1   
//
1   
//
2   
// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
2   
// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   
//
3   
//
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6   
//
6   
//
7   
// Official repository: https://github.com/cppalliance/capy
7   
// Official repository: https://github.com/cppalliance/capy
8   
//
8   
//
9  
9  
10   
#ifndef BOOST_CAPY_FRAME_ALLOCATOR_HPP
10   
#ifndef BOOST_CAPY_FRAME_ALLOCATOR_HPP
11   
#define BOOST_CAPY_FRAME_ALLOCATOR_HPP
11   
#define BOOST_CAPY_FRAME_ALLOCATOR_HPP
12  
12  
13   
#include <boost/capy/detail/config.hpp>
13   
#include <boost/capy/detail/config.hpp>
14  
14  
 
15 + 
#include <coroutine>
15   
#include <memory_resource>
16   
#include <memory_resource>
16  
17  
17   
/*  Design rationale (pdimov):
18   
/*  Design rationale (pdimov):
18  
19  
19   
    This accessor is a thin wrapper over a thread-local pointer.
20   
    This accessor is a thin wrapper over a thread-local pointer.
20   
    It returns exactly what was stored, including nullptr. No
21   
    It returns exactly what was stored, including nullptr. No
21   
    dynamic initializer on the thread-local; a dynamic TLS
22   
    dynamic initializer on the thread-local; a dynamic TLS
22   
    initializer moves you into a costlier implementation bucket
23   
    initializer moves you into a costlier implementation bucket
23   
    on some platforms - avoid it.
24   
    on some platforms - avoid it.
24  
25  
25   
    Null handling is the caller's responsibility (e.g. in
26   
    Null handling is the caller's responsibility (e.g. in
26   
    promise_type::operator new). The accessor must not substitute
27   
    promise_type::operator new). The accessor must not substitute
27   
    a default, because there are multiple valid choices
28   
    a default, because there are multiple valid choices
28   
    (new_delete_resource, the default pmr resource, etc.). If
29   
    (new_delete_resource, the default pmr resource, etc.). If
29   
    the allocator is not set, it reports "not set" and the
30   
    the allocator is not set, it reports "not set" and the
30   
    caller interprets that however it wants.
31   
    caller interprets that however it wants.
31   
*/
32   
*/
32  
33  
33   
namespace boost {
34   
namespace boost {
34   
namespace capy {
35   
namespace capy {
35  
36  
36   
namespace detail {
37   
namespace detail {
37  
38  
38   
inline std::pmr::memory_resource*&
39   
inline std::pmr::memory_resource*&
39   
current_frame_allocator_ref() noexcept
40   
current_frame_allocator_ref() noexcept
40   
{
41   
{
41   
    static thread_local std::pmr::memory_resource* mr = nullptr;
42   
    static thread_local std::pmr::memory_resource* mr = nullptr;
42   
    return mr;
43   
    return mr;
43   
}
44   
}
44  
45  
45   
} // namespace detail
46   
} // namespace detail
46  
47  
47   
/** Return the current frame allocator for this thread.
48   
/** Return the current frame allocator for this thread.
48  
49  
49   
    These accessors exist to implement the allocator
50   
    These accessors exist to implement the allocator
50   
    propagation portion of the @ref IoAwaitable protocol.
51   
    propagation portion of the @ref IoAwaitable protocol.
51   
    Launch functions (`run_async`, `run`) set the
52   
    Launch functions (`run_async`, `run`) set the
52   
    thread-local value before invoking a child coroutine;
53   
    thread-local value before invoking a child coroutine;
53   
    the child's `promise_type::operator new` reads it to
54   
    the child's `promise_type::operator new` reads it to
54   
    allocate the coroutine frame from the correct resource.
55   
    allocate the coroutine frame from the correct resource.
55  
56  
56   
    The value is only valid during a narrow execution
57   
    The value is only valid during a narrow execution
57   
    window. Between a coroutine's resumption
58   
    window. Between a coroutine's resumption
58   
    and the next suspension point, the protocol guarantees
59   
    and the next suspension point, the protocol guarantees
59   
    that TLS contains the allocator associated with the
60   
    that TLS contains the allocator associated with the
60   
    currently running chain. Outside that window the value
61   
    currently running chain. Outside that window the value
61   
    is indeterminate. Only code that implements an
62   
    is indeterminate. Only code that implements an
62   
    @ref IoAwaitable should call these functions.
63   
    @ref IoAwaitable should call these functions.
63  
64  
64   
    A return value of `nullptr` means "not specified" -
65   
    A return value of `nullptr` means "not specified" -
65   
    no allocator has been established for this chain.
66   
    no allocator has been established for this chain.
66   
    The awaitable is free to use whatever allocation
67   
    The awaitable is free to use whatever allocation
67   
    strategy makes best sense (e.g.
68   
    strategy makes best sense (e.g.
68   
    `std::pmr::new_delete_resource()`).
69   
    `std::pmr::new_delete_resource()`).
69  
70  
70   
    Use of the frame allocator is optional. An awaitable
71   
    Use of the frame allocator is optional. An awaitable
71   
    that does not consult this value to allocate its
72   
    that does not consult this value to allocate its
72   
    coroutine frame is never wrong. However, a conforming
73   
    coroutine frame is never wrong. However, a conforming
73   
    awaitable must still propagate the allocator faithfully
74   
    awaitable must still propagate the allocator faithfully
74   
    so that downstream coroutines can use it.
75   
    so that downstream coroutines can use it.
75  
76  
76   
    @return The thread-local memory_resource pointer,
77   
    @return The thread-local memory_resource pointer,
77   
    or `nullptr` if none has been set.
78   
    or `nullptr` if none has been set.
78  
79  
79   
    @see set_current_frame_allocator, IoAwaitable
80   
    @see set_current_frame_allocator, IoAwaitable
80   
*/
81   
*/
81   
inline
82   
inline
82   
std::pmr::memory_resource*
83   
std::pmr::memory_resource*
83   
get_current_frame_allocator() noexcept
84   
get_current_frame_allocator() noexcept
84   
{
85   
{
85   
    return detail::current_frame_allocator_ref();
86   
    return detail::current_frame_allocator_ref();
86   
}
87   
}
87  
88  
88   
/** Set the current frame allocator for this thread.
89   
/** Set the current frame allocator for this thread.
89  
90  
90   
    Installs @p mr as the frame allocator that will be
91   
    Installs @p mr as the frame allocator that will be
91   
    read by the next coroutine's `promise_type::operator
92   
    read by the next coroutine's `promise_type::operator
92   
    new` on this thread. Only launch functions and
93   
    new` on this thread. Only launch functions and
93   
    @ref IoAwaitable machinery should call this; see
94   
    @ref IoAwaitable machinery should call this; see
94   
    @ref get_current_frame_allocator for the full protocol
95   
    @ref get_current_frame_allocator for the full protocol
95   
    description.
96   
    description.
96  
97  
97   
    Passing `nullptr` means "not specified" - no
98   
    Passing `nullptr` means "not specified" - no
98   
    particular allocator is established for the chain.
99   
    particular allocator is established for the chain.
99  
100  
100   
    @param mr The memory_resource to install, or
101   
    @param mr The memory_resource to install, or
101   
    `nullptr` to clear.
102   
    `nullptr` to clear.
102  
103  
103   
    @see get_current_frame_allocator, IoAwaitable
104   
    @see get_current_frame_allocator, IoAwaitable
104   
*/
105   
*/
105   
inline void
106   
inline void
106   
set_current_frame_allocator(
107   
set_current_frame_allocator(
107   
    std::pmr::memory_resource* mr) noexcept
108   
    std::pmr::memory_resource* mr) noexcept
108   
{
109   
{
109   
    detail::current_frame_allocator_ref() = mr;
110   
    detail::current_frame_allocator_ref() = mr;
 
