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26 #include <folly/SharedMutex.h>
27 #include <folly/Synchronized.h>
31 // Some request context that follows an async request through a process
32 // Everything in the context must be thread safe
36 virtual ~RequestData() = default;
37 // Avoid calling RequestContext::setContextData, setContextDataIfAbsent, or
38 // clearContextData from these callbacks. Doing so will cause deadlock. We
39 // could fix these deadlocks, but only at significant performance penalty, so
41 virtual void onSet() {}
42 virtual void onUnset() {}
47 // If you do not call create() to create a unique request context,
48 // this default request context will always be returned, and is never
49 // copied between threads.
50 class RequestContext {
52 // Create a unique request context for this request.
53 // It will be passed between queues / threads (where implemented),
54 // so it should be valid for the lifetime of the request.
55 static void create() {
56 setContext(std::make_shared<RequestContext>());
59 // Get the current context.
60 static RequestContext* get();
62 // The following API may be used to set per-request data in a thread-safe way.
63 // This access is still performance sensitive, so please ask if you need help
64 // profiling any use of these functions.
66 const std::string& val,
67 std::unique_ptr<RequestData> data);
69 // Unlike setContextData, this method does not panic if the key is already
70 // present. Returns true iff the new value has been inserted.
71 bool setContextDataIfAbsent(
72 const std::string& val,
73 std::unique_ptr<RequestData> data);
75 bool hasContextData(const std::string& val) const;
77 RequestData* getContextData(const std::string& val);
78 const RequestData* getContextData(const std::string& val) const;
83 void clearContextData(const std::string& val);
85 // The following API is used to pass the context through queues / threads.
86 // saveContext is called to get a shared_ptr to the context, and
87 // setContext is used to reset it on the other side of the queue.
89 // Whenever possible, use RequestContextScopeGuard instead of setContext
90 // to make sure that RequestContext is reset to the original value when
93 // A shared_ptr is used, because many request may fan out across
94 // multiple threads, or do post-send processing, etc.
95 static std::shared_ptr<RequestContext> setContext(
96 std::shared_ptr<RequestContext> ctx);
98 static std::shared_ptr<RequestContext> saveContext() {
99 return getStaticContext();
103 static std::shared_ptr<RequestContext>& getStaticContext();
105 using Data = std::map<std::string, std::unique_ptr<RequestData>>;
106 folly::Synchronized<Data, folly::SharedMutex> data_;
109 class RequestContextScopeGuard {
111 std::shared_ptr<RequestContext> prev_;
114 RequestContextScopeGuard(const RequestContextScopeGuard&) = delete;
115 RequestContextScopeGuard& operator=(const RequestContextScopeGuard&) = delete;
116 RequestContextScopeGuard(RequestContextScopeGuard&&) = delete;
117 RequestContextScopeGuard& operator=(RequestContextScopeGuard&&) = delete;
119 // Create a new RequestContext and reset to the original value when
120 // this goes out of scope.
121 RequestContextScopeGuard() : prev_(RequestContext::saveContext()) {
122 RequestContext::create();
125 // Set a RequestContext that was previously captured by saveContext(). It will
126 // be automatically reset to the original value when this goes out of scope.
127 explicit RequestContextScopeGuard(std::shared_ptr<RequestContext> ctx)
128 : prev_(RequestContext::setContext(std::move(ctx))) {
131 ~RequestContextScopeGuard() {
132 RequestContext::setContext(std::move(prev_));