.:: Marcos Dione/StyXman's glob ::. (Posts about mapnik)

Debuggung system libraries in Debian

Marcos Dione — Sat, 12 Apr 2025 14:22:22 GMT

"My maps do not render". It's always a simple question like this. The error message was quite cryptic:

Failed to parse color: "D27E01"

This makes sense, that is not a color; #D27E01 is. I thought "I might have made a typo". Searching through my map style gave nothing. Hmm,m that's weird. Maybe the database? I pick a few colors from the data, f.i. for bus or subte/metro/subway/underground lines. Nope, not that either. So where is it coming from?

I fire gdb and thanks to automatic debug symbol loading I get more info that I would otherwise get. First rock I stumble upon: the function where that error is raised is called for every parsed color. In a map style there are gazillion, and remember I'm also parsing some coming from the db. So to set a conditional break point. It's easy, right? Just break parse_color if [condition] and done!

Not so fast. To begin with, strings don't exist in C¹, it's the arrays and \0. Also, arrays don't exist in C, it's pointers and wishful thinking. condition will have to involve strcmp() and == 0. But the parameter is actually a std::string const& str, so it's in the C++ realm. After asking around, guinevere#gdb@libera.chat suggested break parse_color if $_streq(str.data(), "D27E01"), which worked.

The next step was to make sense of the code. mapnik, the library I'm debugging, is the most inscrutable code I have ever seen. Here's a full backtrace of the moment I hit the bug:

