create a test fixture and evaluate BODY there like `progn'.
+Create a temporary directory, then create a temporary buffer and
+set it the variable `buffer-file-name' and `buffer-file-truename'
+to it, let bind `create-lockfiles' to 't' and evaluate BODY."
+  (declare (indent 0))
+  `(filelock-tests--call-with-fixture
+    (lambda () ,@body)))
+
+(defun filelock-tests--call-with-directory-modes (modes body)
+  (let* ((dir (file-name-directory (buffer-file-name)))
+         (original-modes (file-modes dir)))
+    (set-file-modes dir modes)
+    (unwind-protect
+        (funcall body)
+      (set-file-modes dir original-modes))))
+
+(defmacro filelock-tests--with-directory-modes (modes &rest body)
+  "Evaluate BODY forms with directory MODES set.
+The directory is taken from the buffer local variable
+`buffer-file-name'.  Afterwards, restore the modes to what they
+were."
+  (declare (indent 1))
+  `(filelock-tests--call-with-directory-modes
+    ,modes (lambda () ,@body)))
+
+(ert-deftest filelock-tests-lock-buffer-permission-denied ()
+  "Check that locking a buffer in a directory with no write
+permissions does not work."
+  (filelock-tests--with-fixture
+      (should (not (file-locked-p (buffer-file-name))))
+     (filelock-tests--with-directory-modes #o500
+       ;; FIXME: (lock-buffer) should raise some sort of
+       ;; error/warning here that we should verify in test.
+       ;; Currently lock_file() ignores errors from lock_if_free().
+       ;; See the related FIXME in filelock.c.
+       (lock-buffer)
+       (should (not (file-locked-p (buffer-file-name)))))))
+
+(ert-deftest filelock-tests-file-locked-p-permission-denied ()
+  "Check that `file-locked-p' fails if the directory is inaccesible."
+  (filelock-tests--with-fixture
+    (filelock-tests--with-directory-modes #o000
+      (set-buffer-modified-p t)
+      ;; FIXME: Bug#46397: `file-locked-p' currently signals errors,
+      ;; but this prevents the user from killing the buffer.  It also
+      ;; prevents Emacs shutdown.
+      (should
+       (equal
+        (butlast (should-error (file-locked-p (buffer-file-name)))
+                 2)
+        '(file-error "Testing file lock"))))))
+
+(ert-deftest filelock-tests-unlock-permission-denied ()
+  "Check that `unlock-buffer' fails in directories that cannot be
+modified."
+  (filelock-tests--with-fixture
+    (set-buffer-modified-p t)
+    (lock-buffer)
+    (should (file-locked-p (buffer-file-name)))
+    (filelock-tests--with-directory-modes #o500
+      ;; FIXME: Bug#46397: `unlock-buffer' currently signals errors,
+      ;; but this prevents the user from killing the buffer.  It also
+      ;; prevents Emacs shutdown.
+      (should
+       (equal
+        (butlast (should-error (unlock-buffer))
+                 2)
+        '(file-error "Unlocking file"))))))
+
+(provide 'filelock-tests)
+
+;;; filelock-tests.el ends here
-- 
2.30.0