1 // Copyright 2021 gRPC authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #ifndef GRPC_SRC_CORE_LIB_PROMISE_CONTEXT_H
16 #define GRPC_SRC_CORE_LIB_PROMISE_CONTEXT_H
17
18 #include <grpc/support/port_platform.h>
19
20 #include <utility>
21
22 #include "absl/meta/type_traits.h"
23
24 #include <grpc/support/log.h>
25
26 #include "src/core/lib/gprpp/down_cast.h"
27
28 namespace grpc_core {
29
30 // To avoid accidentally creating context types, we require an explicit
31 // specialization of this template per context type. The specialization need
32 // not contain any members, only exist.
33 // The reason for avoiding this is that context types each use a thread local.
34 template <typename T>
35 struct ContextType;
36
37 // Some contexts can be subclassed. If the subclass is set as that context
38 // then GetContext<Base>() will return the base, and GetContext<Derived>() will
39 // DownCast to the derived type.
40 // Specializations of this type should be created for each derived type, and
41 // should have a single using statement Base pointing to the derived base class.
42 // Example:
43 // class SomeContext {};
44 // class SomeDerivedContext : public SomeContext {};
45 // template <> struct ContextType<SomeContext> {};
46 // template <> struct ContextSubclass<SomeDerivedContext> {
47 // using Base = SomeContext;
48 // };
49 template <typename Derived>
50 struct ContextSubclass;
51
52 namespace promise_detail {
53
54 template <typename T, typename = void>
55 class Context;
56
57 template <typename T>
58 class ThreadLocalContext : public ContextType<T> {
59 public:
ThreadLocalContext(T * p)60 explicit ThreadLocalContext(T* p) : old_(current_) { current_ = p; }
~ThreadLocalContext()61 ~ThreadLocalContext() { current_ = old_; }
62 ThreadLocalContext(const ThreadLocalContext&) = delete;
63 ThreadLocalContext& operator=(const ThreadLocalContext&) = delete;
64
get()65 static T* get() { return current_; }
66
67 private:
68 T* const old_;
69 static thread_local T* current_;
70 };
71
72 template <typename T>
73 thread_local T* ThreadLocalContext<T>::current_;
74
75 template <typename T>
76 class Context<T, absl::void_t<decltype(ContextType<T>())>>
77 : public ThreadLocalContext<T> {
78 using ThreadLocalContext<T>::ThreadLocalContext;
79 };
80
81 template <typename T>
82 class Context<T, absl::void_t<typename ContextSubclass<T>::Base>>
83 : public Context<typename ContextSubclass<T>::Base> {
84 public:
85 using Context<typename ContextSubclass<T>::Base>::Context;
get()86 static T* get() {
87 return DownCast<T*>(Context<typename ContextSubclass<T>::Base>::get());
88 }
89 };
90
91 template <typename T, typename F>
92 class WithContext {
93 public:
WithContext(F f,T * context)94 WithContext(F f, T* context) : context_(context), f_(std::move(f)) {}
95
operator()96 decltype(std::declval<F>()()) operator()() {
97 Context<T> ctx(context_);
98 return f_();
99 }
100
101 private:
102 T* context_;
103 F f_;
104 };
105
106 } // namespace promise_detail
107
108 // Return true if a context of type T is currently active.
109 template <typename T>
HasContext()110 bool HasContext() {
111 return promise_detail::Context<T>::get() != nullptr;
112 }
113
114 // Retrieve the current value of a context, or abort if the value is unset.
115 template <typename T>
GetContext()116 T* GetContext() {
117 auto* p = promise_detail::Context<T>::get();
118 GPR_ASSERT(p != nullptr);
119 return p;
120 }
121
122 // Given a promise and a context, return a promise that has that context set.
123 template <typename T, typename F>
WithContext(F f,T * context)124 promise_detail::WithContext<T, F> WithContext(F f, T* context) {
125 return promise_detail::WithContext<T, F>(std::move(f), context);
126 }
127
128 } // namespace grpc_core
129
130 #endif // GRPC_SRC_CORE_LIB_PROMISE_CONTEXT_H
131