#0  mapnik::parse_color (str="D27E01") at ./src/color_factory.cpp:32
#1  0x00007ffff5921295 in mapnik::color::color (this=this@entry=0x7fffffffc0d0, str="D27E01", premultiplied=premultiplied@entry=false) at ./src/color.cpp:38
#2  0x00007ffff614051d in mapnik::evaluate_expression_wrapper<mapnik::color>::operator()<mapbox::util::variant<mapnik::value_null, bool, long, double, icu_76::UnicodeString, mapnik::attribute, mapnik::global_attribute, mapnik::geometry_type_attribute, mapbox::util::recursive_wrapper<mapnik::unary_node<mapnik::tags::negate> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::plus> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::minus> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::mult> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::div> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::mod> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::less> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::less_equal> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::greater> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::greater_equal> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::equal_to> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::not_equal_to> >, mapbox::util::recursive_wrapper<mapnik::unary_node<mapnik::tags::logical_not> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::logical_and> >, mapbox::util::recursive_wrapper<mapnik::binary_node<mapnik::tags::logical_or> >, mapbox::util::recursive_wrapper<mapnik::regex_match_node>, mapbox::util::recursive_wrapper<mapnik::regex_replace_node>, mapbox::util::recursive_wrapper<mapnik::unary_function_call>, mapbox::util::recursive_wrapper<mapnik::binary_function_call> >, mapnik::feature_impl, std::unordered_map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, mapnik::value_adl_barrier::value, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, mapnik::value_adl_barrier::value> > > > (this=<optimized out>, expr=..., feature=..., vars=...) at ./include/mapnik/symbolizer.hpp:284
#3  mapnik::extract_value<mapnik::color>::operator() (this=<optimized out>, expr=...) at ./include/mapnik/symbolizer.hpp:342
#4  apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:332
#5  0x00007ffff61405e8 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#6  0x00007ffff6140640 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#7  0x00007ffff61406a0 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#8  0x00007ffff6140700 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#9  0x00007ffff6140760 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#10 0x00007ffff61407c0 in apply<mapnik::detail::strict_value const&, mapnik::extract_value<mapnik::color> > (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#11 0x00007ffff61407eb in visit<mapnik::extract_value<mapnik::color>, mapnik::detail::strict_value const&> (v=..., f=...) at /usr/include/mapbox/variant.hpp:838
#12 0x00007ffff6140837 in mapnik::util::apply_visitor<mapnik::extract_value<mapnik::color>, mapnik::detail::strict_value const&> (v=..., f=...) at ./include/mapnik/util/variant.hpp:42
#13 0x00007ffff614090b in mapnik::get<mapnik::color, (mapnik::keys)9> (sym=..., feature=..., vars=std::unordered_map with 0 elements) at ./include/mapnik/symbolizer.hpp:335
#14 mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4>::process (this=0x7fffffffc890, sym=..., feature=..., prj_trans=...) at ./src/agg/process_line_symbolizer.cpp:95
#15 0x00007ffff597d682 in mapnik::process_impl<true>::process<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4>, mapnik::line_symbolizer, mapnik::feature_impl, mapnik::proj_transform> (ren=..., 
    sym=..., f=..., tr=...) at ./include/mapnik/symbolizer_dispatch.hpp:43
#16 0x00007ffff597d6e1 in mapbox::util::detail::dispatcher<void, mapnik::point_symbolizer, mapnik::line_symbolizer, mapnik::line_pattern_symbolizer, mapnik::polygon_symbolizer, mapnik::polygon_pattern_symbolizer, mapnik::raster_symbolizer, mapnik::shield_symbolizer, mapnik::text_symbolizer, mapnik::building_symbolizer, mapnik::markers_symbolizer, mapnik::group_symbolizer, mapnik::debug_symbolizer, mapnik::dot_symbolizer>::apply<mapbox::util::variant<mapnik::point_symbolizer, mapnik::line_symbolizer, mapnik::line_pattern_symbolizer, mapnik::polygon_symbolizer, mapnik::polygon_pattern_symbolizer, mapnik::raster_symbolizer, mapnik::shield_symbolizer, mapnik::text_symbolizer, mapnik::building_symbolizer, mapnik::markers_symbolizer, mapnik::group_symbolizer, mapnik::debug_symbolizer, mapnik::dot_symbolizer> const&, mapnik::symbolizer_dispatch<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> > > (
