[BUG]: keep_alive does not applied to properties

[Nuri Jung]

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What version (or hash if on master) of pybind11 are you using?

2.10.4, but 8b48ff8 (the current master) still has the same problem.

Problem description

The problem

Passing keep_alive directly to def_property* family is ignored. The flow:

1. The getter/setters are created without "extra" attributes. Related source locations:
   

   pybind11/include/pybind11/pybind11.h

   Line 1750 in 8b48ff8

    cpp_function(method_adaptor<type>(fget)), 

   
   

   pybind11/include/pybind11/pybind11.h

   Line 1766 in 8b48ff8

    return def_property_readonly_static( 

   
   

   pybind11/include/pybind11/pybind11.h

   Lines 1782 to 1783 in 8b48ff8

    return def_property( 
        name, fget, cpp_function(method_adaptor<type>(fset), is_setter()), extra...); 

   
   

   pybind11/include/pybind11/pybind11.h

   Lines 1790 to 1794 in 8b48ff8

    return def_property(name, 
                        cpp_function(method_adaptor<type>(fget)), 
                        fset, 
                        return_value_policy::reference_internal, 
                        extra...); 

   
   

   pybind11/include/pybind11/pybind11.h

   Lines 1812 to 1813 in 8b48ff8

    return def_property_static( 
        name, cpp_function(fget), fset, return_value_policy::reference, extra...); 

2. All the calls to def_properties* eventually forwarded to def_property_static, and "extra" attributes are processed there, with process_attributes<Extra...>::init:
   

   pybind11/include/pybind11/pybind11.h

   Lines 1816 to 1848 in 8b48ff8

        /// Uses cpp_function's return_value_policy by default 
        template <typename... Extra> 
        class_ &def_property_static(const char *name, 
                                    const cpp_function &fget, 
                                    const cpp_function &fset, 
                                    const Extra &...extra) { 
            static_assert(0 == detail::constexpr_sum(std::is_base_of<arg, Extra>::value...), 
                          "Argument annotations are not allowed for properties"); 
            auto rec_fget = get_function_record(fget), rec_fset = get_function_record(fset); 
            auto *rec_active = rec_fget; 
            if (rec_fget) { 
                char *doc_prev = rec_fget->doc; /* 'extra' field may include a property-specific 
                                                   documentation string */ 
                detail::process_attributes<Extra...>::init(extra..., rec_fget); 
                if (rec_fget->doc && rec_fget->doc != doc_prev) { 
                    std::free(doc_prev); 
                    rec_fget->doc = PYBIND11_COMPAT_STRDUP(rec_fget->doc); 
                } 
            } 
            if (rec_fset) { 
                char *doc_prev = rec_fset->doc; 
                detail::process_attributes<Extra...>::init(extra..., rec_fset); 
                if (rec_fset->doc && rec_fset->doc != doc_prev) { 
                    std::free(doc_prev); 
                    rec_fset->doc = PYBIND11_COMPAT_STRDUP(rec_fset->doc); 
                } 
                if (!rec_active) { 
                    rec_active = rec_fset; 
                } 
            } 
            def_property_static_impl(name, fget, fset, rec_active); 
            return *this; 
        } 

3. ... which in turn calls process_attribute<keep_alive<>>::init:
   

   pybind11/include/pybind11/attr.h

   Lines 644 to 650 in 8b48ff8

    static void init(const Args &...args, function_record *r) { 
        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(r); 
        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(r); 
        using expander = int[]; 
        (void) expander{ 
            0, ((void) process_attribute<typename std::decay<Args>::type>::init(args, r), 0)...}; 
    } 

4. Unfortunately, the process_attribute<keep_alive<>>::init is empty.
   

   pybind11/include/pybind11/attr.h

   Lines 619 to 639 in 8b48ff8

    /** 
     * Process a keep_alive call policy -- invokes keep_alive_impl during the 
     * pre-call handler if both Nurse, Patient != 0 and use the post-call handler 
     * otherwise 
     */ 
    template <size_t Nurse, size_t Patient> 
    struct process_attribute<keep_alive<Nurse, Patient>> 
        : public process_attribute_default<keep_alive<Nurse, Patient>> { 
        template <size_t N = Nurse, size_t P = Patient, enable_if_t<N != 0 && P != 0, int> = 0> 
        static void precall(function_call &call) { 
            keep_alive_impl(Nurse, Patient, call, handle()); 
        } 
        template <size_t N = Nurse, size_t P = Patient, enable_if_t<N != 0 && P != 0, int> = 0> 
        static void postcall(function_call &, handle) {} 
        template <size_t N = Nurse, size_t P = Patient, enable_if_t<N == 0 || P == 0, int> = 0> 
        static void precall(function_call &) {} 
        template <size_t N = Nurse, size_t P = Patient, enable_if_t<N == 0 || P == 0, int> = 0> 
        static void postcall(function_call &call, handle ret) { 
            keep_alive_impl(Nurse, Patient, call, ret); 
        } 
    }; 

Possible solution

Just blindly pass "extra" attributes to the cpp_function constructor. I haven't yet analyzed possible side effects of this solution.

Workaround

keep_alive works when it is passed to the cpp_function constructor directly. If the maintainers choose not to update implementations, this (confusing) behavior should be documented IMO.

Related issues

#4236, #4124, #2618, ...

Reproducible example code

No response

Is this a regression? Put the last known working version here if it is.

Not a regression