Bug 63359 - cairo-1.12.14 - (Incorrectly?) treat libpng warnings as error, causing issues in applications
Summary: cairo-1.12.14 - (Incorrectly?) treat libpng warnings as error, causing issues...
Alias: None
Product: cairo
Classification: Unclassified
Component: png functions (show other bugs)
Version: 1.12.14
Hardware: Other All
: medium normal
Assignee: Carl Worth
QA Contact: cairo-bugs mailing list
Depends on:
Reported: 2013-04-10 07:21 UTC by Richard Grenville
Modified: 2013-04-16 10:16 UTC (History)
0 users

See Also:
i915 platform:
i915 features:

cairo-ignore-libpng-warnings.patch (474 bytes, patch)
2013-04-12 09:17 UTC, Richard Grenville
Details | Splinter Review

Description Richard Grenville 2013-04-10 07:21:23 UTC

When reading a PNG file, cairo treats libpng warnings as errors.

static void
png_simple_warning_callback (png_structp png,
	                     png_const_charp error_msg)
    cairo_status_t *error = png_get_error_ptr (png);

    /* default to the most likely error */
    if (*error == CAIRO_STATUS_SUCCESS)
	*error = _cairo_error (CAIRO_STATUS_NO_MEMORY);

    /* png does not expect to abort and will try to tidy up after a warning */

// ...

static cairo_surface_t *
read_png (struct png_read_closure_t *png_closure)

// ...
    png = png_create_read_struct (PNG_LIBPNG_VER_STRING,

// ...
    if (unlikely (status)) { /* catch any early warnings */
	surface = _cairo_surface_create_in_error (status);
	goto BAIL;

This prevents some PNGs with problems to be loaded with cairo_image_surface_create_from_png(). Not sure if it's intentional or a mistake, but it's causing breakages in fcitx-4.2.7 recently.

How to reproduce the issue:

Here's slightly broken PNG image, included in fcitx-4.2.7: https://raw.github.com/fcitx/fcitx/be9f3af1aa5eb4e037ba6e6dee098edb7e41e9cc/skin/default/cn.png

Small test program to load a PNG with cairo, taken from < http://permalink.gmane.org/gmane.comp.lib.cairo/23741 >:
#include <stdio.h>
#include <cairo/cairo.h>

int main(int argc, char *argv[])
  cairo_surface_t *surface;

  surface = cairo_image_surface_create_from_png(argv[1]);
  if (cairo_surface_status(surface))  
    fprintf(stderr, "Error reading PNG image %s: %s\n",
            argv[1], cairo_status_to_string (cairo_surface_status(surface)));
    fprintf(stderr, "PNG image read.\n");

  return 0;

With <libpng-1.6, cairo loads this image correctly; with libpng-1.6.1, cairo complains "Error reading PNG image cn.png: out of memory". (There's another issue regarding libpng-1.6, #62779, that could cause the same result, and the patch there needs to be applied firstly.)

Background - fcitx-4.2.7 segfaults because of the problem:

After upgrading to libpng-1.6.1, I found fcitx segfaulting immediately after execution. It looks like an issue in cairo and applying the patch from #62779 to cairo-1.12.12 fixed it. But later I found fcitx is still segfaulting sometimes, because it fails to load some particular PNG files with cairo_image_surface_create_from_png() and it isn't handling the failure gracefully. The PNG file used to work correctly with cairo and libpng-1.5, but with libpng-1.6.1 cairo could no longer load it. Further debugging reveals cairo refuses to load the file because libpng-1.6.1 emits a warning:

Breakpoint 5, png_simple_warning_callback (png=0x555555759c00, 
    error_msg=0x7fffffffd870 "iCCP: known incorrect sRGB profile") at cairo-png.c:151

"iCCP: known incorrect sRGB profile" isn't actually a fatal issue in a PNG file, and <libpng-1.6 probably doesn't emit this warning. Yet since cairo treats libpng warnings as errors, it refuses to the load the file.

I've already reported the issue to fcitx devs < http://code.google.com/p/fcitx/issues/detail?id=678 >, but there are many, many, PNGs in the world that are just slightly broken. Maybe cairo could be a little more tolerant on libpng warnings? I'm personally using a custom patch to ignore libpng warnings in png_simple_warning_callback() as a workaround.
Comment 1 Richard Grenville 2013-04-12 09:17:58 UTC
Created attachment 77853 [details] [review]

cairo-ignore-libpng-warnings.patch. Just an ugly patch to ignore libpng warnings right now.
Comment 2 Richard Grenville 2013-04-13 10:20:27 UTC
The PNG images bundled with fcitx have been fixed ( https://github.com/fcitx/fcitx/commit/dc36c1a88f863971f81a76b8d9ec0db23b0d9387 ), yet I was told by the fcitx developer that many other PNG images have an incorrect ICC color profile. He suggested me to search in /usr/share/icons `(find /usr/share/icons -iname '*.png' -type f -exec identify -verbose '{}' +) |& grep 'known incorrect sRGB profile'` and I found like 7-8 images with the same problem under libpng-1.6. Thus with libpng-1.6, cairo will unfortunately still refuse to load many images.
Comment 3 Chris Wilson 2013-04-16 10:16:04 UTC
commit 2dd2c826a5b367d32cf2d48ed69754795990c5db
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Apr 16 10:58:56 2013 +0100

    png: Avoid marking the surface as in error after a png warning

Missed this bug report (amongst the various reports of the libpng16 api breakage), sorry.

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