    v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#17 0x00007ffff597d6f9 in mapbox::util::variant<mapnik::point_symbolizer, mapnik::line_symbolizer, mapnik::line_pattern_symbolizer, mapnik::polygon_symbolizer, mapnik::polygon_pattern_symbolizer, mapnik::raster_symbolizer, mapnik::shield_symbolizer, mapnik::text_symbolizer, mapnik::building_symbolizer, mapnik::markers_symbolizer, mapnik::group_symbolizer, mapnik::debug_symbolizer, mapnik::dot_symbolizer>::visit<mapnik::symbolizer_dispatch<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >, mapbox::util::variant<mapnik::point_symbolizer, mapnik::line_symbolizer, mapnik::line_pattern_symbolizer, mapnik::polygon_symbolizer, mapnik::polygon_pattern_symbolizer, mapnik::raster_symbolizer, mapnik::shield_symbolizer, mapnik::text_symbolizer, mapnik::building_symbolizer, mapnik::markers_symbolizer, mapnik::group_symbolizer, mapnik::debug_symbolizer, mapnik::dot_symbolizer> const&, mapnik::point_symbolizer const&, void> (v=..., f=...) at /usr/include/mapbox/variant.hpp:838
#18 0x00007ffff597d712 in mapnik::util::apply_visitor<mapnik::symbolizer_dispatch<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >, mapbox::util::variant<mapnik::point_symbolizer, mapnik::line_symbolizer, mapnik::line_pattern_symbolizer, mapnik::polygon_symbolizer, mapnik::polygon_pattern_symbolizer, mapnik::raster_symbolizer, mapnik::shield_symbolizer, mapnik::text_symbolizer, mapnik::building_symbolizer, mapnik::markers_symbolizer, mapnik::group_symbolizer, mapnik::debug_symbolizer, mapnik::dot_symbolizer> const&> (f=..., v=...) at ./include/mapnik/util/variant.hpp:42
#19 0x00007ffff598611a in mapnik::feature_style_processor<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >::render_style (this=<optimized out>, p=..., style=0x1cb2e60, rc=..., 
    features=std::shared_ptr<mapnik::Featureset> (use count 3, weak count 0) = {...}, prj_trans=...) at ./include/mapnik/feature_style_processor_impl.hpp:592
#20 0x00007ffff59869af in mapnik::feature_style_processor<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >::render_material (this=this@entry=0x7fffffffc890, mat=..., p=...)
    at ./include/mapnik/feature_style_processor_impl.hpp:552
#21 0x00007ffff5987994 in mapnik::feature_style_processor<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >::render_submaterials (this=this@entry=0x7fffffffc890, parent_mat=..., p=...)
    at ./include/mapnik/feature_style_processor_impl.hpp:453
#22 0x00007ffff598c1e0 in mapnik::feature_style_processor<mapnik::agg_renderer<mapnik::image<mapnik::rgba8_t>, mapnik::label_collision_detector4> >::apply (this=this@entry=0x7fffffffc890, scale_denom=<optimized out>, 
    scale_denom@entry=0) at ./include/mapnik/feature_style_processor_impl.hpp:148
#23 0x00007ffff6d9d858 in agg_renderer_visitor_1::operator()<mapnik::image<mapnik::rgba8_t> > (this=<optimized out>, pixmap=...) at src/mapnik_python.cpp:220
#24 0x00007ffff6dabeea in mapbox::util::detail::dispatcher<void, mapnik::image<mapnik::rgba8_t>, mapnik::image<mapnik::gray8_t>, mapnik::image<mapnik::gray8s_t>, mapnik::image<mapnik::gray16_t>, mapnik::image<mapnik::gray16s_t>, mapnik::image<mapnik::gray32_t>, mapnik::image<mapnik::gray32s_t>, mapnik::image<mapnik::gray32f_t>, mapnik::image<mapnik::gray64_t>, mapnik::image<mapnik::gray64s_t>, mapnik::image<mapnik::gray64f_t> >::apply<mapnik::image_any&, agg_renderer_visitor_1> (v=..., f=...) at /usr/include/mapbox/variant.hpp:332
#25 0x00007ffff6dabf28 in mapbox::util::detail::dispatcher<void, mapnik::image<mapnik::null_t>, mapnik::image<mapnik::rgba8_t>, mapnik::image<mapnik::gray8_t>, mapnik::image<mapnik::gray8s_t>, mapnik::image<mapnik::gray16_t>, mapnik::image<mapnik::gray16s_t>, mapnik::image<mapnik::gray32_t>, mapnik::image<mapnik::gray32s_t>, mapnik::image<mapnik::gray32f_t>, mapnik::image<mapnik::gray64_t>, mapnik::image<mapnik::gray64s_t>, mapnik::image<mapnik::gray64f_t> >::apply<mapnik::image_any&, agg_renderer_visitor_1> (v=..., f=...) at /usr/include/mapbox/variant.hpp:336
#26 0x00007ffff6dabf89 in mapbox::util::variant<mapnik::image<mapnik::null_t>, mapnik::image<mapnik::rgba8_t>, mapnik::image<mapnik::gray8_t>, mapnik::image<mapnik::gray8s_t>, mapnik::image<mapnik::gray16_t>, mapnik::image<mapnik::gray16s_t>, mapnik::image<mapnik::gray32_t>, mapnik::image<mapnik::gray32s_t>, mapnik::image<mapnik::gray32f_t>, mapnik::image<mapnik::gray64_t>, mapnik::image<mapnik::gray64s_t>, mapnik::image<mapnik::gray64f_t> >::visit<agg_renderer_visitor_1, mapnik::image_any&, mapnik::image<mapnik::null_t>&, void> (v=..., f=...) at /usr/include/mapbox/variant.hpp:838
#27 0x00007ffff6dabfa2 in mapnik::util::apply_visitor<agg_renderer_visitor_1, mapnik::image_any&> (f=..., v=...) at /usr/include/mapnik/util/variant.hpp:42
#28 0x00007ffff6da4e41 in render (map=..., image=..., scale_factor=1, offset_x=0, offset_y=0) at src/mapnik_python.cpp:316
#29 0x00007ffff6dac584 in boost::python::detail::invoke<int, void (*)(mapnik::Map const&, mapnik::image_any&), boost::python::arg_from_python<mapnik::Map const&>, boost::python::arg_from_python<mapnik::image_any&> > (f=<optimized out>, 
    ac0=..., ac1=<synthetic pointer>...) at /usr/include/boost/python/detail/invoke.hpp:79
#30 boost::python::detail::caller_arity<2u>::impl<void (*)(mapnik::Map const&, mapnik::image_any&), boost::python::default_call_policies, boost::mpl::vector3<void, mapnik::Map const&, mapnik::image_any&> >::operator() (
    this=<optimized out>, args_=<optimized out>) at /usr/include/boost/python/detail/caller.hpp:233
#31 0x00007ffff6fdf3ae in boost::python::objects::py_function::operator() (this=0xec3670, args=0x7fffe7caf400, kw=<optimized out>) at ./boost/python/object/py_function.hpp:147
