Make REs in magic-(fallback-)mode-alist case-sensitive.

These variables are used for well-defined file formats where relaxed
case matching is not wanted usually.

* lisp/files.el (magic-mode-alist, magic-fallback-mode-alist): Update
the doc string.
(set-auto-mode): Make looking-at for elements of magic-mode-alist and
magic-fallback-mode-alist use case-fold-search == nil.
* lisp/files.el (files-test-magic-mode-alist-re-baseline)
(files-test-magic-mode-alist-re-no-match)
(files-test-magic-mode-alist-re-case-diff): Add.

2 files changed, 37 insertions, 8 deletions

diff --git a/lisp/files.el b/lisp/files.el
index e26b4820f5e..34fdc3031a9 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -2964,9 +2964,9 @@ associated with that interpreter in `interpreter-mode-alist'.")
   "Alist of buffer beginnings vs. corresponding major mode functions.
 Each element looks like (REGEXP . FUNCTION) or (MATCH-FUNCTION . FUNCTION).
 After visiting a file, if REGEXP matches the text at the beginning of the
-buffer, or calling MATCH-FUNCTION returns non-nil, `normal-mode' will
-call FUNCTION rather than allowing `auto-mode-alist' to decide the buffer's
-major mode.
+buffer (respecting case), or calling MATCH-FUNCTION returns non-nil,
+`normal-mode' will call FUNCTION rather than allowing `auto-mode-alist' to
+decide the buffer's major mode.
 
 If FUNCTION is nil, then it is not called.  (That is a way of saying
 \"allow `auto-mode-alist' to decide for these files.\")")
@@ -2998,9 +2998,9 @@ If FUNCTION is nil, then it is not called.  (That is a way of saying
   "Like `magic-mode-alist' but has lower priority than `auto-mode-alist'.
 Each element looks like (REGEXP . FUNCTION) or (MATCH-FUNCTION . FUNCTION).
 After visiting a file, if REGEXP matches the text at the beginning of the
-buffer, or calling MATCH-FUNCTION returns non-nil, `normal-mode' will
-call FUNCTION, provided that `magic-mode-alist' and `auto-mode-alist'
-have not specified a mode for this file.
+buffer (respecting case), or calling MATCH-FUNCTION returns non-nil,
+`normal-mode' will call FUNCTION, provided that `magic-mode-alist' and
+`auto-mode-alist' have not specified a mode for this file.
 
 If FUNCTION is nil, then it is not called.")
 (put 'magic-fallback-mode-alist 'risky-local-variable t)
@@ -3117,7 +3117,8 @@ we don't actually set it to the same mode the buffer already has."
                              ((functionp re)
                               (funcall re))
                              ((stringp re)
-                              (looking-at re))
+                              (let ((case-fold-search nil))
+                                (looking-at re)))
                              (t
                               (error
                                "Problem in magic-mode-alist with element %s"
@@ -3178,7 +3179,8 @@ we don't actually set it to the same mode the buffer already has."
                                            ((functionp re)
                                             (funcall re))
                                            ((stringp re)
-                                            (looking-at re))
+                                            (let ((case-fold-search nil))
+                                              (looking-at re)))
                                            (t
                                             (error
                                              "Problem with magic-fallback-mode-alist element: %s"
diff --git a/test/lisp/files-tests.el b/test/lisp/files-tests.el
index aa5dbe7acf9..df2c3f47ae0 100644
--- a/test/lisp/files-tests.el
+++ b/test/lisp/files-tests.el
@@ -1282,5 +1282,32 @@ renaming only, rather than modified in-place."
   (should (equal (file-size-human-readable 10000 'si " " "bit") "10 kbit"))
   (should (equal (file-size-human-readable 10000 'iec " " "bit") "9.8 Kibit")))
 
+(ert-deftest files-test-magic-mode-alist-re-baseline ()
+  "Test magic-mode-alist with RE, expected behaviour for match."
+  (let ((magic-mode-alist '(("my-tag" . text-mode))))
+    (with-temp-buffer
+      (insert "my-tag")
+      (normal-mode)
+      (should (eq major-mode 'text-mode)))))
+
+(ert-deftest files-test-magic-mode-alist-re-no-match ()
+  "Test magic-mode-alist with RE, expected behaviour for no match."
+  (let ((magic-mode-alist '(("my-tag" . text-mode))))
+    (with-temp-buffer
+      (insert "not-my-tag")
+      (normal-mode)
+      (should (not (eq major-mode 'text-mode))))))
+
+(ert-deftest files-test-magic-mode-alist-re-case-diff ()
+  "Test that regexps in magic-mode-alist are case-sensitive.
+See <https://debbugs.gnu.org/36401>."
+  (let ((case-fold-search t)
+        (magic-mode-alist '(("my-tag" . text-mode))))
+    (with-temp-buffer
+      (goto-char (point-min))
+      (insert "My-Tag")
+      (normal-mode)
+      (should (not (eq major-mode 'text-mode))))))
+
 (provide 'files-tests)
 ;;; files-tests.el ends here