111 + 
}
 
112 +
 
113 + 
/** Resume a coroutine handle with frame-allocator TLS protection.
 
114 +
 
115 + 
    Saves the current thread-local frame allocator before
 
116 + 
    calling `h.resume()`, then restores it after the call
 
117 + 
    returns. This prevents a resumed coroutine's
 
118 + 
    `await_resume` from permanently overwriting the caller's
 
119 + 
    allocator value.
 
120 +
 
121 + 
    Between a coroutine's resumption and its next child
 
122 + 
    invocation, arbitrary user code may run. If that code
 
123 + 
    resumes a coroutine from a different chain on this
 
124 + 
    thread, the other coroutine's `await_resume` overwrites
 
125 + 
    TLS with its own allocator. Without save/restore, the
 
126 + 
    original coroutine's next child would allocate from
 
127 + 
    the wrong resource.
 
128 +
 
129 + 
    Event loops, strand dispatch loops, and any code that
 
130 + 
    calls `.resume()` on a coroutine handle should use
 
131 + 
    this function instead of calling `.resume()` directly.
 
132 + 
    See the @ref Executor concept documentation for details.
 
133 +
 
134 + 
    @param h The coroutine handle to resume.
 
135 +
 
136 + 
    @see get_current_frame_allocator, set_current_frame_allocator
 
137 + 
*/
 
138 + 
inline void
 
139 + 
safe_resume(std::coroutine_handle<> h) noexcept
 
140 + 
{
 
141 + 
    auto* saved = get_current_frame_allocator();
 
142 + 
    h.resume();
 
143 + 
    set_current_frame_allocator(saved);
110   
}
144   
}
111  
145  
112   
} // namespace capy
146   
} // namespace capy
113   
} // namespace boost
147   
} // namespace boost
114  
148  
115   
#endif
149   
#endif