#32 boost::python::objects::function::call (this=0xec3ac0, args=0x7fffe7caf400, keywords=0x0) at libs/python/src/object/function.cpp:221
#33 0x00007ffff6fdf62c in boost::python::objects::(anonymous namespace)::bind_return::operator() (this=<optimized out>) at libs/python/src/object/function.cpp:581
#34 boost::detail::function::void_function_ref_invoker0<boost::python::objects::(anonymous namespace)::bind_return, void>::invoke (function_obj_ptr=...) at ./boost/function/function_template.hpp:193
#35 0x00007ffff6fe465b in boost::function0<void>::operator() (this=<optimized out>) at ./boost/function/function_template.hpp:771
#36 boost::python::detail::exception_handler::operator() (this=<optimized out>, f=...) at libs/python/src/errors.cpp:74
#37 0x00007ffff6da9b67 in boost::python::detail::translate_exception<std::runtime_error, void (*)(std::runtime_error const&)>::operator() (this=<optimized out>, handler=..., f=..., 
    translate=0x7ffff6d9d0f0 <runtime_error_translator(std::runtime_error const&)>) at /usr/include/boost/python/detail/translate_exception.hpp:39
#38 boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::runtime_error const&)> >::operator()<bool, boost::python::detail::translate_exception<std::runtime_error, void (*)(std::runtime_error const&)>, boost::_bi::rrlist2<boost::python::detail::exception_handler const&, boost::function0<void> const&> > (this=<optimized out>, f=..., a=<synthetic pointer>...) at /usr/include/boost/bind/bind.hpp:368
#39 boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::runtime_error, void (*)(std::runtime_error const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::runtime_error const&)> > >::operator()<boost::python::detail::exception_handler const&, boost::function0<void> const&> (this=<optimized out>, a1=..., a2=...) at /usr/include/boost/bind/bind.hpp:1298
#40 boost::detail::function::function_obj_invoker2<boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::runtime_error, void (*)(std::runtime_error const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::runtime_error const&)> > >, bool, boost::python::detail::exception_handler const&, boost::function0<void> const&>::invoke (function_obj_ptr=..., a0=..., a1=...)
    at /usr/include/boost/function/function_template.hpp:137
#41 0x00007ffff6da9bc7 in boost::python::detail::translate_exception<mapnik::value_error, void (*)(mapnik::value_error const&)>::operator() (this=<optimized out>, handler=..., f=..., 
    translate=0x7ffff6d9d250 <value_error_translator(mapnik::value_error const&)>) at /usr/include/boost/python/detail/translate_exception.hpp:39
#42 boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(mapnik::value_error const&)> >::operator()<bool, boost::python::detail::translate_exception<mapnik::value_error, void (*)(mapnik::value_error const&)>, boost::_bi::rrlist2<boost::python::detail::exception_handler const&, boost::function0<void> const&> > (this=<optimized out>, f=..., a=<synthetic pointer>...) at /usr/include/boost/bind/bind.hpp:368
#43 boost::_bi::bind_t<bool, boost::python::detail::translate_exception<mapnik::value_error, void (*)(mapnik::value_error const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(mapnik::value_error const&)> > >::operator()<boost::python::detail::exception_handler const&, boost::function0<void> const&> (this=<optimized out>, a1=..., a2=...) at /usr/include/boost/bind/bind.hpp:1298
#44 boost::detail::function::function_obj_invoker2<boost::_bi::bind_t<bool, boost::python::detail::translate_exception<mapnik::value_error, void (*)(mapnik::value_error const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(mapnik::value_error const&)> > >, bool, boost::python::detail::exception_handler const&, boost::function0<void> const&>::invoke (function_obj_ptr=..., a0=..., a1=...)
    at /usr/include/boost/function/function_template.hpp:137
#45 0x00007ffff6da9c27 in boost::python::detail::translate_exception<std::out_of_range, void (*)(std::out_of_range const&)>::operator() (this=<optimized out>, handler=..., f=..., 
    translate=0x7ffff6d9d120 <out_of_range_error_translator(std::out_of_range const&)>) at /usr/include/boost/python/detail/translate_exception.hpp:39
#46 boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::out_of_range const&)> >::operator()<bool, boost::python::detail::translate_exception<std::out_of_range, void (*)(std::out_of_range const&)>, boost::_bi::rrlist2<boost::python::detail::exception_handler const&, boost::function0<void> const&> > (this=<optimized out>, f=..., a=<synthetic pointer>...) at /usr/include/boost/bind/bind.hpp:368
#47 boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::out_of_range, void (*)(std::out_of_range const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::out_of_range const&)> > >::operator()<boost::python::detail::exception_handler const&, boost::function0<void> const&> (this=<optimized out>, a1=..., a2=...) at /usr/include/boost/bind/bind.hpp:1298
