. /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   // 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 <coroutine>
16   #include <memory_resource> 16   #include <memory_resource>
17   17  
18   /* Design rationale (pdimov): 18   /* Design rationale (pdimov):
19   19  
20   This accessor is a thin wrapper over a thread-local pointer. 20   This accessor is a thin wrapper over a thread-local pointer.
21   It returns exactly what was stored, including nullptr. No 21   It returns exactly what was stored, including nullptr. No
22   dynamic initializer on the thread-local; a dynamic TLS 22   dynamic initializer on the thread-local; a dynamic TLS
23   initializer moves you into a costlier implementation bucket 23   initializer moves you into a costlier implementation bucket
24   on some platforms - avoid it. 24   on some platforms - avoid it.
25   25  
26   Null handling is the caller's responsibility (e.g. in 26   Null handling is the caller's responsibility (e.g. in
27   promise_type::operator new). The accessor must not substitute 27   promise_type::operator new). The accessor must not substitute
28   a default, because there are multiple valid choices 28   a default, because there are multiple valid choices
29   (new_delete_resource, the default pmr resource, etc.). If 29   (new_delete_resource, the default pmr resource, etc.). If
30   the allocator is not set, it reports "not set" and the 30   the allocator is not set, it reports "not set" and the
31   caller interprets that however it wants. 31   caller interprets that however it wants.
32   */ 32   */
33   33  
34   namespace boost { 34   namespace boost {
35   namespace capy { 35   namespace capy {
36   36  
37   namespace detail { 37   namespace detail {
38   38  
39   inline std::pmr::memory_resource*& 39   inline std::pmr::memory_resource*&
HITCBC 40   31906 current_frame_allocator_ref() noexcept 40   32017 current_frame_allocator_ref() noexcept
41   { 41   {
42   static thread_local std::pmr::memory_resource* mr = nullptr; 42   static thread_local std::pmr::memory_resource* mr = nullptr;
HITCBC 43   31906 return mr; 43   32017 return mr;
44   } 44   }
45   45  
46   } // namespace detail 46   } // namespace detail
47   47  
48   /** Return the current frame allocator for this thread. 48   /** Return the current frame allocator for this thread.
49   49  
50   These accessors exist to implement the allocator 50   These accessors exist to implement the allocator
51   propagation portion of the @ref IoAwaitable protocol. 51   propagation portion of the @ref IoAwaitable protocol.
52   Launch functions (`run_async`, `run`) set the 52   Launch functions (`run_async`, `run`) set the
53   thread-local value before invoking a child coroutine; 53   thread-local value before invoking a child coroutine;
54   the child's `promise_type::operator new` reads it to 54   the child's `promise_type::operator new` reads it to
55   allocate the coroutine frame from the correct resource. 55   allocate the coroutine frame from the correct resource.
56   56  
57   The value is only valid during a narrow execution 57   The value is only valid during a narrow execution
58   window. Between a coroutine's resumption 58   window. Between a coroutine's resumption
59   and the next suspension point, the protocol guarantees 59   and the next suspension point, the protocol guarantees
60   that TLS contains the allocator associated with the 60   that TLS contains the allocator associated with the
61   currently running chain. Outside that window the value 61   currently running chain. Outside that window the value
62   is indeterminate. Only code that implements an 62   is indeterminate. Only code that implements an
63   @ref IoAwaitable should call these functions. 63   @ref IoAwaitable should call these functions.
64   64  
65   A return value of `nullptr` means "not specified" - 65   A return value of `nullptr` means "not specified" -
66   no allocator has been established for this chain. 66   no allocator has been established for this chain.
67   The awaitable is free to use whatever allocation 67   The awaitable is free to use whatever allocation
68   strategy makes best sense (e.g. 68   strategy makes best sense (e.g.
69   `std::pmr::new_delete_resource()`). 69   `std::pmr::new_delete_resource()`).
70   70  
71   Use of the frame allocator is optional. An awaitable 71   Use of the frame allocator is optional. An awaitable
72   that does not consult this value to allocate its 72   that does not consult this value to allocate its
73   coroutine frame is never wrong. However, a conforming 73   coroutine frame is never wrong. However, a conforming
74   awaitable must still propagate the allocator faithfully 74   awaitable must still propagate the allocator faithfully
75   so that downstream coroutines can use it. 75   so that downstream coroutines can use it.
76   76  
77   @return The thread-local memory_resource pointer, 77   @return The thread-local memory_resource pointer,
78   or `nullptr` if none has been set. 78   or `nullptr` if none has been set.
