Debuggung system libraries in Debian

Marcos Dione

2025-04-12 16:22

"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, 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.

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 buils; 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. ↩