#48 boost::detail::function::function_obj_invoker2<boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::out_of_range, void (*)(std::out_of_range const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::out_of_range const&)> > >, bool, boost::python::detail::exception_handler const&, boost::function0<void> const&>::invoke (function_obj_ptr=..., a0=..., a1=...) at /usr/include/boost/function/function_template.hpp:137
#49 0x00007ffff6da9c87 in boost::python::detail::translate_exception<std::exception, void (*)(std::exception const&)>::operator() (this=<optimized out>, handler=..., f=..., 
    translate=0x7ffff6d9d150 <standard_error_translator(std::exception const&)>) at /usr/include/boost/python/detail/translate_exception.hpp:39
#50 boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::exception const&)> >::operator()<bool, boost::python::detail::translate_exception<std::exception, void (*)(std::exception const&)>, boost::_bi::rrlist2<boost::python::detail::exception_handler const&, boost::function0<void> const&> > (this=<optimized out>, f=..., a=<synthetic pointer>...) at /usr/include/boost/bind/bind.hpp:368
#51 boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::exception, void (*)(std::exception const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::exception const&)> > >::operator()<boost::python::detail::exception_handler const&, boost::function0<void> const&> (this=<optimized out>, a1=..., a2=...) at /usr/include/boost/bind/bind.hpp:1298
#52 boost::detail::function::function_obj_invoker2<boost::_bi::bind_t<bool, boost::python::detail::translate_exception<std::exception, void (*)(std::exception const&)>, boost::_bi::list3<boost::arg<1>, boost::arg<2>, boost::_bi::value<void (*)(std::exception const&)> > >, bool, boost::python::detail::exception_handler const&, boost::function0<void> const&>::invoke (function_obj_ptr=..., a0=..., a1=...) at /usr/include/boost/function/function_template.hpp:137
#53 0x00007ffff6fe4511 in boost::function2<bool, boost::python::detail::exception_handler const&, boost::function0<void> const&>::operator() (this=<optimized out>, a0=..., a1=...) at ./boost/function/function_template.hpp:771
#54 boost::python::detail::exception_handler::handle (this=<optimized out>, f=...) at ./boost/python/detail/exception_handler.hpp:41
#55 boost::python::handle_exception_impl (f=...) at libs/python/src/errors.cpp:24
#56 0x00007ffff6fdc3c6 in boost::python::handle_exception<boost::python::objects::(anonymous namespace)::bind_return> (f=...) at ./boost/python/errors.hpp:29
#57 boost::python::objects::function_call (func=<optimized out>, args=<optimized out>, kw=<optimized out>) at libs/python/src/object/function.cpp:622
#58 0x0000000000543b8b in _PyObject_MakeTpCall (tstate=0xa7d510 <_PyRuntime+283024>, callable=0xec3ac0, args=<optimized out>, nargs=2, keywords=<optimized out>) at ../Objects/call.c:242
#59 0x000000000055f191 in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at ../Python/generated_cases.c.h:813
#60 0x000000000064db6c in _PyEval_EvalFrame (tstate=0xa7d510 <_PyRuntime+283024>, frame=0x7ffff7fb2020, throwflag=0) at ../Include/internal/pycore_ceval.h:119
#61 _PyEval_Vector (args=0x0, argcount=0, kwnames=0x0, tstate=0xa7d510 <_PyRuntime+283024>, func=0x7ffff7497740, locals=<optimized out>) at ../Python/ceval.c:1814
#62 PyEval_EvalCode (co=0xbcf8d0, globals=<optimized out>, locals=<optimized out>) at ../Python/ceval.c:604
#63 0x000000000066da21 in run_eval_code_obj (tstate=0xa7d510 <_PyRuntime+283024>, co=0xbcf8d0, globals=0x7ffff7434a00, locals=0x7ffff7434a00) at ../Python/pythonrun.c:1381
#64 0x000000000066988c in run_mod (mod=<optimized out>, filename=<optimized out>, globals=0x7ffff7434a00, locals=0x7ffff7434a00, flags=<optimized out>, arena=<optimized out>, interactive_src=0x0, generate_new_source=0)
    at ../Python/pythonrun.c:1466
#65 0x0000000000682983 in pyrun_file (fp=fp@entry=0xb1dcd0, filename=filename@entry=0x7ffff744aa30, start=start@entry=257, globals=globals@entry=0x7ffff7434a00, locals=locals@entry=0x7ffff7434a00, closeit=closeit@entry=1, 
    flags=0x7fffffffd3a8) at ../Python/pythonrun.c:1295
#66 0x0000000000682283 in _PyRun_SimpleFileObject (fp=fp@entry=0xb1dcd0, filename=filename@entry=0x7ffff744aa30, closeit=closeit@entry=1, flags=flags@entry=0x7fffffffd3a8) at ../Python/pythonrun.c:517
#67 0x00000000006820be in _PyRun_AnyFileObject (fp=0xb1dcd0, filename=0x7ffff744aa30, closeit=1, flags=0x7fffffffd3a8) at ../Python/pythonrun.c:77
#68 0x0000000000680ef1 in pymain_run_file_obj (program_name=0x7ffff7434b30, filename=0x7ffff744aa30, skip_source_first_line=0) at ../Modules/main.c:410
#69 pymain_run_file (config=0xa4fc08 <_PyRuntime+96392>) at ../Modules/main.c:429
#70 pymain_run_python (exitcode=0x7fffffffd39c) at ../Modules/main.c:697
#71 Py_RunMain () at ../Modules/main.c:776
#72 0x000000000063d6eb in Py_BytesMain (argc=<optimized out>, argv=<optimized out>) at ../Modules/main.c:830
#73 0x00007ffff7c90ca8 in __libc_start_call_main (main=main@entry=0x63d640 <main>, argc=argc@entry=3, argv=argv@entry=0x7fffffffd5d8) at ../sysdeps/nptl/libc_start_call_main.h:58
#74 0x00007ffff7c90d65 in __libc_start_main_impl (main=0x63d640 <main>, argc=3, argv=0x7fffffffd5d8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffd5c8) at ../csu/libc-start.c:360
#75 0x000000000063cab1 in _start ()

