[cairo-bugs] [Bug 83822] New: device-offset test randomly fails because XCreatePixmap bits are undefined

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=83822

          Priority: medium
            Bug ID: 83822
          Assignee: chris at chris-wilson.co.uk
           Summary: device-offset test randomly fails because
                    XCreatePixmap bits are undefined
        QA Contact: cairo-bugs at cairographics.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: sixtysix at inwind.it
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: xlib backend
           Product: cairo

Created attachment 106221
  --> https://bugs.freedesktop.org/attachment.cgi?id=106221&action=edit
proposed patch

while running

DISPLAY=:2 make test TARGETS=image,xlib

with Xvfb on display :2, to check a fix for bug #74779, occasionally
device-offset.xlib-render-0_0.rgb24 was reported PASS, even if the code
modified was not executed at all by that test.

The problem is that in that test (and few others) a temporary Pixmap is
created on the server and because render-0_0 does not have FillRectangles
an image surface is used to clear the Pixmap, but because the image surface
is already 'clear' it is left untouched and in _cairo_surface_unmap the 'clear'
image is not unmapped because untouched (image->base.serial == 0)

http://cgit.freedesktop.org/cairo/tree/src/cairo-surface.c#n688

this means X's Pixmap is left in its original undefined state and only
occasionally it is clear.

I suggest to increment the serial member of the image_surface when the
'is_clear' state of the image_surface does not correspond to a clear
content of the corresponding xlib_surface, as in the attached patch.

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