. /include/boost/capy /ex /frame_allocator.hpp
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1   // 1   //
2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com) 2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   // Copyright (c) 2026 Michael Vandeberg 3   // Copyright (c) 2026 Michael Vandeberg
4   // 4   //
5   // Distributed under the Boost Software License, Version 1.0. (See accompanying 5   // Distributed under the Boost Software License, Version 1.0. (See accompanying
6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7   // 7   //
8   // Official repository: https://github.com/cppalliance/capy 8   // Official repository: https://github.com/cppalliance/capy
9   // 9   //
10   10  
11   #ifndef BOOST_CAPY_FRAME_ALLOCATOR_HPP 11   #ifndef BOOST_CAPY_FRAME_ALLOCATOR_HPP
12   #define BOOST_CAPY_FRAME_ALLOCATOR_HPP 12   #define BOOST_CAPY_FRAME_ALLOCATOR_HPP
13   13  
14   #include <boost/capy/detail/config.hpp> 14   #include <boost/capy/detail/config.hpp>
15   15  
16   #include <coroutine> 16   #include <coroutine>
17   #include <memory_resource> 17   #include <memory_resource>
18   18  
19   /* Design rationale (pdimov): 19   /* Design rationale (pdimov):
20   20  
21   This accessor is a thin wrapper over a thread-local pointer. 21   This accessor is a thin wrapper over a thread-local pointer.
22   It returns exactly what was stored, including nullptr. No 22   It returns exactly what was stored, including nullptr. No
23   dynamic initializer on the thread-local; a dynamic TLS 23   dynamic initializer on the thread-local; a dynamic TLS
24   initializer moves you into a costlier implementation bucket 24   initializer moves you into a costlier implementation bucket
25   on some platforms - avoid it. 25   on some platforms - avoid it.
26   26  
27   Null handling is the caller's responsibility (e.g. in 27   Null handling is the caller's responsibility (e.g. in
28   promise_type::operator new). The accessor must not substitute 28   promise_type::operator new). The accessor must not substitute
29   a default, because there are multiple valid choices 29   a default, because there are multiple valid choices
30   (new_delete_resource, the default pmr resource, etc.). If 30   (new_delete_resource, the default pmr resource, etc.). If
31   the allocator is not set, it reports "not set" and the 31   the allocator is not set, it reports "not set" and the
32   caller interprets that however it wants. 32   caller interprets that however it wants.
33   */ 33   */
34   34  
35   namespace boost { 35   namespace boost {
36   namespace capy { 36   namespace capy {
37   37  
38   namespace detail { 38   namespace detail {
39   39  
40   inline std::pmr::memory_resource*& 40   inline std::pmr::memory_resource*&
HITCBC 41   113692 current_frame_allocator_ref() noexcept 41   113847 current_frame_allocator_ref() noexcept
42   { 42   {
43   static thread_local std::pmr::memory_resource* mr = nullptr; 43   static thread_local std::pmr::memory_resource* mr = nullptr;
HITCBC 44   113692 return mr; 44   113847 return mr;
45   } 45   }
46   46  
47   } // namespace detail 47   } // namespace detail
48   48  
49   /** Return the current frame allocator for this thread. 49   /** Return the current frame allocator for this thread.
50   50  
51   These accessors exist to implement the allocator 51   These accessors exist to implement the allocator
52   propagation portion of the @ref IoAwaitable protocol. 52   propagation portion of the @ref IoAwaitable protocol.
53   Launcher functions (`run_async`, `run`) set the 53   Launcher functions (`run_async`, `run`) set the
54   thread-local value before invoking a child coroutine. 54   thread-local value before invoking a child coroutine.
55   The child's `promise_type::operator new` reads it to 55   The child's `promise_type::operator new` reads it to
56   allocate the coroutine frame from the correct resource. 56   allocate the coroutine frame from the correct resource.
57   57  
58   The value is only valid during a narrow execution 58   The value is only valid during a narrow execution
59   window. Between a coroutine's resumption 59   window. Between a coroutine's resumption
60   and the next suspension point, the protocol guarantees 60   and the next suspension point, the protocol guarantees
61   that TLS contains the allocator associated with the 61   that TLS contains the allocator associated with the
62   currently running chain. Outside that window the value 62   currently running chain. Outside that window the value
63   is indeterminate. Only code that implements an 63   is indeterminate. Only code that implements an
64   @ref IoAwaitable should call these functions. 64   @ref IoAwaitable should call these functions.
65   65  
66   A return value of `nullptr` means "not specified" - 66   A return value of `nullptr` means "not specified" -
67   no allocator is established for this chain. 67   no allocator is established for this chain.
68   The awaitable is free to use whatever allocation 68   The awaitable is free to use whatever allocation
69   strategy makes best sense (e.g. 69   strategy makes best sense (e.g.
70   `std::pmr::new_delete_resource()`). 70   `std::pmr::new_delete_resource()`).
71   71  
72   Use of the frame allocator is optional. An awaitable 72   Use of the frame allocator is optional. An awaitable
73   that does not consult this value to allocate its 73   that does not consult this value to allocate its
74   coroutine frame is never wrong. However, a conforming 74   coroutine frame is never wrong. However, a conforming
75   awaitable must still propagate the allocator faithfully 75   awaitable must still propagate the allocator faithfully
76   so that downstream coroutines can use it. 76   so that downstream coroutines can use it.