What I get from it is that the color is not coming from the style of the db but it's the result of an expression; see frame #3. Thing is, those parameters have been optimized out into CPU registers and there is no easy way to inspect them as is:

#3  mapnik::extract_value<mapnik::color>::operator() (this=<optimized out>, expr=...) at ./include/mapnik/symbolizer.hpp:342
342     in ./include/mapnik/symbolizer.hpp
(gdb) print this
$1 = <optimized out>
(gdb) print expr
$2 = <optimized out>

So, what options do I have? Well, one is to compile mapnik (and python3-mapnik, because I'm driving it from a Python program; you can see it in the stacktrace since frame #29²) and I tried this first, but even compiling the released code on my Debian failed in ways I could not fix myself without potentially introducing more bugs.

But luckily and ofcoursely³ the sources from Debian did compile; I compiled the debs using CXXFLAGS='-g -O0' CFLAGS='-g -O0' dpkg-buildpackage -us -uc⁴, installed them by hand and now I can do my debug session normally.

Unluckily for this post the error was in the data. When I searched for it I used the wrong key for the Postgres HStore (color vs colour; damn Webster!). It's also a lucky hit, because I was reaching the point were I would had to untangle a mess of C++ templating and I was running out of time. mapnik is definitely the hardest code I had ever had to follow. You get a preview just in that backtrace.

