This error has returned after maybe five years.

I tested 17.1.8, 17.1.9, and 17.2.0 in i686 and x86_64.
The app that crashes is the gtk2 color changer and pixpbuf demos.
I tried no other gtk apps. Glxgears works. The demos crash at startup.
What should have happened: color-chooser should appear or pixbuf demo should appear. To reproduce: run gtk-demos (gtk2) and select those demos.

The program 'gtk2-demo-64' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadLength (poly request too large or internal Xlib length erro'.
  (Details: serial 2460 error_code 16 request_code 72 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

The serial number changes, but the rest is the same.

config for mesa:

        OpenGL:          yes (ES1: yes ES2: yes)

        OSMesa:          libOSMesa

        DRI platform:    drm
        DRI drivers:     i915 i965 nouveau r200 radeon swrast
        DRI driver dir:  ${libdir}/dri
        GLX:             DRI-based

        EGL:             yes
        EGL drivers:     builtin:egl_dri2 builtin:egl_dri3
        GBM:             yes
        EGL/Vulkan/VL platforms:   drm x11 wayland surfaceless

        Vulkan drivers:  no

        llvm:            no

        Gallium drivers: i915 nouveau r600 svga swrast virgl
        Gallium st:      mesa xa xvmc vdpau

        HUD extra stats: no
        HUD lmsensors:   no


        Shared libs:     yes
        Static libs:     no
        Shared-glapi:    yes

        CFLAGS:          -O2 -Wall -std=c99 -Werror=implicit-function-declaration -Werror=missing-prototypes
 -fno-math-errno -fno-trapping-math
        CXXFLAGS:        -O2 -Wall -fno-math-errno -fno-trapping-math
        LDFLAGS:
        Macros:          -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D_GNU_SOURCE
 -DUSE_SSE41 -DUSE_GCC_ATOMIC_BUILTINS -DNDEBUG -DTEXTURE_FLOAT_ENABLED -DHAVE_SYS_SYSCTL_H -DHAVE_STRTOF -D
HAVE_MKOSTEMP -DHAVE_DLOPEN -DHAVE_DL_ITERATE_PHDR -DHAVE_POSIX_MEMALIGN -DHAVE_LIBDRM -DGLX_USE_DRM -DGLX_I
NDIRECT_RENDERING -DGLX_DIRECT_RENDERING -DGLX_USE_TLS -DHAVE_DRM_PLATFORM -DHAVE_X11_PLATFORM -DHAVE_WAYLAN
D_PLATFORM -DHAVE_SURFACELESS_PLATFORM -DHAVE_DRI3 -DENABLE_SHADER_CACHE -DHAVE_MINCORE -DHAVE_ST_VDPAU

        PYTHON2:         python2.7

The config for gtk2 is vanilla.

BTW, mesa 17.2.0 fails one test (no failures in 17.1.8, 17.1.9):

FAIL: main-test
============================================================================
Testsuite summary for Mesa 17.2.0
============================================================================
# TOTAL: 1
# PASS:  0
# SKIP:  0
# XFAIL: 0
# FAIL:  1
# XPASS: 0
# ERROR: 0
============================================================================
See src/mesa/main/tests/test-suite.log
Please report to https://bugs.freedesktop.org/enter_bug.cgi?product=Mesa
============================================================================