Calling push_group on unbounded cairo recording surface

Hello,

I have an unbounded cairo recording surface, on which I want to draw. 
Unforntunately, when I start to use cairo_push_group, my whole drawing 
is gone (ie. cairo_recording_surface_ink_extents gives me only 0s).

While actual usage in my app is quite complicated, I managed to narrow 
the problem to this:

#include <cairo/cairo.h>
#include <cairo/cairo-pdf.h>

void dump_to_file(cairo_surface_t* src)
{
     double imgw = 50, imgh = 50;
     cairo_surface_t* dst = cairo_pdf_surface_create("test.pdf", imgw, 
imgh);
     cairo_t* c = cairo_create(dst);
     double x,y,w,h;
     cairo_recording_surface_ink_extents(src,&x,&y,&w,&h);
     cairo_scale(c, imgw/w, imgh/h);
     cairo_set_source_surface(c, src, -x, -y);
     cairo_paint(c);
     cairo_destroy(c);
     cairo_surface_destroy(dst);
}

void draw(cairo_t *ctx)
{
     cairo_set_source_rgb(ctx, 0, 0.8, 0);
     cairo_set_line_width(ctx, 1);
     cairo_move_to(ctx, 5,5);
     cairo_line_to(ctx, 10, 10);
     cairo_stroke(ctx);
}

int main(int argc, char**argv)
{
     cairo_surface_t* s = 
cairo_recording_surface_create(CAIRO_CONTENT_COLOR_ALPHA, nullptr);
     cairo_t* ctx = cairo_create(s);

     //cairo_push_group(ctx);
     draw(ctx);
     //cairo_pop_group_to_source(ctx);
     //cairo_paint(ctx);

     dump_to_file(s);
     cairo_destroy(ctx);
     cairo_surface_destroy(s);

     return 0;
}

Above code works until I uncomment lines around the call to draw(...). 
But if I provide (big enough) bounding rectangle instead of nullptr, 
then uncommenting push/pop lines makes no difference. So I can either 
have a stack of bounded surfaces or single unbounded surface. How do I 
get a stack of unbounded surfaces? Is there a bug in cairo (I am using 
version 2.12.2 at the moment), or is it a bug in my understanding?

Kind regards,
Daniel
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