I know the code is in C++, it's just for the joke. ↩
To be honest, it surprises me that most of the stack is Python layers; mapnik and agg seem to like very deep call stacks. Maybe it's just I'm not that far deep. ↩
Sue me :) ↩
You have no idea how much time I spent coming up with this invocation. In retrospective, it's obvious, but mapnik seems to be able to use both scons and cmake for configuring the build; I was using the scons style of params while I should have been using the cmake one⁵. ↩
Part of the confusion comes from the fact that I used to compile mapnik from git, and as it's my custom I still had the ./config.sh script I use (together with ./build.sh) to leave a trace of how I compiled something. This script was using scons, while debian/rules uses cmake. I was aware cmake was involved, mostly from the compilation output on the terminal, the classic [ 1%] Building CXX object CMakeFiles/wkt.dir/src/wkt/wkt_factory.cpp.o type of lines, but I thought that for some reason scons was calling it. ↩

Measure your optimnizations

Marcos Dione — Tue, 20 Aug 2024 13:53:57 GMT

One of the parts of having my own map style with hypsometric contour lines is that I have to generate those contour lines. There's a tool in GDAL, particularly the one that actually does everything based on DEM files, called gdaldem that can generate shapefiles with contour lines that mapnik can read. But since my source files are 1x1° files, I will have to generate one layer for each shapefile and that doesn't scale very well, especially at planet size.

So what I do is I convert those shapefiles to SQL files and then I inject them into my database one by one, and then I can use mapnik's own support for filtering by bbox when it's rendering, so that should be faster⁴.

I put the SQL files in my file system, and then I import them by hand as I need them, and I'm running out of space again. A few years ago I had a 1TB disk, and that was enough, and now I am at the 2TB disk, and it's getting small. I have the impression that the new DEMs I am using are bigger, even if I streamlined every layer so it uses as less space as possible.

One of the things I'm doing is converting my processing script into a Makefile, so I can remove intermediary files. My process goes from the original DEM files, that are in LatLon, I project them to WebMerkator. This file becomes the source for the terrain files, which gives the hypsometric tints, and I generate the contours from there, and then I do a compensation for slope shade and hill shade. Notice that I get two intermediary files that I can easily remove, which are first, the reprojected file, because once I have the terrain and contour files, I can remove it, I don't care anymore; and also the compensated file, I don't need it anymore once I have the shade files. The Makefile is covering that part, once the files are generated, the intermediary files are gone.

Going back to the SQL files, I don't inject SQL data directly into my database, because I don't have space for that. So, I just generate this SQL file and I compress it, so it's not using so much space, because SQL is really a lot of text. I've been using xz as the compressor, and I have been blindly using its highest compression level, CL 9. What do I mean with blindly? I noticed it actually takes a lot of time. I just measured it with one tile, and it took 451 seconds. That's 7.5 minutes per degree tile, which is a lot. So I asked myself, what's the compression ratio to time spent ratio?

I took a single file and I compressed it with all the compression levels between 1 and 9, and I took the time and the space in the final file. I made a scatter graph, and it looks like this pretty weird Z figure²:

Here's the raw data¹:

level	time_in_seconds	readable_time	size_in_bytes	comp_ratio
1	57.84	57s	129_486_376	29.21%
2	117.40	1m57s	129_993_440	29.33%
3	252.28	4m12s	130_306_780	29.40%
4	212.26	3m32s	102_359_596	23.09%
5	347.51	5m47s	98_992_464	22.33%
6	344.58	5m44s	99_114_560	22.36%
7	370.20	6m10s	99_043_096	22.34%
8	416.48	6m56s	99_005_352	22.33%
9	451.85	7m31s	99_055_552	22.35%

I'm not going to explain the graph or table, except to point to the two obvious parts: the jump from CL 3 to 4, where it's not only the first and only noticeable space gain, it also takes less time; and the fact that compressions levels 1-3 and 4-9 have almost no change in space gained. So I either use CL 1 or 4. I'll go for 1, until I run out of space again.

All this to say: whenever you make an optimization, measure all the dimensions, time, space, memory consumption, and maybe you have other constraints like, I don't know, heat produced, stuff like that. Measure and compare.

Sorry for the ugly table style. I still don't know how to style it better. ↩
Sorry for the horrible scales. Either I don't know it enough, or LibreOffice is quite limited on how to format the axises³. ↩
No, I won't bother to see how the plural is made, this is taking me long enough already :-P ↩
This claim has not been proven and it's not in the scope of this post. ↩