For SVG, 8192 is the new 256

Prefer librsvg for display of splash.svg
When both librsvg and Imagemagick are available, Emacs should
prefer librsvg to render SVG images.  However, Emacs was using
Imagemagick to render its own splash.svg file because
image-type-from-file-header returned nil for that file.
* lisp/image.el (image-type-from-buffer)
(image-type-from-file-header): Look at the first 8192 bytes of
the image, not just the first 256.  For Emacs’s own splash.svg
file, image-type-header-regexps needs to look at 939 bytes.
8192 bytes is a reasonable number nowadays given typical file
system design.
* test/lisp/image-tests.el (image-tests--emacs-images-directory):
New contant.
(image-type-from-file-header-test): New test.

2 files changed, 13 insertions, 2 deletions

diff --git a/lisp/image.el b/lisp/image.el
index ba87d7f7859..db113020866 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -315,7 +315,7 @@ be determined."
                                         (buffer-substring
                                          (point-min)
                                          (min (point-max)
-                                              (+ (point-min) 256))))))
+                                              (+ (point-min) 8192))))))
                      (setq image-type (cdr image-type))))
             (setq type image-type
                   types nil)
@@ -339,7 +339,7 @@ be determined."
        (file-readable-p file)
        (with-temp-buffer
          (set-buffer-multibyte nil)
-         (insert-file-contents-literally file nil 0 256)
+         (insert-file-contents-literally file nil 0 8192)
          (image-type-from-buffer))))
 
 
diff --git a/test/lisp/image-tests.el b/test/lisp/image-tests.el
index 89b926e629d..621646e5750 100644
--- a/test/lisp/image-tests.el
+++ b/test/lisp/image-tests.el
@@ -22,6 +22,10 @@
 (require 'ert)
 (require 'image)
 
+(defconst image-tests--emacs-images-directory
+  (expand-file-name "../etc/images" (getenv "EMACS_TEST_DIRECTORY"))
+  "Directory containing Emacs images.")
+
 (ert-deftest image--set-property ()
   "Test `image--set-property' behavior."
   (let ((image (list 'image)))
@@ -42,4 +46,11 @@
     (setf (image-property image :width) nil)
     (should (equal image '(image)))))
 
+(ert-deftest image-type-from-file-header-test ()
+  "Test image-type-from-file-header."
+  (should (eq 'svg
+              (image-type-from-file-header
+               (expand-file-name "splash.svg"
+                                 image-tests--emacs-images-directory)))))
+
 ;;; image-tests.el ends here