79   79  
80   @see set_current_frame_allocator, IoAwaitable 80   @see set_current_frame_allocator, IoAwaitable
81   */ 81   */
82   inline 82   inline
83   std::pmr::memory_resource* 83   std::pmr::memory_resource*
HITCBC 84   9933 get_current_frame_allocator() noexcept 84   9982 get_current_frame_allocator() noexcept
85   { 85   {
HITCBC 86   9933 return detail::current_frame_allocator_ref(); 86   9982 return detail::current_frame_allocator_ref();
87   } 87   }
88   88  
89   /** Set the current frame allocator for this thread. 89   /** Set the current frame allocator for this thread.
90   90  
91   Installs @p mr as the frame allocator that will be 91   Installs @p mr as the frame allocator that will be
92   read by the next coroutine's `promise_type::operator 92   read by the next coroutine's `promise_type::operator
93   new` on this thread. Only launch functions and 93   new` on this thread. Only launch functions and
94   @ref IoAwaitable machinery should call this; see 94   @ref IoAwaitable machinery should call this; see
95   @ref get_current_frame_allocator for the full protocol 95   @ref get_current_frame_allocator for the full protocol
96   description. 96   description.
97   97  
98   Passing `nullptr` means "not specified" - no 98   Passing `nullptr` means "not specified" - no
99   particular allocator is established for the chain. 99   particular allocator is established for the chain.
100   100  
101   @param mr The memory_resource to install, or 101   @param mr The memory_resource to install, or
102   `nullptr` to clear. 102   `nullptr` to clear.
103   103  
104   @see get_current_frame_allocator, IoAwaitable 104   @see get_current_frame_allocator, IoAwaitable
105   */ 105   */
106   inline void 106   inline void
HITCBC 107   21973 set_current_frame_allocator( 107   22035 set_current_frame_allocator(
108   std::pmr::memory_resource* mr) noexcept 108   std::pmr::memory_resource* mr) noexcept
109   { 109   {
HITCBC 110   21973 detail::current_frame_allocator_ref() = mr; 110   22035 detail::current_frame_allocator_ref() = mr;
HITCBC 111   21973 } 111   22035 }
112   112  
113   /** Resume a coroutine handle with frame-allocator TLS protection. 113   /** Resume a coroutine handle with frame-allocator TLS protection.
114   114  
115   Saves the current thread-local frame allocator before 115   Saves the current thread-local frame allocator before
116   calling `h.resume()`, then restores it after the call 116   calling `h.resume()`, then restores it after the call
117   returns. This prevents a resumed coroutine's 117   returns. This prevents a resumed coroutine's
118   `await_resume` from permanently overwriting the caller's 118   `await_resume` from permanently overwriting the caller's
119   allocator value. 119   allocator value.
120   120  
121   Between a coroutine's resumption and its next child 121   Between a coroutine's resumption and its next child
122   invocation, arbitrary user code may run. If that code 122   invocation, arbitrary user code may run. If that code
123   resumes a coroutine from a different chain on this 123   resumes a coroutine from a different chain on this
124   thread, the other coroutine's `await_resume` overwrites 124   thread, the other coroutine's `await_resume` overwrites
125   TLS with its own allocator. Without save/restore, the 125   TLS with its own allocator. Without save/restore, the
126   original coroutine's next child would allocate from 126   original coroutine's next child would allocate from
127   the wrong resource. 127   the wrong resource.
128   128  
129   Event loops, strand dispatch loops, and any code that 129   Event loops, strand dispatch loops, and any code that
130   calls `.resume()` on a coroutine handle should use 130   calls `.resume()` on a coroutine handle should use
131   this function instead of calling `.resume()` directly. 131   this function instead of calling `.resume()` directly.
132   See the @ref Executor concept documentation for details. 132   See the @ref Executor concept documentation for details.
133   133  
134   @param h The coroutine handle to resume. 134   @param h The coroutine handle to resume.
135   135  
136   @see get_current_frame_allocator, set_current_frame_allocator 136   @see get_current_frame_allocator, set_current_frame_allocator
137   */ 137   */
138   inline void 138   inline void
HITCBC 139   1364 safe_resume(std::coroutine_handle<> h) noexcept 139   1393 safe_resume(std::coroutine_handle<> h) noexcept
140   { 140   {
HITCBC 141   1364 auto* saved = get_current_frame_allocator(); 141   1393 auto* saved = get_current_frame_allocator();
HITCBC 142   1364 h.resume(); 142   1393 h.resume();
HITCBC 143   1364 set_current_frame_allocator(saved); 143   1393 set_current_frame_allocator(saved);
HITCBC 144   1364 } 144   1393 }
145   145  
146   } // namespace capy 146   } // namespace capy
147   } // namespace boost 147   } // namespace boost
148   148  
149   #endif 149   #endif