77   77  
78   @return The thread-local memory_resource pointer, 78   @return The thread-local memory_resource pointer,
79   or `nullptr` if none is set. 79   or `nullptr` if none is set.
80   80  
81   @see set_current_frame_allocator, IoAwaitable 81   @see set_current_frame_allocator, IoAwaitable
82   */ 82   */
83   inline 83   inline
84   std::pmr::memory_resource* 84   std::pmr::memory_resource*
HITCBC 85   54664 get_current_frame_allocator() noexcept 85   54743 get_current_frame_allocator() noexcept
86   { 86   {
HITCBC 87   54664 return detail::current_frame_allocator_ref(); 87   54743 return detail::current_frame_allocator_ref();
88   } 88   }
89   89  
90   /** Set the current frame allocator for this thread. 90   /** Set the current frame allocator for this thread.
91   91  
92   Installs @p mr as the frame allocator read by the 92   Installs @p mr as the frame allocator read by the
93   next coroutine's `promise_type::operator new` on 93   next coroutine's `promise_type::operator new` on
94   this thread. Only launcher functions and 94   this thread. Only launcher functions and
95   @ref IoAwaitable machinery should call this; see 95   @ref IoAwaitable machinery should call this; see
96   @ref get_current_frame_allocator for the full protocol 96   @ref get_current_frame_allocator for the full protocol
97   description. 97   description.
98   98  
99   Passing `nullptr` means "not specified" - no 99   Passing `nullptr` means "not specified" - no
100   particular allocator is established for the chain. 100   particular allocator is established for the chain.
101   101  
102   @param mr The memory_resource to install, or 102   @param mr The memory_resource to install, or
103   `nullptr` to clear. 103   `nullptr` to clear.
104   104  
105   @see get_current_frame_allocator, IoAwaitable 105   @see get_current_frame_allocator, IoAwaitable
106   */ 106   */
107   inline void 107   inline void
HITCBC 108   59028 set_current_frame_allocator( 108   59104 set_current_frame_allocator(
109   std::pmr::memory_resource* mr) noexcept 109   std::pmr::memory_resource* mr) noexcept
110   { 110   {
HITCBC 111   59028 detail::current_frame_allocator_ref() = mr; 111   59104 detail::current_frame_allocator_ref() = mr;
HITCBC 112   59028 } 112   59104 }
113   113  
114   /** Resume a coroutine handle with frame-allocator TLS protection. 114   /** Resume a coroutine handle with frame-allocator TLS protection.
115   115  
116   Saves the current thread-local frame allocator before 116   Saves the current thread-local frame allocator before
117   calling `h.resume()`, then restores it after the call 117   calling `h.resume()`, then restores it after the call
118   returns. This prevents a resumed coroutine's 118   returns. This prevents a resumed coroutine's
119   `await_resume` from permanently overwriting the caller's 119   `await_resume` from permanently overwriting the caller's
120   allocator value. 120   allocator value.
121   121  
122   Between a coroutine's resumption and its next child 122   Between a coroutine's resumption and its next child
123   invocation, arbitrary user code may run. If that code 123   invocation, arbitrary user code may run. If that code
124   resumes a coroutine from a different chain on this 124   resumes a coroutine from a different chain on this
125   thread, the other coroutine's `await_resume` overwrites 125   thread, the other coroutine's `await_resume` overwrites
126   TLS with its own allocator. Without save/restore, the 126   TLS with its own allocator. Without save/restore, the
127   original coroutine's next child would allocate from 127   original coroutine's next child would allocate from
128   the wrong resource. 128   the wrong resource.
129   129  
130   Event loops, strand dispatch loops, and any code that 130   Event loops, strand dispatch loops, and any code that
131   calls `.resume()` on a coroutine handle should use 131   calls `.resume()` on a coroutine handle should use
132   this function instead of calling `.resume()` directly. 132   this function instead of calling `.resume()` directly.
133   See the @ref Executor concept documentation for details. 133   See the @ref Executor concept documentation for details.
134   134  
135   @param h The coroutine handle to resume. 135   @param h The coroutine handle to resume.
136   136  
137   @see get_current_frame_allocator, set_current_frame_allocator 137   @see get_current_frame_allocator, set_current_frame_allocator
138   */ 138   */
139   // tag::safe_resume[] 139   // tag::safe_resume[]
140   inline void 140   inline void
HITCBC 141   49539 safe_resume(std::coroutine_handle<> h) noexcept 141   49612 safe_resume(std::coroutine_handle<> h) noexcept
142   { 142   {
HITCBC 143   49539 auto* saved = get_current_frame_allocator(); 143   49612 auto* saved = get_current_frame_allocator();
HITCBC 144   49539 h.resume(); 144   49612 h.resume();
HITCBC 145   49539 set_current_frame_allocator(saved); 145   49612 set_current_frame_allocator(saved);
HITCBC 146   49539 } 146   49612 }
147   // end::safe_resume[] 147   // end::safe_resume[]
148   148  
149   } // namespace capy 149   } // namespace capy
150   } // namespace boost 150   } // namespace boost
151   151  
152   #endif 152   #endif